Underground Trona Mine Dewatering

Situation
Hydro Geo Chem evaluated dewatering alternatives for a large, underground trona mine in Wyoming. Mine operations were impeded by excessive water flows in the longwall mining area. Hydro Geo Chem was retained to evaluate the source of the excess water and to develop alternatives for reducing the flows. Two water-bearing geologic units were identified as potential contributors to the excess seepage. One unit was an over-pressured sandstone formation located about 2,000 feet below the mine level. This unit had the potential to contribute seepage through floor fractures. The other unit was a relatively low permeability sandstone located above the mine level which could become hydraulically connected to the mine due to extensive fracturing as the longwall area caved.

Hydro Geo Chem’s Approach
Hydro Geo Chem reanalyzed historical hydraulic testing and borelog data to estimate the permeability of the potential reservoirs and intervening geologic units. Environmental tracers were used to discriminate between the potential sources of seepage. Hydro Geo Chem developed numerical models to evaluate the effectiveness of dewatering alternatives including vertical and horizontal depressurization wells. The models were also used to evaluate future inflows and to track water flows through the mine to identify areas of water accumulation.

Results
The overlying, artificially fractured sandstones were identified as the most likely source of the water flows to the mine based on the environmental tracers and evaluation of borehole logs. The modeling showed that depressurization wells were unlikely to significantly reduce the inflows to the mine because gravity drainage would control long-term seepage rates. The results of this work prevented the mine from investing in an expensive dewatering system that would not have been effective.