Western Cretaceous Coal Seam Project Natural Fractures in Bituminous Coal GasReservoirs. Topical Report, January 1988-December 1991

摘要

The paper discusses the issues of natural fracture (cleat) occurrence andgeometry, and cleat permeability recognition, evaluation and prediction for bituminous coal gas reservoirs. The Darcy flow of gas and water in coal gas reservoirs takes place in cleat permeability avenues during depressurization. Cleat permeability is often the most important characteristic that influences coal gas project success or failure. Often the most important and difficult task associated with coal gas projects is to locate commercial permeability. An hypothesis on the genesis of cleats in bituminous coal gas reservoirs, which incorporates the results of coal gas research and the history of exploration success and failure in several coal basins, is suggested as a starting point in the search for commercial coal gas reservoirs. Coals with interbedded vitreous and non-vitreous lithotypes, and interbedded shale layers, are favored targets because of the greater possibility that these reservoirs are characterized by well developed cleat systems and commercial cleat permeability. The coals must also have sufficient net thickness and adequate reservoir pressure to contain significant gas in-place resources formed during active coalification. The operator should initially find high volatile A to low volatile bituminous rank coals at depths of less than approximately 1,525 meters. These coals are also often characterized by more favorable gas desorption and diffusion characteristics with respect to critical desorption pressure, particularly in areas where reservoir pressure has been maintained after active gas generation.