The Transition Between the Low-Rate and the High-Rate Producing Areas of the Fruitland Coal, San Juan Basin, New Mexico

Abstract

Production histories for coal-bed methane wells producing from the Fruitland Formation of the San Juan basin indicate significant variations in the reservoir quality of the coal seams. These variations can be discussed in terms of overpressured vs. underpressured and/or high permeability vs. low permeability. Correspondingly, the production profiles can be discussed in terms of high rate vs. low rate in various areas of the Fruitland field. A transition from the high-rate area to the low-rate area occurs over a very short distance in the northern townships of New Mexico. The southern boundary of the high-rate area is a meandering line that traverses from T29N, R6W to T32N, R11W on a northwest trend. This boundary is an abrupt transition occuring within 1/2-mi distance between adjacent wells. Prior to Fruitland Coal development, the location of this boundary was predicted by mapping drilling fluid density from wells drilled to deeper targets. The location of this boundary can now be mapped more precisely using bottom-hole pressure and production rate data from Fruitland wells. The geologic nature of this boundary is poorly understood. Well-log correlations across this boundary indicate no stratigraphic discontinuity, such as a pinchout of the primary coal seams. Small-scale faulting could playmore » a role; however, the meandering pattern of the boundary makes this an unlikely explanation. Variations in coal maturity as mapped using vitrinite reflectance data indicate that the boundary corresponds approximately with the 0.75 R[sub o] contour. A model invoking physical changes within the coal corresponding with a vitrinite reflectance of between 0.70 and 0.80 R[sub o] is proposed as a possible explanation for the abrupt change in reservoir quality associated with the boundary between the high-rate and low-rate producing areas.« less