Temperature and electrical conductivity stratification in the underground coal gasification zone and surrounding aquifers at the Majuba pilot plant

Abstract

Underground coal gasification (UCG) is a chemical process that converts coal in situ into a gaseous product at elevated pressures and temperatures. UGC creates an underground cavity that may be partially filled with gas, ash, unburned coal, and other hydrocarbons. A water stratification assessment can help assess the diffusion effects within the underground cavity. In this study we assessed the stratification by comparing the electrical conductivity (EC) profiles of background boreholes to the verification borehole that was drilled after gasification was complete. Stratification was seen in all boreholes, including the cavity borehole. The EC levels were lower in the cavity, which may be due to the dilution induced by injecting surface water during quenching of the gasifier. The thermal gradients showed a steady increase in temperature with depth, with higher temperatures measured in the verification borehole. This temperature increase suggests that heat is still being retained in the cavity, which would be expected. This study serves as the preliminary investigation of the stratification of temperature and EC, and will be followed by in-depth surveys that cover all the groundwater monitoring wells in the different aquifers at the site.