History Matching and Permeability Increases of Mature Coalbed Methane Wells in San Juan Basin

摘要

We discuss detailed well history matches, from the San Juan basin, of coalbed methane (CBM) gas rates, water rates, and pressures. In the sweetspot (fairway) the matching includes permeability that increases strongly with depletion, and is assumed to be exponential, as indicated by independent well tests spaced over time. The permeability increases are well matched by the Palmer-Mansoori (P-M) model. We present statistics of permeability matching to field results by the P-M model in the fairway. First, initial cleat porosities are 0.05-0.17% and consistent with porosities derived from observed water rates. Large permeability increases are not consistent with large cleat porosities. Second, the permeability increase ratio, defined by the maximum permeability reached over the permeability at initial reservoir pressure, varies from 16-170. Third, the matrix shrinkage parameters agree with a recent lab measurement. Fourth, cleat-volume compressibility mostly increases with depletion, when matrix shrinkage becomes dominant and cleats open wider. Last, in many wells north-east of the fairway the perm increase with depletion was found to be similar to that seen in fairway wells. This again suggests cleat porosities less than roughly 0.3%. At low reservoir pressures (<300 psi), we find that the exponential permeability increase flattens. This we have interpreted as coal failure due to stress changes instigated by matrix shrinkage. In fact the P-M model for stress changes can predict the onset of shear failure with depletion, which generally agrees with the timing of the flattening. This adds validity to the general applicability of the P-M model in CBM plays. The modeling is important in being able to better forecast gas rates and estimated ultimate recovery (EUR) for CBM wells, which can remain profitable for decades.