Many different completion techniques, such as vertical-well multiseam completions and multiple lateral wellbores drilled into a single coal seam, have been used to develop coalbed methane (CBM) reservoirs. Stimulation techniques include openhole underream, cavity creation, and hydraulic fracturing. This paper reviews the various techniques that have been used, providing rationales for use of each and commenting on their commercial success, and proposes a general selection-criteria approach that may be useful in the selection of a drilling and completion technique.
Fundamental Reservoir Parameters
Factors to be considered in the selection of the drilling and completion technique include
- Reservoir thickness
- Coal cleat/fracture permeability
- Coal cleat/fracture porosity
- Reservoir pressure
- Gas saturation and gas composition
- Number of seams
- Geologic complexity
- Minimum completeable thickness
- Dip
- Coal competency/hole integrity/risk of collapse
- Surface access
- Economics, capital, and operating costs
In general, the following principles are suggested:
- Coal seams with lower permeability require a greater degree of stimulation, such as hydraulic fracturing or cavitation, to achieve economic production rates and cumulative recovery.
- Thick, highly permeable coal seams require relatively little stimulation, while low-permeability coal seams may require stimulation techniques or horizontal drilling.
- Large numbers of coal seams or highly structured, geologically complex coal seams may limit the optimal candidates to vertical-well-completion options only.
- Surface access or limitations in local services may drive the drilling and completion decision.
Vertical Well, Openhole Underream Completion, Single Seam
The major steps for this drilling and completion technique are
- Drilling the production hole to the top of the coal seam
- Running and cementing casing
- Drilling a hole through the coal seam
- Increasing the diameter of the hole by a technique known as underreaming (Fig. 1)
From a reservoir-engineering perspective, the stimulation effect is achieved because the resulting underreamed hole diameter is larger than the original hole diameter.
This type of technique is best suited for thick, vertically continuous, highly permeable coal seams. The primary advantage of this technique is that it is very inexpensive relative to other options. Disadvantages for this technique are that caving of the formation may cause fill, which, in turn, may cause production problems; completion of deeper coal seams is nearly impossible; and completion of upper coal seams may be difficult and complicated.
Vertical Well, Cased and Openhole, Underream and Perforate Completion, Multiseam
This technique is a variation of the preceding technique. In this technique, a hole is drilled to the top of the main target coal seam and casing is run and cemented as before. After underreaming, a bridge plug is set above the primary completion interval, and additional coal seams are completed according to typical plug-and-perforation techniques.
Vertical Well, Openhole Cavity Completion, Single Seam
This drilling and completion technique (Fig. 2 above) is similar to the vertical-well openhole single-seam underream completion in that a hole is drilled to the top of the coal seam, where 7-in. casing is run and cemented. After the coal seam is drilled, instead of performing the underream technique, air compressors are used to inject air (and sometimes water and air) into the coal seam at a high rate and pressure. After injection, the well is opened to the atmosphere and the high-pressure air is allowed to escape from the coal seam. This process causes individual pieces of coal to cave into the wellbore, after which they are circulated out of the wellbore. This process is repeated many times (perhaps 15 times or more). The latter injection cycles cause less coal to cave than the earlier cycles, and cuttings returns are monitored to determine when injection cycles no longer yield adequate caving to warrant further cycles. At the completion of the cavity process, the well may be left open hole or a perforated liner may be installed.
The stimulation achieved by the cavitation process can be attributed to two main mechanisms—first, the increased diameter of the wellbore caused by the cavitation process; second, coal cleat relaxation in the area beyond the cavity, which increases the aperture of the cleat system, creating an additional stimulated zone.
Vertical Well, Cased and Perforated Hydraulic-Fracture Completion, Multiseam
This technique is by far the most common technique for drilling and completing CBM fields, especially where multiple completable seams are encountered and many or most of them need to be hydraulically fractured to achieve economic flow rates and cumulative recoveries.
The technique involves drilling the production hole to 50–100 ft below the lowest coal seam to be completed and running and cementing production casing. Typical total depths may extend to 4,000 ft. Zones are completed sequentially from bottom to top. The first zone to be completed is perforated (several individual coal seams may be included in each stage) and hydraulically fractured. The zone is then isolated, and the next zone is perforated and hydraulically fractured. Zonal isolation can be accomplished by several techniques such as plug-and-perforation, ball-and-baffle, or multizone-stimulation technology.
Advantages of this technique are that all desired coal seams can be sequentially completed in stages, leaving nothing behind pipe. Coal particles and fines are generally well-controlled behind pipe, minimizing formation caving and associated production problems such as pump and equipment plugging and hole fill-up.
Disadvantages may include somewhat higher costs and longer completion times, depending on the number of hydraulic-fracture stages. Wells may experience initial well-cleanup issues, such as sand and coal-fines production. Operators may control the initial rate of water-level reduction to manage these problems.
