Completion Methods in Coal-Seam Reservoirs
Authors: Holditch S.A. 
Published: December 2007

Wells drilled to produce methane from coal seams are completed with either a perforated casing method, a stable cavity method, or an openhole method. The perforated casing method followed by hydraulic fracturing works best in most coalseam wells. In high-permeability, high-pressure coal seams, the stable cavity method may be preferred, To design the optimum completion, a completion engineer must consider the unique aspects of producing methane from coal.

Introduction

Methane produced from coal seams is an important energy source in the petroleum industry. The increased development of coal-seam reservoirs can be attributed to (1) U.S. income tax credit that is associated with producing coalbed methane, (2) Gas Research Inst. (GRI) research funding to improve technology, and (3) an increased awareness in the industry concerning the importance of coalbed methane. Geologic assessments of coalbed methane reservoirs have been funded by the U.S. DOE and the GRI. Currently, resource assessments are available on the Arcoma basin, Black Warrior basin, Cahaba and Coosa coal fields, Central Appalachian basin, Greater Green River coal region, Illinois basin, Northern Appalachian basin, Pennsylvania anthracite fields. Piceance basin, Powder River basin. Raton basin, Richmond and Deep River basins, San Juan basin, Uintah basin, Valley coal fields, Western Washington coal region, and the Wind River basin. On the basis of detailed studies of these coal areas, GRI estimates that 300 to 400 Tcf of gas in place is contained in the U.S. Table 1 contains the details concerning the gas in place for each basin. Most methane recoverable from coal seams will be in reservoirs that are less than 5,000 ft deep. The low-cost drilling associated with the shallow depths makes coalbed methane development suitable for independent producers. In addition, the need for large acreage positions and the technical challenges involved with producing gas from coal seams also provide incentives for big producing companies to pursue coalbed methane development actively. Because of these inducements, a wide variety of companies develop coalbed methane reservoirs. Several papers on completing wells in coalbed methane formations have been published. In 1980, Steidle et al. published a paper concerning completion techniques from vertical published a paper concerning completion techniques from vertical methane drainage boreholes in Virginia. Lambert et al., Logan et al., and Clark et al. have discussed specific completion problems in specific coalbed methane areas. Lambert et al. problems in specific coalbed methane areas. Lambert et al. discussed completion alternatives that have been evaluated in the Black Warrior basin. Logan et al. and Clark et al. discussed coal-seam well completions in the San Juan and Piceance basins. Much of the published information on coal seams has evolved through research projects sponsored by the GRI. The objective of this paper is to explain the various completion options that are available to an operator developing a coal-seam reservoir. On the basis of the unique properties of coal, the engineer must develop a completion strategy. The strategy should include specific details concerning the site of the perforations and the stimulation treatment needed to maximize gas perforations and the stimulation treatment needed to maximize gas recovery.