Assessment of Coal Bed Gas Prospects

Thomas R. Moore

Coal bed gas is an often overlooked source of clean, methane-rich, H₂S-free natural gas. The economic development of coal bed gas requires a knowledge of coal gas reservoir characteristics and certain necessary departures from conventional evaluation, drilling, completion, and production practices.

In many ways coal seam reservoirs are truly unconventional. Most coals of sufficient rank have generated large volumes of gas that may be retained at depth in varying amounts through adsorption. Coal gas production can take place only when the reservoir pressure is reduced sufficiently to allow the gas to desorb. Gas flow to the well bore takes place through a hierarchy of natural fractures, not the relatively impermeable coal matrix. Economic production is dependent upon critical factors intrinsic to the reservoir, including coal petrology, gas content, internal formation stratigraphy, fracture distribution, hydrogeology, in situ stress conditions, initial reservoir pressure and pressure regime, and the presence or absence of a "free" gas saturation. Further, the coal bed re ervoir is readily subject to formation damage through improper drilling, completion, or production techniques.

This presentation will review the data types critical to the assessment of any coal seam gas prospect, suggest an outline method for screening such prospects, and point out some possible pitfalls to be considered in any coal bed gas development project.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California