Coalbed methane production is initiated by the removal of formation water in quantities sufficient to accomplish a large reduction of fluid pressure in the cleat system. Pressure reduction causes gas to be desorbed from the coal; the cleat system channels this gas to the wellbore. The rate and quantity of water removal is limited by permeability of the cleat system which is usually stimulated to facilitate dewatering and gas production.

Experiments were performed at IGT under in-situ conditions on coal from five U.S. basins. Results showed that water permeability of a given sample decreased by as much as two orders of magnitude over pressure ranges that simulated normal dewatering and production cycles.

Knowledge of the behavior of permeability as a function of net stress allows a better understanding of the following realities of coalbed stimulation: 1) a horizontal drill hole of a given length is much more productive than a propped fracture of the same length; 2) in general, fractures induced in coalbeds are much shorter and wider than designed; and, 3) frac pressures in coalbeds are abnormally high — significantly above overburden in many cases.

Other consequences of a permeability-net stress functionality include: 1) skin factor determinations from injection tests are necessarily more optimistic than those based on drawdown data; 2) overbalanced drilling increases the invasion radius of drilling fluid filtrate, thereby creating the potential for more extensive formation damage than in conventional reservoirs; and, 3) leakoff during fracturing operations is significantly higher than for "conventional" sandstone or limestone beds having identical pre-injection permeabilities.

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