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Sedimentary rocks are those formed at or near the Earth's surface by the depostion (by water, wind or ice), accumulation and lithification of sediment (detrital rock) or by the precipitation from solution (chemical rock) and/or growth in position by organic processes (e.g., carbonate reefs). They are formed at or near the earth's surface at relatively low temperatures and pressures. They typically occur in layers (strata) separated by bedding planes and differences in composition. Sedimentary rocks are the most common rocks exposed on the Earth's surface but are only a minor constituent of the entire crust, which is dominated by igneous and metamorphic rocks. They consist 75% of all rocks exposed at Earth's surface. Many of you will work more with sedimentary rocks than with the other rock types.
1.1.1 Sedimentary Environments
Sedimentary rocks are formed in a variety of environment where sediment accumulates. Sedimentary environments typically are in the areas of low elevation at the surface, which can be divided into continental, shoreline, and marine environments. Each environment is characterized by certain physical, chemical, and biological conditions, thus sedimentary rocks hold clues to ancient environments and Earth history.
1.1.2 Diagenesis and Lithification
Lithification is a complex process whereby freshly unconsolidated deposits of sediments are converted into rock. The main processes of lithification are: • • • Compaction Cementation Crystallization
The physical, chemical and biologic changes that occur under conditions of pressure (up to 1 kb) and temperature (maximum range of 100°C to 300°C) that are normal to the outer part of the Earth's crust to the deposited sediments from the time of their initial deposition to their lithification and after it are known as diagenesis. In very thick sedimentary sequences, diagenesis may grade into metamorphism, and the sediment will change from a sedimentary rock to a meta-sedimentary rock to a metamorphic rock. Some geologists restrict the term to the initial phase of post-sedimentary changes, occurring in the zone where the sediment is still unconsolidated, the process being complete when the sediment has been converted to a more or less compact sedimentary rock. In this usage, the term is equivalent to early diagenesis as used in the U.S. There is no universally excepted definition of the term, and no delimitation (such as the boundary with metamorphism). The main processes of diagenesis are:
1. Compaction 2. Recrystallization 3. Solution 4. Cementation 5. Authigenesis (neocrystallization) 6. Replacement 7. Bioturbation The degree to which each of these processes contributes to the diagenesis of any given sediment is controlled by such factors as: • • • • • • • Composition Pressure (due to burial) Temperature The composition and nature of the pore fluids Grain size Porosity and Permeability The amount of fluid flow
Note that any sediment that has been deposited is subject to diagenesis, not just siliciclastics.
The simplest diagenetic change is compaction. Compaction is the process by which the volume of sediment is reduced as the grains are squeezed together. The reduction in volume as the pore space is reduced helps to hold the rock together by binding the grains closer together and increasing the friction between grains. Intergranular fluid is expelled as volume is reduced. The degree of compaction is controlled by such factors as grain shape, sorting, original porosity, and the amount of pore fluid present.
Recrystallization is a process in which physical or chemical conditions induce a reorientation of the crystal lattices of mineral grains. These textural changes cause the sediment to become lithified. It occurs in response to such factors as pressure, temperature, and fluid phase changes. It also occurs because of solution and reprecipitation of mineral phases already present in the rock.
Solution refers to the process in which a mineral is dissolved. As fluids pass through the sediment, the unstable constituents will dissolve and are transported away or are reprecipitated in nearby pores where conditions are different. The dissolution of soluble minerals by a solution under saturated with respect to that mineral depends on pH, Eh, temperature, pressure, PCO2, ion strength etc. Common minerals to dissolve are evaporates such as halite, sylvite and anhydrite. This process is important because it commonly leads to secondary porosity. Pressure solution is a process that occurs as pressure is concentrated at the point of contact between two grains in the sediment. This causes solution and subsequent migration of ions or molecules away from the point of contact, towards an area of lower pressure where the dissolved phase can be reprecipitated.
Cementation is the process in which chemical precipitates (in the form of new crystals) form in the pores of a sediment or rock, binding the grains together. Some common cements are quartz, calcite and hematite, but a wide variety of cements are known, such as aragonite, gypsum, and dolomite. Pressure solution produces locally derived cement, but many cements consist of new minerals previously in solution in the fluid phase. Cementation reduces porosity by filling in the pore spaces between the grains. Higher pH and higher temperatures favor carbonate cements. Lower pH and low temperatures favor quartz or chert cements. Syntaxial overgrowths are formed when cement growth occurs as and an extension of existing detrital "crystal". The reverse process, called decementation, also is thought to occur. There is evidence that decementation has occurred in calcareous sandstones, in which case the calcareous cement or grains are dissolved in the same manner as the solution of limestones. The frosted and etched surfaces of quartz grains in some friable and loosely cemented sandstones seem to indicate the former presence of a carbonate cement that has been leached.
126.96.36.199 Authigenesis (Neocrystallization)
Authigenesis (neocrystallization) is the process in which new mineral phases are crystallized in the sediment or rock during diagenesis. These new minerals may be produced: • • • By reactions involving phases already present in the sediment (or rock) Through precipitation of materials introduced in the fluid phase, or From a combination of primary sedimentary and introduced components.
This process overlaps with weathering and cementation. It usually involves recrystallization and may result in replacement. Authigenic phases include silicates such as quartz, alkali feldspar, clays and zeolites; carbonates such as calcite and dolomite; evaporite minerals such as halite, sylvite and gypsum, as well as many others. Examples of authigenesis are:
• • • • •
Pyrite from iron minerals under reducing conditions Oxidation of iron minerals under oxidizing conditions (rusting) Alteration of clay minerals Formation of clay minerals by feldspar alteration Alteration of volcanic ash
Replacement involves the essentially simultaneous dissolution of existing minerals and the precipitation of a new mineral in situ. Replacement may be: • neomorphic: where the new grain is the same phase as the old grain, or is a polymorph of it (i.e. albitization; replacing a grain with a more Na-rich plagioclase grain). pseudomorphic: where the old grain is replaced with a new mineral but the relict crystal form is retained, allomorphic: an old phase is replaced with a new phase with a new crystal form
Although there are many replacement phases, dolomite, opal, quartz, and illite are some of the most important phases. Examples of replacement are: • • • Petrifaction of organic matter Silicification of carbonates Replacement of fossil material by pyrite
Bioturbation refers to the physical and biological activities that occur at or near the sediment surface which cause the sediment to become mixed. Burrowing and boring by organisms in this way, can increase the compaction of the sediment and usually destroys any laminations or bedding. During bioturbation, some organisms precipitate minerals that act as cement.
1.1.3 Classification of Sedimentary Rocks
There are three major categories of sedimentary rocks are recognized: 1. Terrigenous clastic sedimentary rocks 2. Carbonates (limestone and dolomite)
coarse clasts. Thus. Varieties of fine clastics include siltstone (average grain size between 1/16 and 1/256 millimeter) and claystone (discrete particles are mostly finer than 1/256 millimeter).2 to 2. and marine currents. and an understanding of their origin is hindered by analytical complexities . 1. a pebble conglomerate is a coarse clastic sedimentary rock whose discrete particles are rounded and range from 4 to 64 millimeters (0. coarse clasts. Shale is any fine clastic sedimentary rock that exhibits fissility. Conglomerates and breccias differ from one another only in clast angularity.e. which is the ability to break into thin slabs along narrowly spaced planes parallel to the layers of stratification. disagreement exists as to what classification schemes are most useful for them. lamination inclined to the main bedding plane).3. Despite the great abundance of the fine clastics. They contain less alumina. and flowage within a layer. Because most of the clasts are rich in silica. Siltstones occur in thin layers rarely thick enough to be classified as formations. they are also referred to as siliciclastic sedimentary rocks. a granite-gneiss pebble conglomerate. in addition to mica. sandstone. and finer-than-silt-sized mudrock (shale. feldspar.1 Terrigenous Clastic Sedimentary Rocks (or Clastic Sedimentary Rocks) Terrigenous clastic sedimentary rocks are composed of the detrital fragments of preexisting rocks and minerals and are conventionally considered equivalent to clastic sedimentary rocks in general. potash. and rock fragment components. Although many shales contain more than 50 percent silt. glaciers. Siliciclastics are further subdivided based on grain diameter into conglomerate and breccia. and water than shales but more silica. The simplest way of classifying coarse clastic sedimentary rocks is to name the rock and include a brief description of its particular characteristics. Noncarbonate chemical sedimentary rocks The next two types could be termed as chemical sedimentary rocks. The term clastic (Greek: "broken") describes the broken and worn particles of rock and minerals that were carried to the sites of deposition by streams. claystone. Claystone generally has less than 1/3 silt where silts are small quartz grains that may be large enough to be seen with a hand lens. somewhat rounded. not all are siltstones. as well as the nature of the material housed between this sand-size "framework" fraction.. siltstones differ from these shales in that they commonly are chemically cemented and show such features as crossbedding (i. Sandstones are abundant in the geologic record and provide an enormous amount of information about depositional setting and origin.5 inches) in diameter. A more precise description reveals the rock types of the mineral fragments that compose the conglomerate--for example. The sediments of chemical sedimentary rocks are soluble material produced largely by chemical weathering and are precipitated by inorganic or organic processes. and mudstone). whereas the latter contain angular. they may contain abundant chlorite and other micaceous clay minerals. Most schemes emphasize the relative abundance of sand-size quartz. cut-and-fill structures.1. Many classification schemes have been developed for sandstones.3. wind. siltstone. The former consist of abraded.
iron-rich sedimentary rocks (iron formations and ironstones). This is a reflection of the great variation in mineral composition. similar to transported quartz or feldspar clasts. siliceous rocks (cherts). hematite. phosphorites. evaporites. In their life processes. For mud logging purposes the Dunham classification is used. because individual grains such as fossil shell fragments exist for some period of time as sedimentary clasts. The simple invertebrate animals secrete calcite-rich skeletons to create massive reefs. They differ in many respects from carbonate sedimentary rocks and terrigenous clastic sedimentary rocks. evaporites (rock salt. siderite. . and the many categories of limestones and dolomites in the geologic record represent a large variety of depositional settings. and invertebrate animals use it to construct their shells or hard parts. and other salts). Dolomites are mainly produced by the secondary alteration or replacement of limestones. consist of the minerals aragonite. They are the most abundant chemical sedimentary rocks.1.3. although they may form thick and widespread deposits. most carbonates bear some textural affinities to the terrigenous clastic sedimentary rocks. and chamosite). and petroleum). However. gypsum. and organic-rich (carbonaceous) deposits of oil. However. Specific varieties discussed below include siliceous rocks (cherts). 1.e. there is no single classification that has been universally accepted. oil shale. natural gas.3. Such rocks as ironstones and banded iron formations (limonite. They make up the bulk of the nonterrigenous sedimentary rocks. Limestones are for the most part primary carbonate rocks.. goethite. They consist of 50 percent or more calcite and aragonite (both CaCO3).1. and coal in sedimentary rocks occur in much less abundance than carbonates and siliciclastic sedimentary rocks. A number of different classification schemes have been proposed for carbonates. phosphate rocks (phosphorites). many plants extract calcium carbonate from water. limestones and dolomites. A considerable amount of calcium carbonate is known to precipitate directly in open warm ocean and quiet water. such as the best-known Great Barrier Reef in Australia.1. Corals are one important example of organisms that are capable of creating large quantities of marine limestone. and other properties of these rock types.2 Chemical Sedimentary Rocks: Carbonates The carbonates. calcite. and organic-rich (carbonaceous) deposits in sedimentary rocks (coal. and dolomite. texture. interlocking mosaic of precipitated carbonate mineral grains. i. the mineral dolomite [CaMg(CO3)2] replaces the calcite and aragonite minerals in limestones during diagenesis.3 Noncarbonate Chemical Sedimentary Rocks The noncarbonate chemical sedimentary rocks include several rock types that are uncommon in the sedimentary rock record but remain important either from an economic point of view or because their deposition requires unusual settings. They are chemical sedimentary rocks in the sense that they possess at least in part a crystalline. Limestones originate by both chemical and biological processes.
it does not refer to refined hydrocarbon products Plastic hydrocarbons – like asphalt Solid hydrocarbons . and viscosity vary widely because they consist of mixtures of thousands of hydrocarbon compounds.1. specifically contains less than 0. surface temperatures Liquid hydrocarbons . Their viscosity generally decreases with temperature. 2 Properties of Oil and Gas 2.hydrocarbons which are gasses in the subsurface where temperatures are high. . They vary greatly in their chemical composition. dark oils . Petroleum exploration is largely concerned with the fluids (gases and liquids). CO2. usually brittle. contain different mixtures of hydrocarbons and other compounds. or crude oil. green) volatile liquids to thick. They vary from light colored (yellow. but condense to liquid when reach cooler. • • • • 2. butane and up to the molecular weight where the fluids are always condensed to liquids Condensates .1. and other less common gases.so viscous that they are difficult to pump from the ground.Classification schemes that incorporate all types of noncarbonate chemical sedimentary rocks do not exist because no triangular or tetrahedral scheme can accommodate all of them.1 Definitions Hydrocarbons are substance made of hydrogen and carbon (among other elements).2.1 gal/1000ft3 of condensible (at surface Temperature and Pressure) material Wet gas .contains largely methane.1 Physical properties Crude oils from different parts of the world.contains ethane. propane. There are several specific forms of hydrocarbons: • • • Dry gas .commonly known as oil. so that oil at surface is less viscous than oil in subsurface. but also H2S. or even from different depths in the same oilfield.2 Properties of Oil 2. liquids and solids. specific gravity. Their physical properties such as color. They grade through three states of matter: gases.1. usually methane.refers to hydrocarbons that are in a solid state at surface conditions. and often shiny and glossy in appearance Gas hydrates .Solids composed of water molecules surrounding gas molecules.
1.1 Introduction Oil is largely composed of carbon and hydrogen and other components that include sulfur. a metric ton is equal to approximately 252 Imperial gallons or about 7.2. . In general. barrels. with light oils ranging above 25º.2.there are hundreds of different compounds that can be generated from carbon and hydrogen. viscosity and API gravity are inversely related. Crude oils below 20º API gravity are usually considered heavy.2. For oil having the same gravity.1. 2. crude oil is measured in metric tons. The pour point is the temperature below which crude oil becomes plastic and will not flow and it affects the recovery and transport of oil.1. 2.1. oxygen. heavy oils.2. Liquids lighter than water.2 Hydrocarbons which contain only hydrogen and carbon Hetero-compounds contain elements in addition to hydrogen and carbon Hydrocarbon Content Almost all crude oil have 82 to 87 percent carbon by weight and 12 to 15 percent hydrogen.3 Chemical Composition 2. However an important temperature reading called the pour point is always determined.1.3. The weight per barrel of API 30º light oil would be about 306 pounds.2. whereas the conventional crude oils with API gravities between 20º and 25º are regarded as medium.1 Specific gravity Based on specific gravity and relative mobility crude oils are generally classified as tar sands.1. hydrogen and other elements.2. one barrel of oil is equivalent to 42 gallons.1. The more viscous bitumens generally vary from 80 to 85 percent carbon and from 8 to 11 percent hydrogen. These compounds divided into: • • 2. The limited chemical composition is misleading . and medium and light oils. In many other countries. such as oil.2 Boiling and Freezing Points It is impossible to refer to common boiling and freezing points for a crude oil because the individual compounds have widely differing boiling points and solidify at different temperatures.2 U.2 Measurement systems In the United States.3. 2. with an arbitrarily assigned API gravity of 10º. The widely used American Petroleum Institute (API) gravity scale is based on pure water.S.2. Pour points range from 32º C to below -57º C.1. have API gravities numerically greater than 10.
these have higher boiling points than the branched alkanes Branched chain. but different structures): • • Straight chain. Its most common member. aromatics and olefins. called "normal alkanes" e. While those with more than 15 carbon atoms are viscous liquids and solids. molecules with identical compositions. They have a sweet smell thus they are named aromatics. They constitute only a small percentage of most crude oils.Crude oil can be grouped into four basic chemical series: paraffins. also called the methane (CH4) series. They compose approximately 40% of oil.with the general formula CnH2n .six carbon ring . Figure 1 4 most common hydrocarbon chemical series (from Encylopaedia Britannica). benzene (C6H6). Those with 5 to 15 carbon atoms are liquids. normal butane. and the crude oils in which this series predominates are called asphalt-base crudes. The aromatic series has a composition based on the benzene ring . Most crude oils are mixtures of these four series in various and seemingly endless proportions. is present in all crude oils.g. The residue of the refinery process is asphalt.e. The paraffin (alkane) series of hydrocarbons. Largest molecule recorded from crude oil contains 78 carbons. No two crude oils from different sources are completely identical.g.6 and is an unsaturated closed-ring series. called "isoalkanes" e. . naphthenes. Alkanes with less than 5 carbons are gas. The series comprises the most common hydrocarbons in crude oil. is a saturated straight-chain series that has the general formula CnH2n + 2. The naphthene (cycloalkane) series has the general formula CnH2n and is a saturated closed-ring series composed of 5 and 6 member rings. All are liquid at surface temperature and pressure. There are two types of alkane isomers (i. isobutane The paraffins that are liquid at normal temperatures but boil between 40º and 200º C (approximately between 100º and 400º F) are the major constituents of gasoline.
3. respectively. Monoolefins. Sodium chloride also occurs in most crude oils and is usually removed like sulfur. two. usually in quantities of less than 0.CO2.1. three. propylene. and butylenes are examples of acyclic monoolefins.05 percent (by weight) up to 5 percent or more in heavy Mexican or Mississippi oils..1. Crude oil also may contain a small amount of decay-resistant organic remains. They are rare but are formed in huge quantities during the cracking of crude oil to gasoline.3. • Ethylene.3 Hetero-Compounds (Non-hydrocarbon) Sulfur is the third most abundant atomic constituent of crude oils. the greater is its sulfur content. triolefins. in which the double bond is located between carbon atoms forming part of a cyclic (closed-ring) or of an open chain grouping. because sulfur oxides released into the atmosphere during the combustion of oil would constitute a major pollutant. Among the most common metallic elements in oil are vanadium and nickel. 2. or some other number. such as siliceous skeletal fragments. Krypton.1. the heavier oils contain the most oxygen. resins.Helium. Argon. The excess sulfur is removed from crude oil during refining.1 percent by weight. The total sulfur in crude oil varies from below 0. etc. wood. spores. in which the number of double bonds per molecule is. Nitrogen is present in almost all crude oils. The common gasses in reservoirs can be divided based on their origins: • • Inorganic . coal. and various other remnants of former life. diolefins. one.1 Definitions Given a strict definition by the petroleum industry of gas being "a mixture of hydrocarbons and varying quantities of non-hydrocarbons that exists either in the gaseous phase or in solution with crude oil in natural underground reservoirs".2. Radon Mixed inorganic and organic .3 Properties of Gas 2. Generally. H2S . 2. For this reason. Many metallic elements are found in crude oils. They are classified in either or both of the following ways: • Cyclic or acyclic (aliphatic) olefins. The oxygen content of crude oil is usually less than 2 percent by weight and is present as part of the heavier hydrocarbon compounds in most cases.The olefin series (also called alkene series) consists of unsaturated hydrocarbons containing one or more pairs of carbon atoms linked by a double bond. respectively. the higher the specific gravity of the crude oil. which apparently occur in organic combinations as they do in living plants and animals.
It can be (and usually is) physically separated from the liquid when the fluids are produced. Propane. • • • • • 2. It is free gas (not dissolved) that sits on top of.Hydrogen.2 Components of Natural Gases To be written at a future date.Free gas that is trapped without a significant amount of crude oil. Butane The various descriptive terms for natural gas: • Dissolved gas .Also known as the "gas cap".3.Contains no H2S Sour Gas (and Oil) .• Organic . They include the wet gases. Natural gas liquids . in the field and at gas processing plants. Non-associated gas . . Sweet Gas (and Oil) . Ethane. Methane.That portion of natural gas that is dissolved in liquid phase in the subsurface.Contains H2S. and are liquified. natural gasoline. and in contact with. and condensate.The liquids that can be.1. Associated gas . crude oil in the reservoir.
