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Sub-hydrostatic pore pressure in coalbed and sand aquifers of the Powder River Basin, Wyoming and Montana, and implications for disposal of coalbed-methane-produced water through injection

Department of Geophysics, Stanford University, Stanford, CA 94305-2215, U.S.A.

^* Correspondence should be addressed to: heross{at}stanfordalumni.org

Coalbed methane (CBM) production in the Powder River Basin (PRB), Wyoming, is associated with the production of large volumes of water. Locally, water in coalbeds from the PRB has high saline and sodium contents, making it unsuitable for agricultural use and potentially environmentally damaging if discharged at the surface. One option for the disposal of CBM-produced water is injection into aquifers. For injection to be feasible, however, the porosity and permeability of the sands need to be high, the pore pressure ideally needs to be sub-hydrostatic, and the aquifer cannot be in hydraulic communication with coalbeds or aquifers used for irrigation use. In order to determine if pore pressures in the aquifers are low enough to allow for significant water injection (and to determine whether the coals and nearby sands are in hydraulic communication), we have compiled pore pressures in 250 wells that monitor water levels in coalbeds and adjacent sands within the PRB.

All 250 wells have pore pressures below hydrostatic pressure, suggesting that injection of produced water should be feasible. Through the analysis of pore pressure changes with time for both the coals and their overlying/underlying sands, we find after 8 to 13 years of water-level monitoring that none of the sands more than 200 ft (61 m) vertically from producing coals appear to be in hydraulic communication with the coalbeds. Therefore, injection of CBM-produced water should be carried out in sands at least 200 ft (61 m) from adjacent coalbeds to be sure that the disposed water does not rapidly migrate back into the coalbeds.

In addition, we constructed two 3D stochastic reservoir models of conceptualized sand units to determine the rates at which water can be injected into shallow (300 ft (91 m)) and deep (1000 ft (305 m)) sub-hydrostatic aquifers. We find that for shallow sands we can inject water at a rate of 160 bbl/day, whereas for deeper sands, whose pore pressures are lower than the shallower sands, the rate is 435 bbl/day. Both of these rates are higher than the average water production rate from CBM wells in the PRB of 100 bbl/day. This implies that for deep aquifer injection sites, it would take only one injection well to dispose of the water production from approximately four CBM wells.

Key Words: Powder River Basin • coalbed methane • coalbed methane-produced water • pore pressure • sub-hydrostatic • hydraulic communication • coalbed methane-produced water disposal • injection