Well completion is one process or method applied when the borehole has been drilled through the reservoir section of interest for production or injection, the method by which fluid communication will occur between the reservoir and the borehole, after completion, has to be decided.
There are 3 alternative approaches for the well completion of the reservoir zone:
- Open hole completion
- Pre-drilled / pre-slotted liner or screen completion (uncemented).
- Casing or liner with annular cementation and subsequent perforation
Open Hole Well Completion
Such completions are sometimes referred to as “barefoot” completions and the technique is widely applied. Since no equipment requires to be installed there are savings in both costs and time. However this type of completion does mean that the entire interval is open to production and hence it often provides no real selective control over fluid production or injection. It is therefore not recommended for production or injection wells where distinctive variations in layeral permeability will detrimentally control the sweep efficiency on zones under water flood or gas injection. Further, in an oil well if water/gas breakthrough or migration into the wellbore occurs it is difficult to isolate unless the entry is at the base of the well where isolation with a cement plug may be successful. The possibility of interzonal cross flow or zonal back pressure dictating multizone depletion cannot be corrected with this type of completion. This lack of zonal control for production or injection is a major limitation on the application of this technique.
Open hole well completion should only be applied in consolidated formations as the borehole may become unstable once a drawdown in applied to induce the well to flow. In such cases either total collapse of the formation or the production of sand may occur. Currently open hole completions are applied in a range of environments
a) Low cost / multi well developments
b) Deep wells, consolidated with depletion drive
c) Naturally fractured reservoirs
d) Some horizontal and multi lateral wells
Screen or pre-slotted liner well completion
In this technique, once the drilling through completed reservoir section has been completed, a wire-wrapped screen or steel pipe which has slots or alternative sand control screen, is installed (Figure Below).
The principal purpose of the screen or liner is to prevent any produced sand from migrating with the produced fluids, into the production flow string. The success of the completion in controlling sand production is dependent upon the screen or slot sizes and the sand particle sizes. The screen will only become 100% effective if it totally restrains sand production which requires that the slot size be equal to the size of the smallest particles. However, in such cases the slots may quickly become plugged and impede flow resulting in a loss in productivity. This system is sometimes used in inclined/high angle angles to prevent major borehole collapse or facilitate the passage of logging tools.
This technique also suffers from the same inability for zonal control of production or injection as exists in the open hole completion and may only effectively control sand production over a limited range of conditions. However, it is a low cost technique since the cost of a screen to cover the reservoir interval is much less than the cost of a casing string run to surface plus the cost of cementing and perforating. However in the case of using premium sand exclusion screens, the cost saving will be reduced. The technique is therefore only of application as an alternative to the open hole completion in situations where the reservoir rock consists of relatively large and homogenous sand grains.
Cemented and Perforated Casing/liner Well Completion
The final choice is to install either a casing string which extends back to surface or a liner which extends back into the shoe of the previous casing string, which would then be cemented in place by the displacement of a cement slurry into the annular space between the outside wall of the casing and the borehole wall (Figure below “Cemented And Perforated Casing/Liner Well Completion”). Subsequently, to provide flow paths for fluid to enter the wellbore from the formation, or vice versa, the casing and cement sheath will be perforated at selected locations using explosive charges contained in a perforating gun.
The integrity and selectivity of the completion depends to a great extent on an effective hydraulic seal being located in the casing-formation annulus by the cement. For the completion to be effective, a successful primary cement job must provide zonal isolation behind the casing. The absence or failure of the cement can lead to either fluid migration behind the casing to surface, into another zone or into perorations from which it was assumed to be isolated. If required the perforations can subsequently be closed off by a cement squeeze operation.
This type of completion involves considerably greater costs and time than the previous options. The cost of a full length of casing from the surface to the base of the well can be considerable, to which must be added the cost of perforating, cementing and the additional time necessary to complete the borehole in this way. The use of a liner helps to reduce the required length of tubular and hence the overall costs. However the ability to control the depletion of individual zones, isolate the inflow of undesirable produced fluids and control the injection of fluids into zones are essential to a large number of developments and this has resulted in the cemented and perforated liner or casing being the most widely applied bottom hole completion technique in situations where enhanced reservoir management capabilities are required.
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