1 Death. some was deposited on the beds of inland seas. lakes. warm up. Oil and gas are derived almost entirely from decayed animals.3 Petroleum Generation and Accumulation 3. Kerogens are dark-colored and insoluble in water and petroleum solvents. It may consist of compacted organic material. From the first living cell to ourselves millions upon millions of plant and animal species have sprung up and then vanished. the oldest layers being buried beneath ones that are more recent. The plants. nitrogen and oxygen. vast amounts of plant material accumulate. and were thus protected from bacterial action. plants and bacteria. but have not yet. This causes fats. Here the remains of living things are “cooked”. and any organic material they contain." Wallace G. waxes and oils from algae. They combined to form amino acids. and then accumulates in layers over many millions of years. The continuous action of plate tectonics at work in the Earth's mantle breaks up these layers and precipitates them still more deeply into the Earth's crust. lagoons. To alleviate the confusion that exists with regard to use of the term "source rock. Organic matter mingled with sediment (sand. In these areas. and S. Terms such as "effective source rocks" or "potential source rocks" are commonly used interchangeably. They have an indefinite and complex mixture of compounds with large molecules containing mainly H and C but also O. the higher the temperatures and pressures rise. hydrogen. The various types of source rocks depend largely on their state of maturity and the distinctions . nitrogen and oxygen) created the conditions for life. expelled hydrocarbons. producing a stagnant environment that is unfit for larger grazing and scavenging animals. N. river deltas and other oxygen-poor aquatic milieus.2 Maturation The further these sedimentary layers sink in the sedimentary basin.). However. By their sheer mass. hydrogen. these sedimentary layers sink naturally. most organic waste is destroyed and digested by bacteria. Many special circumstances have to be in place for hydrocarbons to form. bacteria spores and cuticle (leaf’s skin) to link and form dark specks of kerogen. It is the source of most hydrocarbons generated. Decay and Burial Billions of years ago. 3. "migratable. Being made up of carbon. Bacteria breaking down this material may use up all the available oxygen. etc. Dow presented a list of source rock definitions in 1977 that were slightly modified by Colin Barker in 1979. The presence of four simple elements (carbon. salt. bacteria and the chemicals derived from their decay become buried in silts and muds and are preserved. Temperatures within the Earth’s crust increase with depth causing sediments. the sun fuelled life on earth." or "commercial" amounts of oil or gas is called a source rock. Potential source rocks are those that can. A unit of rock that has generated and expelled "significant". billions upon billions of creatures have lived and died.
A trap that is formed after a source rock became inactive for example will not contain oil or gas generated from that source rock even though geochemical data appear favorable. The distinction between source rocks and potential (immature) source rocks are essential in petroleum system studies and when correlating oils to their source rocks.are important in petroleum system studies. A unit of rock that has the capacity to generate oil or gas in commercial quantities but has not yet done so because of insufficient catagenesis (thermal maturation). Active source rocks cannot occur at the surface. A source rock that was once active but has temporarily stopped generating prior to becoming spent. Active source rocks have oil window maturities and are at or close to their maximum burial depth. The term "commercial" is variable and the terms "migratable" or "significant" are often substituted. Cannot be defined by geochemical data alone but requires geological information as to the thickness and aerial extent. A source rock that is in the process of generating oil or gas. Oil-rock correlations are best done between oils and active or inactive source rocks. Active source rocks cannot occur at the surface. A spent oil source rock can still be an active or inactive source for gas. Inactive source rocks are usually associated with areas of overburden removal and will generate hydrocarbons again if reburied. The distribution of active source rocks is essential in petroleum system studies. Limited source rock Potential source rock Active source rock Active source rock Inactive source rock Spent source rocks . The distribution of active source rocks is essential in petroleum system studies. We feel that proper and consistent use of these source rock terms will help mitigate the confusion that exists in the literature and in interpreting and using source rock data. A source rock that is in the process of generating oil or gas. Below is a list of source rock definitions from Dow and slightly modified by Barker: Term Source rock Definition A unit of rock that has generated oil or gas in sufficient quantities to form commercial accumulations. A source rock that has completed the oil and gas generation process. Source rock is synonymous with "effective source rock". A unit of rock that contains all the prerequisites of a source rock except volume. Active source rocks have oil window maturities and are at or close to their maximum burial depth.
2. kerogen is converted into oil and gas by thermal processes known as catagenesis. shorter hydrocarbon chains break away to give light oil and then gas. oil.2. 3. long chains of hydrogen and carbon atoms break from the kerogen.Table 1 Table of source rock definitions. Diagenesis produces the following products: kerogen (the solid remaining). The hydrogen/carbon ratio decreases.2. it begins to release hydrocarbons.3 Metagenesis With continued burial. This process is called thermal degradation and cracking.2 Catagenesis As sediments are more deeply buried. oil containing dissolved gas. The H/C ratio continues to decline in the solid as the fluid H/C ratio increase. the sediments are subjected to extreme pressure and thermal stress. methane. water. Gas condensate is light oil. 188.8.131.52 Formation and Maturation of Kerogen There are three major steps involved in the formation and maturation of kerogens from organic matter. . H 2S.1. forming waxy and viscous heavy oil. The most important hydrocarbons are gas. carbon dioxide (bicarbonate at most pH). The major change to the solid is that its oxygen content is reduced. 3. The rate and types of hydrocarbons released depend on the rate of heating and the length of time available for heating. Initially liquids are released and later at higher temperatures gas.1 Diagenesis Aquatic and terrestrial organic matter that is preserved in sediments is converted to kerogen by biological and very low temperature processes termed diagenesis. As the source rock becomes hotter. These are: • • • Diagenesis Catagenesis Metagenesis 3. Light oil is the more valuable crude. At higher temperatures. Diagenesis happens at shallow depths. organic matter is metamorphosed into methane and graphite by a process called metagenesis. Fluid hydrocarbons are released from the solid matter. but the O/C ratio doesn't change much. which is gaseous at high underground temperatures and pressures.2 Oil and Gas Windows As the kerogen is subjected to deeper burial and increased pressure and temperature. When the source rock starts to generate oil or gas it is said to be mature. It includes biogenic decay (bacteria) and abiotic reactions.1. and gas condensate. but the hydrogen/carbon ratio isn't changed greatly. 3.2.
Oil. they must have moved away after formation. Process of oil and gas formation: As hydrocarbon chains separate from the kerogen during oil and gas generation. Since they accumulate at shallower depths compared to the source rock. 3. The buoyancy of the fluids drives it and the migration occurs when the hydrocarbons are fluid. 3. It can be triggered both by: • Natural compaction: Most sediments accumulate as a mixture of mineral particles and water. (some authors have put it at 65°C and 150°C) called the "oil window" Gas generation: 120 to 225° C. Hydrocarbons typically found in the highest portion of laterally continuous porous and permeable beds. It implies upward and lateral migration 4. This is called migration. Hydrocarbons form at depth through increased temperatures and pressures. As they harden to become rock. respectively. they take up more space and create • . Due to their physical properties. Many things can trigger migration. 2. this is also expelled. gas generation are: • • Oil generation: 60°C to 120°C. Migration is a slow process. Secondary Migration – It is the movement of hydrocarbons within the reservoir. called the "gas window" These temperature ranges for the beginning and end of oil generation (the oil window) depend largely on the type of organic material in the source rock. That implies they are free to move laterally and vertically. gas and water are stratified according to their densities. Empirical evidences for the temperature for oil vs.3 Migration Many observations indicate that hydrocarbons found in reservoir beds did not originate there: 1.Special environment where oil and gas is generated is called the oil window and the gas window. There 2 types of migration: • • Primary Migration – It is the release of oil from kerogen and its movement in the narrow pores and capillaries of the source rock to the reservoir rock. some water is expelled and dispersed. If the rocks contain oil or gas. with oil and gas Traveling perhaps only a few kilometers over millions of years. They must have flowed in after the porosity formed. Very often oil and gas have a mind of their own. Hydrocarbons are found in secondary porosity. they have a tendency to move about from the source rock and away from it.
1 Porosity Porosity and permeability are the most important physical properties of these reservoirs. Cap rocks must be sufficiently impermeable to act as a barrier to further migration. and capped by a seal called a cap rock. being lighter than water. is an opening in a rock.4.1 Morphological and Genetic Classification of Porosity There are two genetic types of porosities: 1. which may extend for great distances. Extensive marine shales and dense evaporite beds are ideal regional cap rocks. 3. φ . they ultimately seep out through the surface. or solidify as bitumen Much oil is dispersed in isolated blobs through large volumes of rock. Secondary porosity Primary porosity is formed when the sediment is deposited. Probably the world’s best regional cap rock is the Upper Jurassic Hith Anhydrite of the Middle East. This means that liquids and gas can freely move about cracks and pore spaces between the rock particles that are interconnected. gas and oil displace water and settle out in layers due to their low density. gas and water migrate through permeable rocks. Evaporites are ideal because they bend without breaking even at low temperatures and pressures.higher pressure in the source rock. Oil and gas also migrate along large fractures and faults. Impermeable rocks like clays. However. Strictly speaking it is a ratio: Porosity = Open space / total volume of the rock plus opening Porosity is often multiplied by 100 so that it can be expressed as a percentage. Oil. such as porous sandstone or fractured limestone. Water is always present below and within the oil and gas layers. 3. cemented sandstones or salt act as seals. . such rocks are called impermeable.4. For a regional cap rock to be effective. Secondary porosity is formed after sedimentary deposition. Fluid cannot flow through rocks where these spaces are very small or are blocked by mineral growth. Primary porosity 2.4 Accumulation Oil and gas fields need to be trapped in permeable reservoir rocks. it should not be strongly fractured. when large amounts become trapped in porous rocks. 3. Oil and gas. This way. Porosity. oil and gas ooze through minute pores and cracks in the source rock and thence into rocks where the pressure is lower. tend to rise toward the Earth's surface. If there's nothing to stop them. .1. and are large enough to permit fluid movement.
g. Different types of logs can be used for porosity measurements depending on lithology (Sonic. Seismic . Closed pore . measuring the volume of the gas.the pore opens to only one throat passage.the pore opens to more than one throat passage. 3. 3. and independently measuring the volume of the rock and porosity. specifically the size of the pore openings. Caternary . Relative permeability is the ratio of the effective permeability to the total permeability of the rock.where there is no connection with other pores and they cannot produce any type of hydrocarbons Morphological types (1) and (2) are called effective porosity because they allow hydrocarbons to move into and out of them.4.2 Permeability 3. it is the best type because the hydrocarbons in the reservoir can be flushed from rocks 2. Neutron and Density) 2.1 Definition Permeability is a measure of the ability of fluids to pass through a porous medium.2. represents a property of the porous reservoir only. Cul-de-sac . The unit of measure used in the oilfield for intrinsic permeability is the darcy which is defined as: (1cP)(1cm3/sec)/(1cm2)/(1atm/1cm) = 9.2 Porosity Measurements Porosity can be measured using three techniques: 1. gas expansion) 3. This is a more difficult variable to measure in reservoir rocks. Direct measurements of cores – This generally involves filling pore space with gas. the pores cannot be flushed. .There are three types of morphologies to the pore spaces: 1. so they are reported as millidarcy (md). K. 3.4. The effective permeability is the permeability of one of the phases at some saturation. but they can produce hydrocarbons by release of pressure (e.4. Wireline logs – It is one of the primary uses of wireline logs. Intrinsic permeability. but in many ways may be more important than porosity.Through a change in acoustic impedance (v*r) density decreases with increasing porosity. Morphological type (3) is ineffective porosity because no hydrocarbons can move in or out of the rock.87 * 10-9 cm2 Most petroleum reservoirs have permeabilities less than 1 Darcy.1.
