Mature Field Production Management of the Fruitland Coal, San Juan Basin, U.S.A.

R.R. Gierhart, W.L. Pelzmann, and W.C. Riese
BP America Production Co, Houston, TX

Production of methane from the Fruitland Formation coals in the San Juan Basin of Colorado and New Mexico, USA, began in 1977. Little was known at that time about reservoir attributes and performance. Field development orders therefore specified a 320 acre spacing. Petroleum system models for this formation described it as a through-flowing aquifer in which methane was trapped at the molecular level within the coal matrix, and held there by hydrodynamic pressure.

Subsequent work has determined that the reservoir attributes of the coals are unique with regard to a number of attributes. Although no structural trap is apparent between production and outcrop, the coals have been demonstrated to not be a through-flowing system. Individual reservoir performance units vary in thickness from a few inches to a few feet in thickness, and in areal extent from 40 acres to more than 320. Initial gas-in-place calculations which were based on desorption isotherms have been found to be inaccurate: pressure data must be gathered to allow material balance calculations of resource volumes, and indicate that less than half of the resource is adequately developed in parts of the basin. Permeabilities, which are difficult to measure in core, must be calculated from performance data, and are found to vary with time.

These collective observations and analyses have caused reservoir spacing orders to be rewritten to allow 160 acre spacings field-wide, and 80 acre spacings in some areas. We expect that 40 acre spacings will be required for the effective development of the resource in some areas.