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

Abstract

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.

Hydrostatic pore water pressure revisited

“…Geological underpressuring, while less commonly documented than overpressuring, is known from several basins and is told by pressures significantly below pressures defined as “hydrostatic”. In the Alberta basin, pressures are 200–300 psi below 0.433 psi/ft gradient datumed on the land surface (Parks & Tóth, 1995), a similar underpressure and datum scenario is reported from the Powder River Basin, Wyoming (Ross & Zoback, 2008), while in the Barents Sea and Svalbard there are pressures 600–800 psi below a 0.447 psi/ft gradient datumed on Mean Sea Level (Birchall et al, 2020). Underpressures are generally thought to result from elastic dilation of reservoirs in response to unloading (e.g., Birchall et al, 2020; Neuzil & Pollock, 1983).…”

“…In the simplest interpretation of hydrostatic pressure resulting from a column of fluid that extends directly upwards from the depth of interest in deep wells, the top of the column is often described as the “surface” (e.g., Ross & Zoback, 2008). For offshore wells this datum is usually mean sea level (MSL).…”

Application of Cross-Plotting Techniques for Delineation of Coal and Non-Coal Litho-Units from Well Logs

“…Identification of lithologies above and below coal seams is important to know the variation in permeability, velocity and elastic properties of rocks immediately overlying and underlying CBM reservoir for identification of the best targets for CBM exploration and production [1][2][3][4][5].…”