4. but such reservoirs are relatively rare. It can be measured with Drill Stem Tests (DST) or estimated with wireline log response. but not all the available pore space is occupied by petroleum.2 Measuring Permeability Although permeability is one of the more important aspects of a reservoir. Accumulations of petroleum also occur in shales and igneous and metamorphic rocks because of porosity resulting from fracturing. it is unlikely that the vast quantities of oil now present in some reservoir rocks could have been generated from material of which no trace remains. It also can be measured directly on core samples with permeameters. Most petroleum accumulations have been found in clastic reservoirs (sandstones and siltstones). insoluble organic matter.4. A certain amount of residual formation water cannot be displaced and is always present. recrystallization. weathering during temporary exposure at the Earth's surface. and grading and packing of the sediment grains and on the manner of their initial consolidation. or induced.2. 3. Petroleum traps provide what is known as closure at the level of the petroleum-bearing reservoir. Reservoir rocks may be divided into two main types: (1) those in which the porosity and permeability is primary. allowing commercial exploitation. Next in number are the carbonate reservoirs (limestones and dolomites). and further cementation.4 Traps A petroleum trap concentrates the petroleum fluids at particular locations. fracturing. Primary porosity and permeability are dependent on the size. These secondary factors may either enhance or diminish the inherent conditions. shape. the site where commercial amounts of oil originated apparently is not always identical to the location at which they are ultimately discovered.3 Reservoir Rocks A reservoir rock is a subsurface body of rock having sufficient porosity and permeability to store and yield hydrocarbons at a commercial rate. The porosity and permeability of carrier and reservoir beds are important factors in the migration and accumulation of oil. Therefore. it is difficult to measure.4. if any. Porosities in reservoir rocks usually range from about 5 to 30 percent. or inherent. and (2) those in which they are secondary. Therefore. Secondary porosity and permeability result from post-depositional factors.3. such as solution. There are two main types of traps: • • Structural traps Stratigraphic traps . 3. It contains little.
In reality most traps are formed by more complex sequence of events.Figure 2 Examples of traps. then sealed by an impermeable rock that was deposited later over the eroded structure. For example. The trap in e was formed by more complex events. and cannot be classified so rigidly. for example that sea trap structures can exist 125 million years ago. Examples of structural traps are: • • • Fault traps Anticlinal traps (80% of the world’s oilfields are in this category) Rocks are domed over rising salt masses A fault trap occurs when the formations on either side of the fault have been moved into a position that prevents further migration of petroleum. It could take. an impermeable formation on one side of the fault may have moved opposite the petroleum-bearing formation on the other side of the fault.1 Structural Traps Structural traps are traps formed by Earth movements. structures must exist before hydrocarbon generation ceases. In order to trap migrating oil and gas.4. but the were not filled with oil until 100 million years later. . The impermeable layer prevents further migration of petroleum. in (e) the reservoir rock was first folded and eroded. For example. 3.4.
Figure 4 An anticline trap (from Maverick Energy).4. and an overlying bed of impermeable rock prevents its escape. Petroleum migrates into the highest part of the fold.4. If reservoir beds beneath an unconformity are tilted. then a trapping geometry results. An unconformity is a surface of erosion.Figure 3 A fault trap (from Maverick Energy). An anticline is an upward fold in the layers of rock. The giant Midway-Sunset oil field in the San Joaquin Valley is an example of an unconformity trap.2 Stratigraphic Traps Stratigraphic traps are traps that result when the reservoir bed is sealed by other beds or by a change in porosity or permeability within the reservoir bed itself. and impermeable beds above the unconformity form a seal. . much like an arch in a building. Prudhoe Bay field in Alaska is another. impermeable rock layer (unconformity). Examples of stratigraphic traps are: • A tilted or inclined layer of petroleum-bearing rock is cutoff or truncated by an essentially horizontal. (80% of the world’s petroleum fields are in this category). 3.
where the reservoir thickness decreases to zero). then a trap may form at the updip pinchout of the reservoir (i. then a trap may result. some might say just a wrinkle. If a porous reservoir rock is encased within an impermeable seal. on the . • Petroleum-bearing formation pinches out.Figure 5 An unconformity acting as a seal. The Gatchell sand in the East Coalinga Extension field in the northern San Joaquin Valley is an example of an updip pinchout. or from more clay or cement in the pore spaces.e. • Change in porosity and permeability in the reservoir itself. If a reservoir rock suddenly loses porosity and permeability in the updip direction and becomes a seal.. The updip change may result from decreasing grain size. North Coles Levee field in the San Joaquin Valley is a small anticline. Figure 6 A stratigraphic trap showing a pinch out. • Reservoir bed is surrounded by impermeable rock. Figure 7 A stratigraphic trap surrounded by impermeable formation (from Maverick Energy). such as shale or salt.
refine. lard and castor oil and from whales were not enough. Paraffin. the demand for lubricants increased.east side of the giant Elk Hills field anticline. so much oil was discovered and produced that the price of kerosene dropped to nearly nothing. Mobil. internally as laxative) and waterproofing (for caulking boats). Hydrocarbons was then used in warfare. the Arabs and Persians distilled crude oil to obtain flammable products for military purposes. Thus when crude oil prices are low. This was the first oil bust. . "Colonel" Edwin L. As industries grew in the last century. and caused a second oil boom. Probably because of the Arab invasion of Spain. 4 The Search for Hydrocarbons 4. and First World War. That is they explore.000 years ago. In 1847. The supplies of natural oils and greases from beef and mutton tallow. Texaco. and the development of mass-produced automobiles. Drake drilled a 59-foot well and produced oil at Oil Creek. Shell. which previously had been used in lamps. BP. Some geologists think that a permeability barrier prevents the oil at North Coles Levee from leaking updip into Elk Hills. and market the products. James Young began retorting oil from oil shales in Scotland. 3. This greatly expanded the demand for oil products initiating the first oil boom. Exxon. With the second oil boom. Pennsylvania. the industrial art of distillation into illuminants became available in Western Europe by the 12th century. Gulf.4. Socal were known as The Seven Sisters. used for candles. Thousands of "independent" oil companies were born. large multinational oil companies developed. In 1859. used for lamps were some of the products. and Babylonians used crude oil and asphalt ("pitch") collected from large seeps at Tuttul (modern-day Hit) on the Euphrates for many purposes more than 5. medication (externally on wounds and rheumatism.1 History of Use The ancient Sumerians.5 A Final Word Only about 2% of the organic matter dispersed in sediments becomes petroleum. and kerosene (coal oil). The majors and some of the minor oil companies are vertically integrated. produce. Of these only 0. Because of this discovery. This marked the beginning of the modern oil business where large volumes of hydrocarbons were extracted from the subsurface.5% of it gets accumulated in reservoirs. Assyrians. Kerosene became cheaper than whale oil. The oil bust continued until the mass production of the internal combustion engine. Early in the Christian era. The auto created another large demand for hydrocarbon products.
Geologists work with a variety of other specialists. which are formed over many millions of years by stretching of the crust combined with sagging. By the 1970's. They attempt to reconstitute a scenario that may have been written long time ago by tracing the history of these deposits and their lateral variations in order to identify places where oil might have become "trapped. . Two more oil busts happened between the mid 80’s and the end of the 20th century. like geophysicists.000.2.2. Nigeria. a worldwide recession. the Organization of Petroleum Exporting Countries (OPEC) was founded. explore and scrupulously record any clue to the possible presence of hydrocarbons below ground. but only 26 are significant producers and seven of these accounts for more than 65 percent of total known oil.2 Exploring For Oil and Gas 4. when crude oil prices are high. OPEC countries produced 2/3 of the world’s oil. Aside from exploration work. Ecuador. The North Sea is an example. In 1960. and drop in consumption of oil. Qatar. but no significant commercial discoveries have been made. 4. The combined drop in consumption and the increase in production from the North Sea (largely from Norway) caused a major slump in oil prices and a bust in the early 1980's an.2 The Job of the Geologist The geologist's job is to observe.they make money in the refining business. 4. Venezuela. and appraising fields following discovery." To do this they examine rocks and take samples to ascertain their nature and date the strata from which they were taken. and geochemists. structural geologists. Thus they were able to control the price of crude oil. meanwhile. No other basin has more than one such field. Nearly all oil and gas is found in such basins. Gabon. Kuwait. Algeria. Iran. Saudi Arabia. Dubai. they make money in the exploration/production business. Supergiant fields are those with 5. Another 240 basins have been explored. About the same time. the price increased rapidly through the 1970's. Indonesia. By limiting production.000. they are also involved in selecting the best exploration permits. locating explorations wells over these traps. The Arabian-Iranian sedimentary basin is the main producing basin and it contains more than 20 supergiant fields. major oil deposits were discovered in the North Sea. reservoir engineers. biostratigraphers. It was composed of countries whose economy was based on oil exports. Libya. The result was conservation. These countries include Iraq. use all of their colleagues' data in order to describe the geometry and characteristics of the oil reservoir.1 Sedimentary Basins Layers of sedimentary rock that are thickest in the middle of basins cover large areas of the continental crust. Petroleum geologists. There are approximately 600 sedimentary basins and about 160 of these have yielded oil. and United Arab Emirates.000 or more barrels of recoverable oil. sedimentologists.
