By Dr. Robert Puls
Oklahoma Water Survey
Norman, OK 73072
Hydraulic fracturing, together with horizontal drilling, has changed the energy landscape worldwide. With the explosion of oil and gas resource extraction in unconventional formations in the United States, there is now accelerated activity in China, England, Germany, Poland, Spain and elsewhere. With the expansion of high volume hydraulic fracturing, there have been increasing concerns about its potential impacts on drinking water resources in the vicinity of these operations. Risks to fresh water resources exist where hydraulic fracturing operations are occurring, however better management and operational practices can significantly reduce risks to these resources throughout the lifecycle of these operations. This includes site selection and pad and well construction, production well completion, and wastewater management and well closure.
It is now well known that contaminated ground water is at best difficult to remediate and at worst, impossible. This will depend on the scale of the impact, hydrogeological conditions, and type of contamination. Ground-water cleanups, even if ‘successful’, take years or decades to accomplish and are often cost-prohibitive. Ground water protection is the responsible and cost-effective policy regardless of potential contamination source.
Hydraulic fracturing is a well stimulation technique used to maximize production of oil and natural gas in unconventional reservoirs, such as shale, coalbeds, and tight sands. During hydraulic fracturing, water containing a very small concentration of specially engineered fluids containing chemical additives and proppant (e.g. sand) are pumped under high pressure into the production well to create and hold open fractures in the formation. These fractures increase the exposed surface area of the rock in the formation and, in turn, stimulate the flow of natural gas or oil into the wellbore for recovery at the surface. Over the last 10-15 years, industry has coupled hydraulic fracturing with horizontal well drilling. This has enabled increased efficiencies to tap unconventional formations such as shale.
In 2010 the U.S. Environmental Protection Agency (EPA) initiated a study at the suggestion of Congress, to study the relationship between hydraulic fracturing and drinking water resources. Case studies were deemed an important component of the study. Approximately 40 sites were initially reviewed where ‘complaints’ had been registered with states and EPA. In a very real sense, these represented worse case scenarios among thousands of locations where hydraulic fracturing had occurred over a number of years. An analysis of these sites (most of which were
actually visited) is presented in Figure 1. These sites were classified as to the type of suspected water resource impairment. Clearly, the most significant risk to water resources, based on this limited analysis, was from surface wastewater management and production well construction. The sites were located in ten different states. Twenty-eight were in shale (five different plays), eight were in tight sands, and four were in coal bed methane formations.
HYDRAULIC FRACTURING CHEMICALS
Hydraulic fracturing chemicals together with water are injected under high pressure to create fractures and carry the proppant to hold open the induced fractures. Chemical additives are used to optimize this process; each chemical having a unique purpose (Table 1). For example, biocides are added to prevent microbial growth and clogging of flow paths for the gas and oil; friction reducers to minimize friction between the injected fluids and the pipes to effectively deliver the
TABLE 1. An example of the volumetric composition of a fracturing fluid.
|Example Compounds||Purpose||Percent Composition (by Volume)||Volume of Chemical (Gallons)ª|
|Proppant||Silica, quartz sand||Keep fractures open to allow gas flow out||9.51||285,300|
|Acid||Hydrochloric acid||Dissolve minerals, initiate cracks in the rock||0.123||3,690|
|Friction reducer||Polyacrylamide, mineral oil||Minimize friction between fluid and the pipe||0.088||2,640|
|Surfactant||Isopropanol||Increase the viscosity of the fluid||0.085||2,550|
|Potassium chloride||Create a brine carrier fluid||0.06||1,800|
|Gelling agent||Guar gum, hydroxyethyl cellulose||Thicken the fluid to suspend the proppant||0.056||1,680|
|Scale inhibitor||Ethylene glycol||Prevent scale deposits in the pipe||0.043||1,290|
|pH adjusting agent||Sodium or potassium carbonate||Maintain the effectiveness of other components||0.011||330|
|Breaker||Ammonium persulfate||Allow delayed breakdown of the gel||0.01||300|
|Crosslinker||Borate salts||Maintain fluid viscosity as temperature increases||0.007||210|
|Iron control||Citric acid||Prevent precipitation of metal oxides||0.004||120|
|Corrosion inhibitor||N,N-dimethyl formamide||Prevent pipe corrosion||0.002||60|
fluids and proppant to the targeted formation zone without reduction in applied pressures; weak acids to dissolve minerals and improve fracture initiation effectiveness; and surfactants to increase the viscosity of the injected fluids for efficient delivery to the targeted fracture zone. The types and concentrations of chemical additives and proppants vary depending on the conditions of the specific well being fractured, creating a fracturing fluid tailored to the properties of the formation and the needs of the project. Table 1 represents the volumetric composition of a fluid used in a fracturing operation in the Fayetteville Shale (GWPC and ALL Consulting, 2009; API, 2010b). There is an increasing trend among operators to fully disclose the chemicals used in hydraulic fracturing (www.FracFocus.org) and reduce the additives to the minimum necessary to accomplish their objectives.
In general, the total volume of chemicals used in hydraulic fracturing represents less than 1% of the total volume injected. However, the volume of water used can range from 1 million to 6 million gallons depending on the depth of the well and the length of the horizontal leg of the well. Many horizontal legs are one mile in length and well depths range from less than 1000 feet to more than 12,000 feet in the Haynesville shale. By far, the greatest risk to drinking water wells is not from migration of chemicals from the frac zone, but rather management of fluids on the surface (well pad, transportation of chemicals, etc.)
