Well completion commonly refers to the process of finishing a well so that it is ready to produce oil or natural gas. In essence, completion consists of deciding on the characteristics of the intake portion of the well in the targeted hydrocarbon formation. There are a number of types of completions, including:
- Open Hole Completion
- Conventional Perforated Completion
- Sand Exclusion Completion
- Permanent Completion
- Multiple Zone Completion
- Drainhole Completion
The use of any type of completion depends on the characteristics and location of the hydrocarbon formation to be mined.
Open Hole Completion
Open hole completions are the most basic type and are used in formations that are unlikely to cave in. An open hole completion consists of simply running the casing directly down into the formation, leaving the end of the piping open without any other protective filter. Very often, this type of completion is used on formations that have been ‘acidized’ or ‘fractured.’
Conventional Perforated Completion
Conventional perforated completions consist of production casing being run through the formation. The sides of this casing are perforated, with tiny holes along the sides facing the formation, which allows for the flow of hydrocarbons into the well hole, but still provides a suitable amount of support and protection for the well hole. The process of perforating the casing involves the use of specialized equipment designed to make tiny holes through the casing, cementing, and any other barrier between the formation and the open well. In the past, ‘bullet perforators’ were used, which were essentially small guns lowered into the well. The guns, when fired from the surface, sent off small bullets that penetrated the casing and cement. Today, ‘jet perforating’ is preferred. This consists of small, electrically-ignited charges, lowered into the well. When ignited, these charges poke tiny holes through to the formation, in the same manner as bullet perforating.
Sand Exclusion Completion
Sand exclusion completions are designed for production in an area that contains a large amount of loose sand. These completions are designed to allow for the flow of natural gas and oil into the well, but at the same time prevent sand from entering the well. Sand inside the well hole can cause many complications, including erosion of casing and other equipment. The most common methods of keeping sand out of the well hole are screening or filtering systems. These include analyzing the sand experienced in the formation and installing a screen or filter to keep sand particles out. The filter may be either a type of screen hung inside the casing, or a layer of specially-sized gravel outside the casing to filter out the sand. Both types of sand barriers can be used in open holes and perforated completions.
Permanent completions are those in which the components are assembled and installed only once. Installing the casing, cementing, perforating, and other completion work is done with small diameter tools to ensure the permanent nature of the completion. Completing a well in this manner can lead to significant cost savings compared to other types.
Multiple Zone Completion
Multiple zone completion is the practice of completing a well so that hydrocarbons from two or more formations may be produced simultaneously, yet separately. For example, a well may be drilled that passes through a number of formations as it descends; alternately, it may be more effective in a horizontal well to add multiple completions to drain the formation efficiently. Although it is common to separate multiple completions so that the fluids from the different formations do not intermingle, the complexity of achieving complete separation can present a barrier. In some instances, the different formations being drilled are close enough to allow fluids to intermingle in the well hole. When it is necessary to prevent this intermingling, hard rubber ‘packing’ instruments are used to maintain separation among different completions.
Drainhole completions are a form of horizontal or slant drilling. This type of completion consists of drilling out horizontally into the formation from a vertical well, providing a ‘drain’ for the hydrocarbons to empty into the well. In certain formations, drilling a drainhole completion may allow for more efficient and balanced extraction of the targeted hydrocarbons. Drainhole completions are more commonly associated with oil wells than with natural gas wells.