Oil companies rarely have the necessary equipment and personnel to conduct field measurements themselves. seismic reflection techniques are used. as companies do not need them continuously. They initially use gravimetry.4 Seismic Survey Once the contours and depth of the sedimentary basin have been ascertained. Granite near the surface will have a stronger pull than the same lump deeper down. so measurements help to build up more information about the layers of rock. seismic measurement is in fact easier at sea than on land. but it is essential to discovering oil and gas fields. There is more data obtained offshore than onshore. to give some idea of the nature and depth of strata depending on their density. Magnetomer surveys are generally performed from the air.2.3. and a comparison of their results serves to enrich the geologist's findings. They are used occasionally.3. These tools measure gravity.2 Gravimetry Gravitometers measure the strength of the Earth's gravitational pull. 4. less costly than seismic and are used to identify potentially oilbearing sedimentary basins. the geologist's observations then serve to formulate initial hypotheses: yes.3. and later seismic surveys.Oil companies sometimes call in outside experts.2. They therefore contract with outside geophysical service . Sedimentary rocks are nearly non-magnetic and igneous rocks have a stronger magnetic effect. Igneous rocks like granite are denser than sedimentary rocks. measurements can be made to work out the thickness of the sedimentary layers that may contain oil. Computer engineers help geologists in using software to visualize cross-sections. 4. 4. This is not the same all over the Earth because of the different densities of the rocks. maps and other geological data.2. Varieties of methods are used at this stage. including academics and private consultants with leading edge techniques or equipment. They give an idea of the depth distribution of crystalline terrains that have NO chance of containing any oil. These methods measure very precisely the travel time of sound waves generated by a shock applied to the ground or sea. Combined with aerial and satellite photographs. 4.3.2. Because of these different effects on the magnetic field. magnetometry. there could be oil down there. and it could be worthwhile looking further. These methods are more expensive than gravimetry and magnetometry. below ground. The first two are regional in scale. seismic refraction and more often.1 Introduction The geophysicists study the physical properties of the subsoil. Since ships can travel easily in all directions.3 Magnetometry Magnetometers measure very small changes in the strength of the Earth's magnetic field.2.3 The Job of the Geophysiscist 4.
The bit cuts and breaks up rock material as it penetrates the formation. Geophysicists create an image of the subsurface deposits and their deformations. They serve to determine whether certain strata are likely to contain hydrocarbons. and the resulting seismic maps and vertical sections are created. The best location for the siting of the drill rig is determined based on the existing state of knowledge of underground conditions and the topography of the terrain. By plotting all of the points identified on a grid covering the topographical map.4 Arriving At a Decision Geologists. By collating and comparing their experience. their conclusions are the result of a team effort. to help geologists identify traps. The geophysicist at the surface listens to the echo of these waves and records them. Isochronic lines are points on the ground at which the waves take exactly the same time to be reflected / refracted at the surface.2. The shock wave is reflected or refracted by the geological strata back to the surface. All these results are aggregated and studied.companies to conduct seismic surveys. If the point from which the sound wave was generated. production and reservoir engineers all supply data to economists and financial planners. This is generally sited vertically above the thickest part of the stratum thought to contain hydrocarbons. they are going to have to drill down to that zone. where it is picked up by a highly sensitive microphone called a geophone (or hydrophone at sea). petroleum architects. they interpret the image to extrapolate the geometry of the trap.1 Introduction: Drilling with Mud Drilling is accomplished by rotating the drill string by means of a power-driven rotary table or Top Drive System. The “shock” is caused by dropping a heavy weight. and the speed at which it travels through the different rocks are known. geophysicists. one can map the depths of the main strata. In coordination with the geologist. The geophysicist's seismic recordings are fed into powerful computers. a mechanical vibrator. The terrain is mapped by means of isochronic lines.3.3 Mud Engineering 4. 4. If the decision is that it is worth taking a gamble. with a bit attached to the bottom of the pipe. 4. a bubble of high-pressure air. Two and three-dimensional images of the underground strata. then one can deduce the depth of the stratum from which the wave was reflected. or by an explosion. together with drilling. expertise and findings. By juggling figures. The oil companies' in-house geophysicists oversee these campaigns. parameters and probabilities. The way in which the waves are propagated varies as they pass through the different strata. . they seek to work out a possible strategy for developing the reservoir in the event of confirmation of the presence of hydrocarbons.
The standpipe is a steel pipe mounted vertically on one side of the mast. At the surface. drilling mud flows into the shakers (8) and into a ditch to a settling pit where the cuttings settle to the bottom (9). The functions and corresponding properties of a drilling mud are: 1. This is connected to the swivel via the gooseneck.. and economically. Carry and release the cuttings at the surface (viscosity/gel strength) 3. Drilling fluid swirls in the bottom of the hole picking up material broken by the bit. A drilling fluid must fulfill many functions in order for a well to be drilled successfully. Drilling fluid or mud is pumped from the mud suction pit (1) by the mud pumps (2) through a discharge line to a standpipe (3). Numerous types of mud are available for various hole conditions. Release cuttings when processed by surface equipment 5. Suspend cuttings and weight material in suspension when circulation is stopped (gel strength) 4. safely. Allow cuttings to settle out at the surface 6. Remove drilled cuttings from the borehole (viscosity) 2. In the rotary drilling method the casing pipe is not introduced until after the drilling operations are completed. carrying the cuttings to the surface and clearing the hole (7). solids and chemicals for which mud engineers are contracted by the operating company to maintain. 4. cool and lubricate the bit and continuously remove cuttings from the borehole. Prevent the borehole from collapsing or caving in (mud density) 8. From the standpipe the mud passes through a reinforced rubber hose. The walls of the hole are held in place by the pressure of the mud pump against the sides of the hole. Today.2 Functions of Drilling Fluids Initially. local structural conditions. From the settling pit the fluid overflows into the mud suction pit from which it is picked up through the suction hose of the mud pump (2) and recirculated through the drill pipe. The drill pipe and bit deepen the hole as the operation proceeds. all enter into the choice of a particular mud. the primary purposes of drilling fluids were to clean. But today what started as a simple fluid has become a complicated mixture of liquids. the drilling fluid must permit the securing of all information necessary for evaluating the productive possibilities of the formations penetrated. The fluids' characteristics must be such that good cores. wireline logs and drill returns logs can be obtained. Control subsurface pressures (mud density) 7. Factors such as depth. shows the mud circulatory system.Error: Reference source not found. called the kelly hose or rotary hose. and then flows upward in the annulus (space between the walls of the hole and the drill pipe). types of formations.3. etc. As the mud enters the swivel (4) it flows down the drill stem (5 and 6) and exits at the bit. Protect producing formations from damage that could impair production .
2.1 Mud Weight The pressure exerted by the mud column at the bottom of the borehole is a function of the mud density and column height. Ensure maximum information from the formation drilled 13. A blowout is an uncontrolled kick. the mud density may be reduced (“cut”) so severely that a blowout will occur. then a kick will occur.9. It can be seen that items #1-3 are best served if the drilling fluid has a high viscosity.3. without damage to the circulation system or upon the formation adjacent to the hole Occasionally. On the other hand. whereas items #4-5 are best accomplished with a low viscosity.2.3. and lubricate the drill bit and drill string. This is the simplest and one of the most essential duties of drilling mud (additive content) 10. Three types of formation can be recognized: . these functions require the drilling fluid to act in conflicting ways. it is not practical or economical to have the mud weight too high. If circulation is lost.1. Excessive mud weights result in: • Low rates of penetration: The rate of penetration is reduced rapidly as the pressure differential between the mud hydrostatic pressure and the formation pressure increases. drilling mud and any formation fluids they may contain are also lost. Fracturing of weak formations that may cause the loss of drilling fluids (lost circulation): If this situation is not addressed the hydrostatic level will be severely reduced causing formation fluids to enter the hole and causing a blowout. Clean. If the pressure exerted by the mud column falls below that which is necessary to hold back formation pressures. cool. Help support part of the weight of the drill string/casing (density) 12. then the cuttings. It must be sufficient to prevent a kick. A kick is situation wherein formation fluids enter the well. Seal porous and permeable zones with an impermeable filter cake (water loss) 11. The way in which a lost circulation zone behaves generally indicates the type of porosity of the formation into which the fluid is being lost. • Density is also important in preventing unconsolidated formations from caving into the borehole. The effects of mud weight on mud logging are: • Hydrostatic pressure in excess of formation pressure might cause a loss of circulation.1 Controlling Subsurface Pressures 4. Items #6 & 8 are often mutually exclusive because drilled solids will tend to pack into the pore spaces of a producing formation. Do all of the above. If this condition is remains unchecked for even a short period of time. 4.
making cuttings analysis difficult. Coarse. Partial returns are maintained. Other than in extreme cases.1 Hydrostatic Pressure Hydrostatic pressure is the pressure that exists due to the mud weight and vertical depth of the column of fluid. like shales. If pore openings are large enough. depending on permeability.519 x MW (lbs/gal) x TVD (feet) Or Hp or P (bars) = 0. Fissured or fractured formations: Fractures may be natural or induced and opened by the hydrostatic pressure. The size and shape of the fluid column have no effect. after which full returns are maintained. 4. Hp or P (psi) = 0.3.0981x MW (g/cc) x TVD (meters) Where: MW = Mud Density . regular seepage loss. this is a slow. • Formation pressures that approximate or are greater than the hydrostatic pressure may cause a kick to occur. Losses of drilling mud are large and continuous.2. cavings may occur.1. Cavernous and vugular formations: Loss is usually sudden and of a finite amount. then loss of whole mud occurs.2.3. permeable unconsolidated formations: There is normally some loss by filtration into these formations. 2.2. until an impermeable filter cake if formed.1.2 Pressure Control Terminology The following terms must be understood when discussing pressure-control terminology: • • • • • • Hydrostatic Pressure Pressure Gradient Equivalent Mud Weight Apparent and Effective Mud Weight Pore Pressure Overburden Pressure 4. In low permeability formations. 3.1.
