DEVELOPMENT OF SITING CRITERIA FOR COALBED NATURAL GAS WATER INFILTRATION BASINS

Coalbed Natural Gas (CBNG) represents a significant new domestic energy source; however there are environmental challenges associated with its development. CBNG forms from coal through biogenic and thermogenic processes, and is sequestered within a coalbed when there is sufficient hydrostatic pressure to cause the methane to become adsorbed onto the surface of the coal. In order to produce economic quantities of CBNG it is necessary to reduce the hydrostatic pressure within the coal seam by removing coalbed water. Environmentally sound management of produced water is a major environmental issue associated with the development of CBNG. In Montana, the quality of this produced water is significantly different than surface water quality. For this reason it is anticipated that only a small percentage of the total produced water will be able to be assimilated by surface waters without impacting their beneficial uses.

One proposed method of CBNG water management is to place the water in unlined basins, thereby allowing the water to infiltrate and recharge underlying aquifers. Potential problems with this method of water management include the possibility that the water will intersect an aquatard and flow horizontally to an outcrop or sub-crop, or that the water quality may be degraded through subsurface chemical reactions. For these reasons this study uses field data to develop models of the physical flow of water through the variably saturated media beneath infiltration basins, and the chemical evolution of the CBNG water as it travels along its flow path. These models allow for the development of siting and monitoring criteria for infiltration basins. These criteria will aid in the environmentally responsible use of infiltration basins, and prevent the siting of infiltration basins in unsuitable locations.