--> A Basin-Scale Coalbed Methane Producibility Model: Comparison of the San Juan and Sand Wash Basins, by W. R. Kaiser, D. S. Hamilton, A. R. Scott, and R. Tyler; #90986 (1994).

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Abstract: A Basin-Scale Coalbed Methane Producibility Model: Comparison of the San Juan and Sand Wash Basins

W. R. Kaiser, Douglas S. Hamilton, Andrew R. Scott, Roger Tyler

Studies in the San Juan and Sand Wash basins show that permeability, hydrodynamics, coal distribution and rank, gas content, and structural grain control the producibility of coalbed methane. High productivity requires that these geologic and hydrologic controls be synergistically combined. That synergism is evident in a comparison of the prolific San Juan basin and marginally producing Sand Wash basin. In the San Juan basin, ground water flows basinward through high-rank, high-gas-content coals toward lower rank coals at a structural hingeline (no-flow boundary) along which exceptionally high production occurs. At this point, flow turns upward and coalbed wells typically produce >1000 Mcf/day. A giant conventional hydrodynamic trap is postulated to exist along the hingeline and im lies that non-sorbed free and solution gas are important sources of coalbed methane. Gas is dissolved or entrained and swept basinward ahead of an advancing flux of meteoric water to provide additional sources of gas beyond that sorbed on the coal surface. Because flow direction and flow boundary are orthogonal, the largest possible area of flow is

intercepted, maximizing the trapping of gas. In the Sand Wash basin, flow is basinward through low-rank, low-gas-content coals toward areas of higher thermal maturity. At the basin margin, wells have yielded little gas and large volumes of water for a basinwide cumulative gas/water ratio of 15 cu ft/bbl. The most prospective coals lie in the deeper basin associated with upward flow and higher rank along a major fault system. Out of a comparison between the two basins, a basin-scale coalbed methane producibility model is evolving. The model's essential elements are (1) ground-water flow basinward through coals of high rank and high gas content orthogonally toward no-flow boundaries (regional hingelines, fault systems, facies changes, and/or discharge areas) and (2) conventional trappin of gas along them.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994