Groundwater environmental impact caused by in-situ pyrolysis of oil shale

Based on a sample of oil shale from the Lucaogou Formation in Shichanggou, Xinjiang, China, the mechanism of changes in the physical and mechanical properties of oil shale during in-situ pyrolysis was systematically analyzed, and combined with the kinetics of the pyrolysis reaction, a constitutive model of permeability change during the in-situ pyrolysis of oil shale was established. By leaching experiments, the changes in the physicochemical parameters and pollutant concentration of oil shale immersion solution under different pyrolysis temperatures were studied. Basing on the theoretical permeability value and pollutant concentration under in-situ pyrolysis conditions, a hydrogeological model was established to simulate groundwater pollution caused by the in-situ pyrolysis of oil shale. The results showed that the permeability of oil shale after in-situ pyrolysis increased by three orders of magnitude, changing from a water-proof layer before pyrolysis to a weakly permeable layer after pyrolysis. However, the permeability of oil shale after complete pyrolysis at 600 °C was only 2.062 mD and still very low, and the pollutants had low concentration and were mainly concentrated in the pyrolyzed oil shale layer. The in-situ pyrolysis of oil shale would not pollute the groundwater in the mining area, but the pH of groundwater would gradually increase with rising pyrolysis temperatures. At 600 °C, pH even increased to 11.68, which is strongly alkaline. It is suggested that the pyrolysis temperature should be 400–500 °C.