the Power Law or Modified Power Law formulae. Pressure gradient = P/TVD = 0. But if the mud is circulated. can be estimated by calculating the pressure loss in the annulus using Bingham’s.3.432 psi/ft The pressure gradient for typical formation water (MW = 8.33 ppg) is: Pressure Gradient = 8.33 x 0.6 ppg is an average used worldwide. 4.0519 x MW The pressure gradient for fresh water (MW= 8. but may not fit local conditions.2. this value should be used until a local value is determined.TVD = True Vertical Depth 4. additional pressure is placed against the formation due to frictional effects in the mud. and will exert a hydrostatic pressure equal to the hydrostatic pressure. called Effective Mud Weight (EMW) or Effective Circulating Density (ECD). This additional pressure.2 Pressure gradient Pressure gradient is the rate of change of hydrostatic pressure with depth for any given unit of fluid weight.6 X 0.3 Apparent and Effective Mud Weight or Effective Circulating Density The mud weight measured at the pits is the apparent mud weight going into the hole. However.2. That is.1.2. Bingham’s formula for calculating ECD is: Pressure loss (psi) = (PV x V x L) / (60000 x (dh-dp)2) + (YP x L) / (200 x (dh-dp)) Where.2.1.0519= 0.0519 = 0. This is a static pressure.446 psi/ft The value 8. L = section length (ft) YP = yield point (lb/100 ft2) (dh-dp) = hole diameter minus pipe outside diameter (inches) PV = plastic viscosity .6 ppg) is: Pressure Gradient = 8.3.
0519 x TVD When pipe is pulled out of the hole. If permeability also exists and formation pressure is greater than mud hydrostatic pressure. Bottomhole Circulating Pressure (BHCP) = ECD x 0.3. Mud Weight for safe trip = Wo + 2 x (pressure loss / (0. ECD. In impermeable formations (such as shales). EMW. a kick will occur.1. excessive pore pressure is confined and will produce sloughing or caving. MWe = MWo .2.4 Pore Pressure Pore pressure is the pressure that is exerted by fluids contained in the pore space of the rock and is the strict meaning of what is generally referred to as formation pressure.V = annular velocity in laminar flow (ft/min) In calculating the ECD. the pressure on the formation is reduced by an amount of similar magnitude to the pressure loss in the annulus. is the equivalent mud weight for the sum of the hydrostatic pressure plus the pressure loss in the annulus.2.3. International Logging uses computers to calculate the ECD using the Power Law formula.2.0519 x TVD) Therefore the Bottomhole Circulating Pressure is. All rocks have porosity to some extent. That is.0519 x TVD)) 4.pressure loss / (0. 4.2 Removing and Suspending the Cuttings The most important factors of the drilling mud involved in carrying the cuttings up the borehole and suspending them when circulation is stopped are: • Annular Velocity (the speed at which the mud travels up the borehole) .0519 x TVD) Where. each section of annulus should be considered separately and losses summed to total loss. ECD = MWo + pressure loss / (0 0. That is. or effective circulating density. We = equivalent weight during POOH A safe rule of thumb is to use an actual mud weight required to balance the formation pressure and add to this a mud weight equivalent to twice the annular pressure loss. The effective mud weight.
2 Gel Strength Gel strength refers to the ability of the drilling fluid to develop a gel as soon as it stops moving. This value is normally maintained between 40 and 50. .2. It is the resistance that the drilling fluid offers to flow when pumped. 4.2. • 4. The measurement of Funnel Viscosity is sec/qt (seconds per quart). Lubrication.2. the drilling fluid tends to retain any entrained gas as it passes through the surface mud cleaning equipment. and the elapsed time to empty the funnel is recorded in seconds.3. gel strength should be low enough to: • • • • Allow the cuttings to be removed at the surface Permit entrained gas to be removed at the surface Minimize swabbing when the pipe is pulled from the borehole Permit starting of circulation without high pump pressures The gel strength is most commonly determined with a Fann VG (Viscosity/Gel) Meter and is expressed in lbs/100ft2 (pounds per 100 square feet). Swabbing of the borehole may also introduce extraneous gas anomalies.2. In general.2. Also. thus recycling and contaminating the cuttings samples. It affects the ability of the drilling fluid to lift the cuttings out of the borehole. Its purpose is to suspend the cuttings.3 Cooling and Lubricating the Bit and Drill string Practically any fluid that can be circulated through the drill string will serve to cool the bit and drill string. Drilling muds ordinarily have gel strengths between 5 and 30 lbs/100ft2. Fine cuttings may be held in suspension so they cannot be removed at the shale shakers and settling pits. however.3 The Effect of Viscosity and Gel Strength on Drill Returns Logging Below are the effects of viscosity and gel strength on drill returns logging: • If the viscosity or gel strength (or both) is too high.3. cuttings consisting of clays or other dispersible material may be dissolved.3. 4. mud solids in the borehole. The funnel is filled with one quart of drilling fluid. chemicals and other materials.184.108.40.206 Viscosity Viscosity Gel Strength Viscosity is the resistance to flow. commonly requires special mud characteristics that are gained by adding oil.• • 4. It is dependent on the amount and character of the suspended solids. and not permit them to settle around the bit when circulation is halted. with the effect that the gas may be recycled several times. Viscosity is ordinarily measured in the field using a Marsh Funnel.
thereby reducing the diameter of the borehole that increases the possibility of sticking the drill string and the swabbing effect when pulling the drill pipe. .1 Water-Based Muds 4. only gross changes can be observed. making it difficult to interpret wireline logs. shale problems occur because of wellbore stresses within shale or wetting by the liquid mud phase. A plastering effect.3.3. At this time. however. control of the filtration rate (water loss) is necessary for two reasons: 1.3.1. depositional forces. For these reason’s cake toughness rather than thickness or fluid loss should be a more significant criterion for evaluating dynamic filtration changes. Dynamic filtration is a balance between pressure differential. or the ability to line permeable portions of the borehole with a thin. A number of reactive and nonreactive solids are added to obtain special properties. High water loss can cause deep invasion of the formations. therefore.3.1 Components of a Water-Based System This is the most commonly used type of mud system.3 Types of Drilling Fluids There are four basic drilling fluid types: • • • • Water-based muds Low solids muds Oil-based muds Air. it is the pattern of change in these values that could be significant.3. API and high temperature-high pressure fluid loss values have no engineering value since no correlation can be established between them and dynamic filtration. erosive forces across the filter cake and erodability of the cake’s matrix. Shales do not possess permeability. Nevertheless. is also produced. Neither of these conditions will be affected by fluid loss.4. tough filter cake.3. However. A poor quality filter cake may cause excessive water loss and produce an excessively thick filter cake.4 Sealing Porous and Permeable Zones with an Impermeable Filter Cake The hydrostatic pressure of the column of drilling fluid exerted against the walls of the borehole helps prevent the caving of unconsolidated formations. It is a three-component system consisting of water. 2. 4. and reactive and inert solids that are suspended in water. there is no way to objectively measure cake toughness. Gas.2. Mist systems 4.
It is sometimes referred to as “gel”.3.3.2 Reactive Solids The reactive solids found in drilling fluids are: • • • • • • Clays Dispersants Filtration Control Agents Detergents. 220.127.116.11.1.1. gel strength and water loss. It affects the viscosity.1. reducing the attraction between particles.for saltwater muds Natural formation clays that hydrate and enter the mud system Dispersants reduce viscosity by adsorption onto clay particles.for fresh water muds Attapulgite . Some commonly used filtration agents are: . Examples of dispersants are: • • • • • Tannins Quebracho Phosphates Lignite Lignosulphonates Filtration Control Agents control the amount of water loss into permeable formations by ensuring the development of a firm impermeable filter cake. Seawater is commonly used in offshore drilling and saturated saltwater may be used for drilling thick evaporite sequences to prevent them from dissolving and causing washouts.1 Water This may be fresh water or salt water.3. Emulsifiers and Lubricants Defoamers Sodium and Calcium Compounds Clays are the basic material of mud. Common clays are: • • • Bentonite . Saturated saltwater is also used for shale inhibition.
long chain molecules that can be polymerized into different lengths or “grades”. attapulgite and granular (squeeze) If none of these materials successfully plug the lost circulation zone. Defoamers are used to prevent mud foaming at the surface in mud tanks or surface cleaning equipment. .1.) Anti-friction Material Weight Materials are finely ground.C.pregelatinized to prevent fermentation Sodium Carboxy-Methyl Cellulose (CMC) . leather fiber Granular: walnut shells (nut plug). They are also used for a spotting fluid in order to free stuck pipe. The L. Common weight materials are: • • • Barite Hematite Galena Lost Circulation Material (L. Polymers . high-density minerals held in suspension to control mud density.C. time setting clays. It is available in many sizes and types to suit particular circulation loss situation. used under special conditions • Detergents. Emulsifiers and Lubricants assist in cooling and lubricating. medium.organic colloids. The grades depend on the desired viscosity.3 Inert Solids Inert solids added to the mud system are: • • • Weight Material Lost Circulation Material (L. Sodium Compounds precipitate or suppress calcium or magnesium that decreases the yield of the clays.M. 4. the zone must be cemented off. drispac.M.• • Starch . coarse) Reinforcing Plugs: bentonite with diesel oil (gunk). fine. materials commonly used are: • • • • Fibrous: wood fiber.3.C. mica (fine. coarse Flakes: cellophane.) is added to the mud system in order to plug the lost circulation zone.3. Calcium Compounds inhibit formation clays and prevent them from hydrating or swelling.1.M.for example cypan.
