A Geologic and Hydrochemical Investigation of the Suitability of Central Utah\u27s Navajo Sandstone for the Disposal of Saline Process Water and CO2

Salt water is produced from the Ferron Sandstone Member of the Mancos Shale in central Utah as part of the production of coalbed methane (CBM) and is disposed of by injection predominantly into the Navajo Sandstone between 4,500 feet to 7,300 feet and is considered to be a hazardous waste. Local government agencies are concerned about the potential impacts on shallow groundwater because of this disposal method. Water samples were gathered from four shallow water-supply wells, and nine salt water disposal (SWD) wells to compare hydrochemistries as an indicator of potential mixing. Shallow water-supply wells are likely recharged by local precipitation while the source of CO2 is from atmospheric and/or soil CO2 gas and comparatively, are low in total dissolved solids. Carbonate mineral dissolution is the source of CO2 in the SWD wells and is exceptionally high in TDS. The SWD water appears to be old water and displays an evaporative signature. A geologic analysis was conducted for the Drunkards Wash gas field using 479 digital gas well logs. Three subsurface faults were identified with one fault in the north and the other two in the central part of the gas field near the eastern and western flanks. These faults were further confirmed by comparing average monthly gas and water production from the first 24 months in these faulted areas to adjacent control areas. Areas near faults reveal two to six times greater gas production than that of the associated control areas, and water production is greater by nearly an order of magnitude. This difference is likely due to the fracturing associated with the damage zone near the faults allowing for increased flow of gas and water. Due to the high injection pressures the vertical hydraulic gradient has been reversed from downward to upward. However, due to the thick sequences of shale separating the disposal aquifers and the shallow aquifers the estimated time required for the disposal waters to migrate to the surface would be at least 2,000 years. I conclude that the saline waters produced from the Ferron Sandstone are being safely sequestered in deeply buried, extensive and geologically-sealed aquifers