Well completion is the process of making the well ready for production, economic success of a well depends in large part on how the well is completed. Once the well has been drilled, everything took into places the construction of the well need to be completed with specific design and proper manner. The steps taken to transform a drilled well into a producing well, include casing, cementing, perforating, gravel packing and installing a production tree. Drilling process and the completion closely related to each other, the completion starts when drilling bit makes contact with the productive reservoir. If the drilling wasn’t success leading to unsuccessful completion, then we cannot have a good well.
A successful completion must first make the maximum mechanical connection between the wellbore and the reservoir that let oil or gas flow into the well, keep water out of the well and keep the formation from collapsing into the wellbore or reservoir.
Casing and cementing
The first step in completing a well is to case the hole. The decision to case and cement a well for production relies on formation evaluation (FE) using open-hole logs, which give information of the depth of productive formations. After a well has been drilled, consisting of steel pipe that is joined together to make a continuous hollow tube, the casing is run into the well and cementing process starts to fill the annulus between the casing and the actual drilled wellbore with cement slurry, and left to harden. Cementing job is not always a flawless operation and may resulting some issues that undermine the strength and integrity of the casing. The bond of the cement with the wellbore and cement with the casing is evaluated by a cement evaluation wireline tool. This tool determines that whether the cement job is optimal, if it is not further remedial work must take into place.
The different levels of the well define what diameter of casing will be installed. Referred to as a casing program, the different levels include production casing, intermediate casing, surface casing and conductor casing.
Choosing type of completion
Open hole completion
There are two fundamental types of well completion at the reservoir level, known as open hole completion and cased hole completion. If the well with an open hole completion, a production casing is run through the wellbore until it sits directly on top of the reservoir, but does not run through the reservoir. Basic open hole may or may not have a production tubing. Advantages of open hole completion is saving the cost of cementing and perforation operations, which are expensive processes. The drawbacks include: difficulty to control the production flow of associated sand, water, gas and other formation fluids, hence the produced fluid need more treatment when it comes up to the surface; remedial work on the well is more difficult after production has commenced.
Cased hole completion
In cased hole completion, production casing is run along the entire length of the well and through the reservoir. In this case perforating operations have to be done to create connection between the wellbore and productive formations. The cased hole completion effectively acts as a control mechanism for safe production of desired hydrocarbons and as a barrier preventing of unwanted fluids, gases, and solids into the wellbore, therefore, it allows to control produced fluid, enhances well integrity and remedial work is easier to carry out. Drawback of cased hole completion mainly is high cost of this type of completion due to costs of procuring casings, perforation guns, cement, and other completions equipment.
The production tubing then is run into the hole, it is generally much smaller in diameter than the production casing, with packers that are installed just above the perforation zone to ensure the produced fluid reaches the surface only through the production tubing. Unlike casing, production tubing hangs from the wellhead and is not cemented into place. This tubing is then easy to remove if any well problems develop or production issues happen in the future. Its strength, material and size-weight/unit length and internal diameter are chosen according to expected production rates, production types, pressures, depths, temperatures and corrosive potential of produced fluids.
Installing the production tree:
The last step in completing a well is installing the wellhead known as the Christmas tree sits on top of the well, it is an assembly of valves, spools, choke manifold and fittings. Its functions include: help support the weight of the production tubing, control the flow rate and pressure of the well fluids, seal the well, and also connects the well with pipeline systems.
Process of completing a well
In summation, designing a completion must take into consideration of characteristics for every single well, that is the volumes of fluids to be produced, downhole and surface condition, production zone depths, production rates, well location and surrounding environment. Engineers then have to choose which design of the well is the most feasible and reasonable option to complete, and to achieve the ultimate goal, that is to recover, at a reasonable cost, as large a percentage of the original oil in place (OOIP) as possible.
Open hole completion is often favored in horizontal wells, where running a production casing along the entire length of the well might be too expensive, or technically unfeasible. Cased hole completion, on the other hand, might be the better option for vertical wells when there is low formation integrity.
In some wells, the reservoir pressure is insufficient for the free flow. Therefore, The wells must be equipped with additional pumps or gas lift systems.