RISK MANAGEMENT PRACTICES TO PROTECT DRINKING WATER
Lessons learned from analyzing a number of sites around the U.S. are focused on the following activities:
- Site selection and pad and well construction
- Production well completion (hydraulic fracturing)
- Wastewater management and well closure
It is to industry’s advantage to protect ground water, because of the expense in remediating contaminated ground water. Each state has requirements and guidance for surface water and ground water protection. These include requirements for surface impoundment construction and decommissioning, well construction, especially regarding protection of drinking water aquifers, and safeguards for all aspects of resource extraction.
Site Selection and Pad and Well Construction
It is recommended that water supply wells in the immediate area of planned production wells be sampled to establish baseline water quality. A minimal number of analytes to consider for sampling are listed in Table 2. Communication with local residents and state and local authorities is recommended to engender trust and establish good communications among all stakeholders.
If operating in a new area, the site should be well characterized with respect to abandoned wells, natural fractures and faults, the location of the base of the drinking wáter aquifers, and the presence of shallow natural gas zones. It is further recommended that closed loop systems be used during drilling for well construction. Drilling muds need to be sampled to determine if they constitute a ‘hazardous waste’ and if so, disposed of in accordance with state regulations. Perhaps the most important recommendation during well construction is to verify cement and casing for leaks. There are well established procedures for insuring well integrity (e.g. pressure tests, cement bond logs) and these practices should be followed, documented and reported.
TABLE 2. Recommeded basic list of analytes for water quality baseline monitoring.
|General Parameter||Inorganic/Metal Analytes||Organic Analytes||Microbiology|
|Alkalinity||Arsenic||Oil & Grease||Total coliform/E. Coli|
|Total dissolved solids||Iron|
Production Well Completion (Hydraulic Fracturing)
It should be obvious that hydraulic fracturing should not be permitted in or immediately adjacent to an underground source of drinking water. Tanks, rather than lined pits, should be used for stoage of flowback/produced water. Recycling of flowback/produced water is now commonly practiced and should continue to be encouraged. However, it is important to monitor the infrastructure used in the recycling process (e.g. pits, tanks, transmission lines, etc.) to guard against leaks in the system.
Disclosure of chemicals used in the hydraulic fracturing process is essential for establishing trust with the community where the activities occur as well as from a human and ecological health standpoint. Operators should use the minimum suite of chemicals to allow effective resource extraction and use ‘green frac’ chemicals where possible. Chemical usage should be tracked and volumes injected reported. There should be monitoring of downhole, annulus, and wellhead pressures. Contingency plans are essential for accidents as they would be for any industry. Plans for blowouts where fluid contact with drinking water resources is possible should be in place and personnel should be trained to handle such events quickly and efficiently. It is recommended that follow up water quality testing be performed following completion of production well construction and hydraulic fracturing. This should be done within 3 to 6 months after these activities are completed and the well is in the production phase.
Wastewater Management and Well Closure
Recycling of flowback/produced water makes sense from both a water availability standpoint and from a wastewater management/disposal standpoint. Having secondary containment in place on the well pad insures that any spills, leaks, blowouts can be contained on site and prevent water resource contamination offsite. Routine monitoring of the site is also important. Air emissions should be minimized using best available technology. Production wells should be shut-in during non-producing periods and wells should be plugged with materials optimized for the local hydrogeological conditions.
A number of studies have shown that cement seals around wells eventually fail. This fact coupled with the higher pressures being used today as well as the potential for repeated fracturing of the same well over the entire lifetime of the well argues for increased research investments in improved cement formulations to extend longevity. Improved understanding of the microbiology of waters at depth and in flowback and produced waters can result in better management of biofouling issues downhole and with wastewater management and recycling operations. The continued development of ‘green chemicals’ is also encouraged. Finally, issues surrounding methane migration from shallow zones into aquifers should be further researched. A recent report (Groat and Grimshaw, 2012) by the Energy Insitute of the University of Texas suggests that shallow methane deposits may migrate to aquifers from energy inputs to the subsurface system; that is, via vibrations and pressure pulses during drilling of the production well. This pathway would be in addition to the already acknowledged pathway from poor well construction techniques. While the effects from vibrations and pressure pulses may be transitory, research is needed to characterize and mitigate this impact.
API (American Petroleum Institute). (2010b, July 19).
Freeing up energy—hydraulic fracturing: Unlocking America’s natural gas resources. Washington, DC: American Petroleum Institute.
Retrieved December 2, 2010, from http://www.api.org/policy/exploration/hydraulicfracturing/upload/HYDRAULIC_FRACTURING_PRIMER.pdf.
Groat, C.G. and T.W. Grimshaw, 2012.
Fact-Based Regulation for Environmental Protection in Shale Gas Development.
GWPC (Ground Water Protection Council) & ALL Consulting. (2009).
Modern shale gas development in the US: A primer. Contract DE-FG26-04NT15455. Washington, DC: US Department of Energy, Office of Fossil Energy and National Energy Technology Laboratory.
Retrieved August 2, 2010, from http://www.netl.doe.gov/technologies/oil-gas/publications/EPreports/Shale_Gas_Primer_2009.pdf.