FLOW TRENDS IN A BASIN-WIDE FRACTURED RESERVOIR: STRUCTURAL CONTROL OF WATER AND METHANE PRODUCTION IN THE BLACK WARRIOR COALBED-METHANE PROVINCE OF ALABAMA

The co-extraction of water and coalbed methane from the Black Warrior Basin, a Pennsylvanian-age foreland basin at the juncture of the southern Appalachian and Ouachita fold-thrust belts, represents a basin-scale experiment in production from a fractured reservoir and provides an excellent opportunity to examine the controls on the regional distribution of fracture permeability. The basin is bounded on the southeast by the Appalachian thrust front and is cut by numerous northwest-trending normal faults which have maximum lengths on the order of 10 km and maximum displacements on the order of 100 m. None of the bedrock units, including the coal, have significant matrix permeability; the greatest permeability is provided by the coal cleat. Commercial methane production from coal beds requires production of the water along with the methane, thus peak daily production of water or methane are both used to measure fracture permeability. Based on the results of about 3500 methane wells, drilled approximately in a grid pattern, map-scale structure in the basin exerts a significant control on the magnitude and distribution of both methane and water production, and the specific relationship changes with distance from the Appalachian thrust front. Far from the thrust front, the normal faults are zones of low water and gas production that segment the reservoir into blocks having significantly different transmissivity. The most transmissive blocks are half grabens, with horsts and full grabens being the least transmissive. Moderate deformation (in half grabens) may enhance fracture transmissivity relative to areas of no deformation (horsts), or too much deformation (full grabens). Close to the thrust front, production can be enhanced in and near fault zones. In addition, there are nearly E-W trends of exceptional production that cannot be correlated to mappable faults. Together, the two productive trends have the orientation of conjugate strike-slip faults bisected by the Appalachian maximum compressive stress direction. Near the thrust front, production is commonly enhanced at fault tips and at releasing overlaps. The die out of compressional deformation with distance from the thrust front can explain the changing character of the productivity trends.