September Distinguished Lecturer: Tight Coalbed Methane A Giant Worldwide Resource
The development of coalbed methane (CBM) has been limited to moderate- to high-permeability
reservoirs (>1 md). However, a significant resource of natural gas exists within low-permeability coals (<1
md). Worldwide CBM resources are estimated to range from 3,500-7,000 Tcf, and EIA estimates >800 Tcf are technically recoverable. As of 2010, however, only 60-70 Tcf of CBM reserves were proved. Vast CBM resources are untapped.
Because of the coal depositional process, and the nature of gas storage and transport mechanisms, a
large percentage of CBM exists in low permeability ("tight") coals. Coals are typically laterally-extensive
deposits, often covering entire basins. In addition, coals are usually deposited in a cyclic process, producing
large gross intervals containing many coal seams. Thermal maturity typically increases with depth within a basin, and since gas adsorptive capacity increases with thermal maturity, large gas accumulations are found in the deeper parts of basins. However, coals are naturally-fractured reservoirs, and permeability of coals is stress-sensitive. Therefore, permeability tends to decrease with depth and strongly affected by structural folding and tectonic forces . Commercial development to-date has been primarily restricted to shallow, high-permeability coals for this reason.
Horizontal drilling and enhanced CBM techniques have been successful in recovering gas from tight coals, but with limited commercial success to-date. Better understanding of coal geology and Geomechanics will lead to identification of sweet spots that can be successfully developed. Advancements in horizontal drilling technology and potentially, ECBM technology, will reduce development costs and facilitate commercial development. R&D is required to advance these technologies.