2.0 to 9. They are formulated by conditioning the mud with commercial calcium sulfate (plaster).3. There are two types of high pH water-based muds in use: • • Lime treated high pH muds are made by adding caustic soda.1. This would include spud muds.2 High pH Water-Based Muds High pH water-based muds are muds with a pH greater than 9. organic colloid treatments and some completion fluids. These may be prepared from fresh water or brine. Source water here may be from open sea or bays. There are four types of low pH water-based muds in use: • Fresh water low pH muds are muds containing only small concentrations of salt. 4. red muds. lime. clay and an organic thinner.3.3. Saturated saltwater low pH muds are muds where the liquid phase is saturated with sodium chloride.3. Fresh water high pH muds are muds treated with products that bring the pH level above 9.3 Oil-Based Muds These mud systems have desirable properties as completion fluids or when drilling production wells because they do not react with clays and their filtrate will not damage the .2 Types of Water-Based Muds There are two types of water-based mud systems in use: • • 4. bentonite-treated muds. where torque and differential sticking are a problem. It may either have a water or oil base. The most frequently used materials are inert polyurethane spheres.1. • • • 4. More frequently it is used on high angle directional wells.2.1. Brackish water low pH muds are muds using seawater.5.2 Low Solids Muds A low solids system is a mud wherein solids content is less than 10% by weight or the mud weight is less than 9.Anti-friction materials are added to the mud system to reduce torque and decrease the possibility of differential sticking.5 ppg. 4. These muds normally have a pH greater than 11.3.3. They are also called gypbased muds. These would include most alkaline-tannin treated muds.3. brackish water or hard water. 4.1 Low pH High pH Low pH Water-Based Muds Low pH water-based muds have a pH of 7. Gyp treated low pH muds are gypsum-treated muds.3.3. although other salts may be present.3.
in which diesel or crude oil is dispersed in a continuous phase of water.4 Measurement While Drilling Measurement-While-Drilling (MWD) tools are necessary in directional drilling. horizontal drilling. Their use in exploration wells. The air is forced down through the drilling pipe and out through the holes at the bottom of the rotary drill bit. is limited because it complicates geological evaluation. These may contain up to 50% by volume of water in the liquid phase. Viscosity is controlled by the addition of diesel fuel to thin. other than in certain troublesome evaporite and clay sections. The MWD operator can keep a constant check on the direction in which a downhole motor is facing thereby making deviation control easier.4 Air. defines oil-base mud as “a special type drilling fluid where oil is the continuous phase and water the dispersed space. and asphalt compounds or organo clays to thicken. As a rule. 4. oil muds will either form without emulsions with formation water or from other sources of water contamination. The air line is connected to the swivel hose at the top of the kelly or top head drive.” Oil-base muds . . Invert Emulsion (water/oil) System . Gas. tool face position) via mud pulse telemetry. A small stream of water is often introduced into the air system to help cool the drill bit and control dust. They utilize instruments that transmit the survey data (azimuth.” • • When oil-based mud systems are in use. The air serves to cool the drill bit and force cuttings up and out of the hole. This information is then available for surface readout. Weight is increased by the addition of barites. Oil-base muds are differentiated from invert emulsion muds (both water-in-oil emulsions) by the amounts of water used.Bulletin D 11 of the API. esp. 4.3.in which water is dispersed in a continuous phase of diesel/crude oil. method of controlling viscosity and thixotropic properties. wall-building properties and fluid loss. natural gas or mist is occasionally used as a drilling fluid (at times with a foaming agent to improve carrying capacity). They are expensive and difficult to run and maintain. inclination. air receiver and unloading system replaces the mud pump. special considerations must be made regarding formation evaluation. Mist Systems Compressed air. Air drilling is used principally in hard clay or rock formations or in areas where there is little formation water.formations. not emulsions at the start of their use in drilling. The four basic types of oil-based mud systems used are: • • Emulsion (oil/water) System .3.The IADC identifies oil muds as. Oil muds . Oilbase mud contains blown asphalt and usually 1 to 5% water emulsified into the system with caustic soda or quick lime and an organic acid. “usually a mixture of diesel fuel and asphalt. Air drilling is basically the same as mud drilling except that a compressor assembly including cooling system.
There are two MWD systems in use today: • • Probe-based Collar-based Probe-based tools are generally used in slim hole and short radius applications. They provide inclination information to ensure that the drill string remains within the reservoir or returns to the reservoir whenever the bit exits the formation.5. Changes in borehole diameter move the arms in and out and send signals to the surface. They take the place of wireline logs because wireline tools could not reach the entire length of horizontal wells. The numerous logs offered by wireline companies today gather data in many different ways under many different conditions.1 Caliper Logs A caliper is a tool that measures diameter. eliminating the costly downtime of the earlier survey methods. Geosteering is a horizontal drilling application that utilizes resistivity measurements to identify bed boundaries and fluid changes. which would be available after several hours to several weeks after the borehole is drilled if wireline logs were used. and a recorder plots a graph at the surface as the sonde is raised to the surface. radioactivity logs. resistivity logs. that the target will be intersected and to prevent severe doglegs. porosity. Correlation between the curves gives a clear picture of lithology. permeability. an instrument called a sonde is put on bottom. while the collar-based systems provide a full range of formation evaluation (FEMWD) services. but not all. Wellbore diameter may vary widely due to lateral bit movement. The data gathered by these tools is from a formation that has not undergone significant alteration. spontaneous potential logs. MWD tools provide constant inclination information to ensure the drill string does not deviate from the planned path. since they affect the interpretation of other log data. A specific combination is usually chosen for the types of formation data needed. . To log a well by wireline (actually conductor line). logging devices can be run in a single sonde in one wireline trip. Wireline well logging is the indirect analysis of downhole features by electronic methods. Caliper logs. and saturation up and down the wellbore. mud cake. They are also used in geosteering.5 Wireline Logging Mud logging and core analysis are direct methods of formation evaluation. It also provides the necessary formation evaluation logs (LWD). These variations are important. 4. or acoustic logs may be included in a typical logging run. They allow surveys to be taken at every connection. Many. Arms. springs. 4. or pads are held against the sidewall as the device is pulled out of the hole by wireline. A caliper logging sonde measures the inner diameter of the borehole. or flexure (rock bowing into the wellbore due to overburden stress). caving formations.
This value is useful in calculating the formation water resistivity (Rw).2 Spontaneous Potential Logs The spontaneous potential. high permeability. The SP curve is recorded in millivolts against depth. all of these formulas describe the same thing: the flow of electrical current. as well as relative permeabilities of formation rocks. Usually drilling fluids contain less salt than formation fluids. spontaneous potential causes weak current to flow from the uninvaded to the invaded zone. natural electrical currents that flow in formations adjacent to the wellbore. 4. such as shale. High porosity. or SP. Because the SP log is so simple to obtain and provides such basic information. then it can be inferred that it holds petroleum. Oil and gas raise resistivity. As freshwater filtrate invades a permeable formation. current flows from the uninvaded rock into the wellbore through any impermeable formation.3 Resistivity Logs Resistivity logs record the resistance of a formation to the flow of electricity through it. Some well log formulas use resistivity while others use conductivity. and these changes are logged at the surface.5. above and below the permeable layer. . Resistivity is expressed in ohm-metres. like salt. log is the most common and widely run log. It records the weak. Resistance to this flow depends on: • • • How much water the formation can hold How freely the water can move How saturated the formation is with water rather than hydrocarbons In this way. conductivity in mhos/meter. Differences in formation characteristics change the flow of current through the sheet. However. Conductivity is the inverse of resistivity. some. A set of guard electrodes in the sonde focus current sideways into a flat sheet.4. 4. However. since hydrocarbons are poor conductors. are excellent conductors when dissolved in water. and high water saturation each lower resistivity. When a layer of rock or mud cake separates two areas of differing salt content. the sheet of current passes through formation rock.5. The SP log can be visually interpreted to show formation bed boundaries and thickness. If well logs show a formation to be very porous and permeable but also to be highly resistive. Common resistivity logs include the lateral focus log. Most minerals are nonconductors when dry. a weak current will flow from the higher salt concentration to the lower. As the sonde is raised. it is included in almost every logging run. which may be very salty.1 Lateral Focus Log The lateral focus log uses a sonde that sends current outward through the rock in a specific pattern.3.5. resistivity is directly related to other formation characteristics. and the microresistivity log. More importantly. the induction log.
but cannot wash them out of impermeable formations. or less porous. 4. a formation is. salt content.4. Some sondes measure the levels of both gamma rays and unabsorbed neutrons. which generates a current monitored by instruments at the surface. a curve is graphed in API standard gamma ray units to show gamma ray emissions from the formation beds. It has two curves: one showing resistivity in mud cake.5. The sonde sets up a doughnut-shaped magnetic field around the wellbore. such as a Geiger counter.2 Induction Log The induction log. fluid density. the induction log is very accurate for investigating thin formation beds. involves inducing a current in formation beds.5. Like the lateral focus log.3 Microresistivity Log The microresistivity log is designed to show resistivities very close to the wellbore. Porosity and permeability can be calculated from the microresistivity curves.4. The detected amounts of these elements give information about water and hydrocarbon saturations. carbon. like the neutron log. uses radiation bombardment. the more gamma rays it absorbs. the less rock there is to stop gamma rays. oxygen. Over time. Analysis of the curve allows easy location of shales and other rock types and predicts permeability in low-permeability formations. All neutron logs give good porosity readings. Other neutron logs can be calibrated to the gamma rays emitted by certain elements such as hydrogen. Traces of radioactive elements are deposited in formation sediments. such as limestone and clean sandstone. The density log responds to bulk densities in formation beds. As the sonde is raised through formations. and rock types. an invaded zone is indicated and a possible reservoir has been found. 4.3.5. The denser. and pore space volume. The gamma ray log is useful for correlation with the neutron log. The gamma ray sonde contains a gamma ray detector. radioactivity logs record natural and induced radioactivity. silicon. 4. As neutron radiation bombards the rock around the wellbore. reflecting rock matrix density.4.4 Radiocativity Logs Just as resistivity logs record natural and induced electrical currents. water leaches them out of porous. the more porous a formation is. hydrocarbon density. or calcium.5. A radioactive source is loaded into the sonde and sent downhole. Bulk density is the total density of a rock. On the other hand.3. changes in the current are logged. Mathematical formulas for such figures as porosity. but with gamma rays instead of neutrons.4. When the two curves are not identical. chlorine.3 Density Logs The density log. As the sonde is raised in the hole. the other showing resistivity less than one-half foot away in the formation.5.5. permeable rock. 4.1 Gamma Ray Logs Gamma ray logs show radiation from these impermeable formations.2 Neutron Logs The neutron log records induced radiation in formation rock. 4. the rock gives off gamma rays from the neutrons it has absorbed. such as shale and clay-filled sands. and oil-shale yield can be solved with data taken from density curves. .
since the rock could be brought up and examined almost exactly as it occurred downhole. permeability.5. Below is a picture of E-log sondes ready to be picked up and made up for a logging job in Bangladesh. a few well-planned core samples may be taken to tie together and clarify information from the driller's log and the mud log as well as from wireline well logs taken later.6 Coring The purpose of getting cores is to obtain rock samples of sufficient size for: • • • • • Making reservoir analysis tests to determine porosity. a short piece of pipe called a biscuit cutter was driven into the hole bottom. slow down the pulses. Shale and clay. formation evaluation would be simpler.5. Today most coring is done by barrel and sidewall coring methods. and core analysis has become an exact science. 4. But coring is expensive. porosity can be figured from acoustic logs. shows differences in travel times of sound pulses sent through formation beds. continuous core. porosity.4. then jerked out and brought to the surface to obtain a formation sample.5. Instead. Using information about formation types from other logs. as well as porous rock. 4. permeability and residual oil and water saturations (primary purpose of coring) To establish the physical character of formation boundaries To furnish paleontologic data for age and facies correlation To determine the structural attitude of the beds To provide sedimentary and petrologic data from which depositional and diagnostic environment analysis can be made If every well could be drilled as one long. also called the sonic log. . 4.6 Nuclear Magnetic Resonance (NMR) Logs To be published at a future date. Originally. but a specific combination is usually chosen for the types of formation data needed.5 Acoustic Logs Sound travels through dense rock more quickly than through lighter. Correlation between the curves gives a clear picture of lithology.7 Typical logging runs A wide variety of logs can be taken using a single sonde. more porous rock. The acoustic log. and saturation up and down the wellbore.
. and mineral composition.7 Formation Testing 4. barrel coring is done on fewer than 10 percent of all wells cored. since it is costly and time-consuming. The formation test is the final proof of a well's profitability. Porosity and permeability may be studied by several techniques. 4. they are not as good as barrel cores for determining porosity. hard evidence given by formation tests. Instead of guessing about the type of completion needed . The coring tool consists of an annular (doughnut-shaped) diamond bit to cut the core and a hollow barrel to catch it. 4.1 Barrel Cores When the bit is about to enter a formation of special interest. Perforated completions are tested cased-hole.the oil company can take advantage of the accurate. or fluid saturations. Despite its advantages over. They are often taken along with wireline well logs.1 Introduction Formation testing may be done before or after casing. Core diameters range from 11/8 to 5 inches. cementing. inexpensive way to sample a selected formation. are tested only open-hole. Core analysis for barrel cores may be very detailed.or even about whether the well will be productive . and tests may be done in a lab. with 4 inches being the most popular size for core analysis. for rock type. removing the core is sometimes dangerous since it is possible to swab the well and cause a blowout. a 60-foot core is average.6. whether barrel or sidewall.6. The core samples.3 Core Analysis The wellsite geologist checks core samples.7. or plugs. Cores and logs tell which formations should produce and where to perforate them. Core analysis can tell more about local reservoir structure than any other formation evaluation method. but predictions are not the best data on which to base an expensive completion. However. Sidewall cores may be tested for the presence of oil. Core samples may be of any length. of course. Open-hole testing followed up by cased-hole testing is also standard in some areas. and perforating. with the sidewall cores inside them.4. sidewall coring. A sidewall-coring gun is lowered on wireline to the formation chosen for sampling. where it may fire thirty or more hollow bullets into the wellbore wall. and retrieved with the gun. but they do not reveal as much as barrel cores because of their small size and their mud filtrate content. a service company may be brought in to take a barrel core. sidewall coring is a quick. Also. Sidewall coring is more commonly used than barrel coring. but cores of over 90 feet are hard to handle. 4. physical characteristics.2 Sidewall Cores Sidewall cores are taken generally as a follow-up once the well has been drilled.6. Since sidewall plugs are small and may also have been crushed somewhat. permeability. The bullets are attached to the gun by cables so that they may be pulled out. are usually less than 1 inch in diameter and up to 4 inches long. Fossils help to identify beds. The few wells that are completed open-hole.
A second test chamber is usually opened for a second flow period. has become a kind of temporary. this technique has been commonly used for formation testing. The final shut-in pressure is recorded after the second flow period. permeable formations. The pad valve is opened. Since the test chambers each hold less than an ounce. a pressure gauge. one or two perforations are made to allow flow into the tool. and well test packages may be used on either open or cased holes. The tool may be run with a logging sonde or with a bottomhole pressure gauge. and formation fluids enter the tool and register initial shut-in pressure. drill stem test tools. such as drill stem tests.7. either together with other testing methods or alone in well-known fields as a checkup on flow rates. Wireline formation tests are useful for investigating oil and gas shows. forming a hydraulic seal from mud in the wellbore. 4.7. a representative reservoir sample may be obtained in the sample chamber. For a flow period. and testing chambers and sampling chambers interconnected by valves. The test tool is made up of a rubber pad with a valve in it. which involve the running of drill pipe or tubing. They allow general predictions for zone productivity and may be used in planning more sophisticated formation tests. or DST. and confirming porosity and permeability data from other logs. In cased holes. inexpensive way to measure pressures at specific depths. In porous.2 Wireline Formation Tests A wireline formation test is a quick. taking quick readings of hydrostatic and flow pressure. The wireline formation tester is actually run on conductor line. A backup shoe kicks out to press the pad against the formation sidewall. which may be used for open or cased holes: • • Single-packer DST tool Straddle-packer DST tool . 4. partial completion of the well that provides data on several feet or several hundred feet of producing formation. After a valve is opened to equalize pressure.3 Drill Stem Tests The Drill Stem Test. A sample chamber may be opened to draw a few gallons of formation fluid. The zone to be tested is located by wireline depth measurement or by SP log.Wireline formation testers. The term wireline distinguishes this test from the others. The tool may then be retrieved. fluids drawn into them are almost 100 percent mud filtrate. a test chamber valve is opened and a small piston draws fluids at a steady rate while pressure in the chamber is logged at the surface. a getaway shot is fired to release the tool. unless it is designed to make more than one test per trip downhole. Although originally designed to sample formation fluids. DST tools come in two basic types. Any or all of these three types of formation tests may be used on a well in considering it for completion.
The main goal of drill stem testing is pressure data. through the tool. The pressure charts are metal plates in the recorders that slowly move under a stylus as it etches a pressure curve. The distance from packer to packer depends on the thickness of the test zone. and up the drill stem to the surface.7. the mud in the wellbore is well circulated and conditioned. for instance . The strength of this first flow is estimated. if any. The number and placement of DST pressure recorders are matters to be decided by experience and judgment.the two recorders will produce pressure charts with telltale differences. All test assembly parts are then added. where it is exposed to pressure inside the perforated pipe.1 DST Procedures Once a test zone has been chosen. the proper length of anchor pipe is tripped in to situate the test assembly at the chosen wellbore depth. DST assemblies normally contain two pressure recorders. An instrument records temperature by etching a curve on a metal plate. Sometimes drill collars are made up above the tool as a precaution against collapse if no cushion is used. Perforated pipe is made up below the packer. The tool is opened. usually nitrogen. In cased holes. the lower packer being basically the same as the upper packer except turned upside down. where it is exposed to pressure in the annulus. The straddle-packer DST tool isolates the formation bed or beds between two packers. Cushions can also be drawn off slowly after the tool is opened to prevent formation fluids from flowing so suddenly that the formation rock is damaged. Water is easier to obtain and is most commonly used. After the pressure charts are loaded and the DST tool is laid out and ready to run. though. Formation fluids enter perforated pipe between the packers and flow upwards. Then the cushion. and the tubing-casing annulus is monitored for pressure changes that warn of poor packer seating. vary less than pressures across a field and can be easily calculated from mathematical formulas. Wellbore conditioning gives better DST results and also helps to prevent blowouts caused by the loss of mud weight or by the loosening of a packer in excessively thick mud cake.The single-packer DST tool is used to isolate formations from below the tool to the bottom of the hole. One is usually inside the tool. the rest of the drill stem is carefully tripped in. 4. Cuttings or junk not circulated out of the hole may also damage the DST tool before the test even begins. Formation fluids enter the wellbore and flow through the perforations. The other is usually below the perforated pipe. Temperatures. where the smooth metal sidewall of the casing may not provide as secure a seating as in open holes. A cushion of water or compressed gas. Finally. When the DST tool is in place. The cushion supports the drill pipe against mud pressure until the test starts. either by observing the blow in a bubble bucket (for small wells) or by checking data from a surface computer or other . DST tools include slips to grip the casing and substitute a sturdier packer to bear the increased set-down weight needed to secure a tight seal. is often placed in the drill stem when the DST tool is run in. The tool is very similar to the single-packer tool. This curve can be analyzed for many kinds of production data.3. is bailed or bled off to reduce pressure so that formation fluids may flow up the drill stem. the packer seal is set and the test is ready to begin. If something goes wrongif the perforations become plugged. Temperature readings may also be taken during the test.
If they are saved.surface readings (for deeper. Each round includes a flow period followed by a shut-in period to record hydrostatic formation pressure.7. they may be analyzed later for saturation and other properties. Most DST’s include two flow and shut-in periods. the valves of the tool are closed to trap a clean fluid sample. Some longer tests have three rounds. 4.2 DST Interpretation DST data come from two main sources: the pressure charts and the fluid sample taken by the tool. The curve is a record of pressure over time. From this estimate the number and lengths of the flow and shut-in periods for the rest of the test are chosen.3. The short first round clears out any pressure pockets in the wellbore and removes mud from the drill stem. When the test is over. the second round being given more time than the first. the drill string and tool are carefully tripped out. Then the DST tool is unseated. Produced fluids are caught in a holding tank or burned off as they reach the surface. Fluids in the drill stem are reverse-circulated out to keep crude from being spilled over the rig while tripping out. stronger wells). Any fluids caught at the surface and saved in a holding tank have probably lost their gas content and are less representative of reservoir saturations. Finally. and the fluid sample and the charts are retrieved. The pressure curve on the DST chart is used to calculate pressure behavior plots. .
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