Groundwater Compartmentalization and Fracture Flow - Rollins Sandstone, Colorado

Groundwater and methane gas inflows from fault-fracture systems in the West Elk coal mine, Colorado, USA have occurred with increasing severity. Inflows of 6, 160 and 500 L/s discharged almost instantaneously from three separate faults encountered in mine workings about 460 m below ground level. The faults are about 600 m apart. The δ²H and δ¹⁸O compositions of the fault-related inflow waters and the hydrodynamic responses of each fault inflow indicate that the groundwaters discharge from hydraulically isolated systems. 14C data indicate that the groundwaters are as much as 10,500 years old. Discharge temperatures are geothermal (» 30°C) which may indicate upwelling from depth. However, calculations of geothermal gradients, analysis of solute compositions of groundwater in potential host reservoirs, geothermometer calculations, and results of packer testing indicate that the fractured groundwater reservoir is the Rollins Sandstone (120 m thick) directly beneath the coal seams. The packer test also demonstrates that the methane gas is contained in the coal seams. A geothermal gradient of 70-80 °C/km, related to an underlying intrusion, is believed to be responsible for the slightly elevated discharge temperatures.

Large discharge volumes, as great as 8.2 x 10⁵ m³ from the 14 South East Headgate fault (14 SEHG), rapid declines in discharge rates, and vertical and horizontal permeability (matrix permeability generally < 0.006 Darcy) indicate fracture flow. An in-mine pump test demonstrates that the 14 SEHG fault has excellent hydraulic communication with fractures 50 m from the fault. A conceptual model describes a series of parallel, hydraulically separate groundwater systems associated with fault specific damage zones. The faults are about 600 m apart. Groundwater stored in fractured sandstone is confined above and below by clayey layers.