Causes of Variable Production Rates of Pottsville Formation Coalbed Gas Well Virginia Mine Field, Black Warrior Basin, Alabama
Walter B. Ayers Jr. and Paul A. Ferguson
In 1991, 27 coalbed gas wells were drilled, creating Virginia Mines field. In early 1993, average production rate was 55 Mcf/d per well, less than the geometric mean of 63 Mcf/d for wells in this region of the Warrior Basin. To clarify controls on gas production rates, we evaluated the production trends and the geologic setting.
Strata on the southeast side of the project dip steeply northwestward off the Birmingham Anticlinorium toward the synclinal axis of the basin, which plunges approximately 2° southwestward. Northeast-trending normal faults having throws great as 115 ft divide the project area into horsts and graben. Virginia Mines coalbed gas wells are completed in 13 to 16 ft of coal in 2 coal groups. Closure pressure (minimum stress) varies with structural setting and is 1,500 to 2,300 psi in the Black Creek group and 950 to 1,900 psi in overlying the Mary Lee group. Fracture gradient is greatest (commonly > 1.0 psi/ft) on the southeast of the project, along basin margin, suggesting that induced fractures have complex (T-shaped) geometries.
Peak gas production at Virginia Mines occurred within the first 2 months and ranged from 40 to 180 Mcf/d; production rates fell sharply to 30 to 80 Mcf/d in the 18th month. Gas production rates are highest in the northern part of the project a lowest on the southeast side of the project and in one major fault block. Water production rates were 95 to 330 b/d initially but decreased an average of less than 40 bid by the 18th month. Rapid gas and water decline rates are attributed to ineffective stimulations due to high fracture gradients and to low permeability caused by high in-situ stress. In-situ stress differences, in turn, reflect a highly variable stwctural setting. To improve production from low-rate wells will require new completion and stimulation techniques. Such techniques co ld have far-reaching implications, because coal beds in many other areas have high in-situ stresses.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California