ABSTRACT: Suggested Reservoir Goals for Exploration of Coal Gas Resources

CLOSE, JAY C., Resource Enterprises, Inc., Salt Lake City, UT

Successful coal natural gas production has been achieved in areas with the following interdependent geologic and reservoir characteristics. Bituminous coals with interbedded bright vitrain and clarain lithotypes, and interbedded noncoal layers, are preferable targets because of the greater possibility that well-developed cleats and high cleat permeability will be encountered. Coal/noncoal interbeds can be recognized on openhole high resolution bulk density and microlaterolog resistivity wireline log suites. Most cleats in bituminous coals are hypothesized to have formed as a result of simultaneous moisture loss, devolatilization, fluid pressure, and strain accommodation processes during and just after paleotectonic stress events. The coals must have sufficient thickness and reservoir ressure to contain significant gas in place resources formed during active coalification. High volatile A and greater rank bituminous coals with a great frequency of closely spaced, open interconnected cleats are often characterized by rapid gas desorption and diffusion characteristics. These same coals can have a gas content that is equal or nearly equal to the isotherm gas storage capacity, particularly in areas where reservoir pressure has been maintained over geologic time after thermogenic gas generation. In these cases the critical desorption pressure lies on or nearly on the storage capacity curve. Favorable gas and water relative permeability relationships have been documented in coals characterized by a fracture frequency of greater than 3 cleats/cm, greater than 5 md cleat abso ute permeability, 1 to 2% cleat porosity, and greater than 300 SCF/ton as-received gas content. Since cleat permeability is highly stress-sensitive, a maximum depth of approximately 1500 m with respect to minimal lateral compressional stress is desirable.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)