Drilling and Completion Technique Selection for Coalbed Methane Wells
Authors: Wilson Adam 
Published: February 2015

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 The paper has not been peer reviewed.

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 completable 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.