Vertical Well, Openhole Underream With Intercepting Single or Multiple Surface-to-Inseam Openhole Horizontal Wells, Single Seam
In this technique (Fig. 3), a vertical well is drilled with the “vertical well, openhole underream, single seam” method described earlier. A target is then placed in the underream section, and the surface-to-inseam horizontal well is drilled, typically from 1 km away, and intersects the underreamed section of the vertical well. Sensors in the drillstring are used to detect the target. Two or three passes may be required to hit the target. Typically, a perforated or slotted plastic liner is inserted into the open and unstimulated horizontal well to prevent collapse of the coal.
After cleanout of the openhole section of the vertical well, artificial-lift equipment is installed, with the pump typically set in or just above the underream section.
Advantages of this technique are that a high recovery of gas in place can be achieved in a short period of time relative to that achieved by vertical wells, and it can be used in areas where hydraulic-fracturing capability is lacking. Disadvantages include an inability to complete more than one coal seam with each set of inseam wells. Inseam-well-stability issues can cause partial or complete loss of an inseam-well section.
Vertical Well, Openhole Underream With Intercepting Surface-to-Inseam Openhole Multilateral Horizontal Wells
This technique is similar to the preceding technique in that a vertical well is drilled to the top of the coal seam and production casing is run and cemented. The coal seam is then drilled and underreamed. At this point, a nearby surface-to-inseam well is drilled to a depth near the top of the coal seam. A tight-radius turn is made, and a horizontal inseam well intersects the underreamed portion of the vertical well (Fig. 4). The inseam well is then drilled through the coal, typically for approximately 0.7 miles. The drillstring is then retracted, and lateral wells are drilled into the coal seam in a pinnate pattern.
Production is by pump in the vertical well. This type of drilling and completion technique has the same advantages and disadvantages as the surface-to-inseam technique previously discussed, with two additional disadvantages.
It is not possible to install plastic liners in the multiple lateral well sections, and, in relatively thin coal seams and where geologic complexity exists, core-hole drilling may be required to locate the inseam-well sections properly.
Process for Determination of Proper Completion Technique
In general, ranked from high to low permeability (from greater than 50 md to less than 0.1 md), appropriate drilling and completion techniques may be suggested as follows:
- Vertical well, openhole underream, single seam
- Vertical well, openhole cavity, single seam
- Vertical well, cased and perforated hydraulic fracture, multiseam
- Surface-to-inseam horizontal wells with vertical-well intercepts, single seam
- Multilateral with vertical-well intercepts
The author suggests the following general workflow as a guide to developing the optimal drilling and completion strategy.
Gather Critical Reservoir Parameters. No attempt to develop an optimal drilling and completion strategy for a CBM resource can succeed unless it takes into consideration all of the critical reservoir parameters. Time and care should be taken to compile as accurate a description of the CBM resource as possible, using all the available data. Where parameters are unknown, best estimates must be made on the basis of experience and use of the best analog available. Each parameter should be ranked with respect to certainty and economic effect.
Using Reservoir Simulation, Perform Spacing Optimization Study for Each Style of Completion To Be Considered. Some drilling and completion techniques may be ruled out immediately—for example, it will be difficult to develop a stack of 10 to 15 relatively thin coal seams with horizontal wells. Thus, with experience, it may be possible to use general guidelines to screen likely techniques. Once the likely drilling and completion techniques have been identified, reservoir simulation should be used to study and optimize spacing between wells or inseam laterals. For each case, incremental economics should be run to determine which spacing is optimum for each drilling and completion candidate.
Run Field-Development Economics for Each Completion Style Based on Optimized Spacing. With optimal spacing estimated, full field-development scenarios can be studied for each drilling and completion technique. The development plan should give consideration to total number of wells, location of pipelines, and central processing facilities. Net present value economics should be run, including capital and operating cost, gas prices, and reasonable timing assumptions.
Perform Incremental Economic Analysis To Determine the Best Completion Technique. Once development scenarios have been economically evaluated, incremental economics can be run to compare one drilling and completion technique to another. For example, an area that would require two surface-to-inseam wells intersecting a vertical well may require four vertical wells. Each surface-to-inseam well is more expensive, but fewer of them are required. The production profiles for the two drilling and completion techniques are different; therefore, one may have better net present value than the other. The surface-to-inseam option may develop only one seam, but the vertical-well scenario can develop an additional small seam, with the resulting additional resource having an effect on project economics.
This article, written by Editorial Manager Adam Wilson, contains highlights of paper IPTC 17153, “Drilling and Completion Technique Selection Methodology for Coalbed Methane Wells,” by J. Caballero, SPE, ExxonMobil Development Company, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2013 International Petroleum Technology Conference. Reproduced by permission.