Coalbed methane exploration must evolve to adequately reflect a growing geologic and reservoir understanding in domestic and international coal basins. Currently accepted exploration strategies in the San Juan Basin of New Mexico and the Warrior Basin of Alabama are basin specific and remain unproven outside these traditional methane plays. Future development will require modified exploration strategies to locate commercial prospects in the more complex geologic settings of other less favorable domestic and overseas basins. Coalbed methane operators presently focus exploration efforts on locating areas of improved permeability, over-pressured hydrologie conditions, high methane storage capacity coals and a maximum ratio of gas desorption pressure to initial reservoir pressure. The effects of each of these reservoir properties and more importantly their interaction has not been thoroughly examined.

A reservoir simulation study was undertaken to investigate the effects of stress setting as it relates to permeability, pore pressure gradient and coal rank in terms of isotherm characteristics and gas content. A series of 162 well performance simulations were conducted evaluating the interaction of depth, coal permeability in high, normal and relaxed stress settings, gas contents for high, medium and low rank coals and finally, normal and abnormal hydrologie conditions. The results indicate that 1) there is an optimum target depth for a coal of a given rank under a specific stress setting, 2) the optimum producing depth increases with decreasing stress setting, 3) low rank coals exhibit superior production than higher rank coals under similar reservoir conditions and 4) the ratio of desorption pressure to initial reservoir pressure is of less significance than the absolute value of the desorption pressure. Based on these findings, fundamental inadequacies of existing exploration criteria require modification, particularly to the interaction of specific coal reservoir properties.

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