Coal Bed Methane Development -- Structure Is Still Important

By

Richard W. Davis

Geotechnical Corporation, Laramie, WY

Many producers tend to ignore geologic structure when designing coal bed methane (CBM) production facilities. Well placement is often based solely upon the calculated extent of drawdown cones. Structural features, in general, are ignored as either not particularly important (folds) or are indeterminable (fractures). Data developed during a study of Powder River Basin CBM producing fields using conventional geologic analysis in combination with numerical analysis of coal bed permeability variations shows neither assumption to be true. Analysis of CBM migration patterns indicates that structural dip encountered by the migrating gas can strongly influence CBM migration patterns, causing a large part of the desorbed methane to move through the outer margins of drawdown cones and bypass wells on the flanks of folds and to move toward an accumulation center at the top of the structure. Production sites, from best to worst, are on anticlinal highs, synclinal lows, or on anticlinal flanks. Dominant fracture patterns also have a strong influence on CBM production, causing some wells in dipping beds to be better producers than others, depending upon dip, fracture permeability, and fracture orientation. In respect to fractures, if located on dipping beds, the best locations are where bed strike parallels maximum fracture permeability.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming