STRUCTURAL CONTROL OF COALBED METHANE PRODUCTION IN THE BLACK WARRIOR BASIN, ALABAMA

Stratigraphy, structure, and hydrology exert a strong influence on coalbed methane production. Investigation of geologic, engineering, and production data from more than 6,000 coalbed methane wells in the Black Warrior Basin of Alabama indicates that productivity is controlled more strongly by geologic structure than by stratigraphic architecture or drilling and completion practice. Large differences in water and gas production typically exist among closely spaced wells, indicating significant interwell heterogeneity.

Coal-bearing strata in the Black Warrior coalbed methane fields have effectively no matrix permeability to water, thus all flow is through natural fractures. Closely spaced cleats make coal the only permeable rock type in the coalbed methane target zone, although interference testing demonstrates that joints and fault-related shear fractures can influence production performance. The Black Warrior basin has enjoyed a spectrum of extensional and compressional deformation, and structural geometry is thought to be a fundamental control on the abundance and openness of fractures in the Black Warrior coalbed methane reservoirs and, hence, is an important determinant of well performance.

Fault zones are critical discontinuities that are typically associated with limited fluid production and tend to partition the reservoirs. Half grabens and compressional fold hinges are associated with productivity sweet spots in several fields. Adjacent to the frontal structures of the Appalachian orogen, linear trends of high and low productivity are aligned with the normal and conjugate paleostress orientations for Appalachian compression, suggesting that orogenic fracture zones also play an important role in controlling gas and water production.