| As an important alternative energy source,oil shale has attracted global attention due to its huge reserves and rich levels of comprehensive utilization.Currently,there are two main mining techniques for oil shale:ex-situ mining（surface dry distillation）and in-situ mining.Compared to surface dry distillation,in-situ mining has advantages such as less disruption to the formation system,less land occupation,and lower water consumption,and has become the main mining technology both domestically and internationally.Since in-situ mining of oil shale is still in the experimental stage,there are few relevant results regarding the impact of the thermal cracking process on the underground water environment.Therefore,tracking the in-situ thermal cracking mining process and studying the mechanism of its impact on the underground water environment in real-time is essential for achieving coordinated development between oil shale energy development and utilization and environmental protection,and has both theoretical and practical significance.The paper is based on the National Natural Science Foundation of China project“Study on BTEX Release and Contamination of Aquifer during In-situ Oil Shale Mining（42002260）”,and it adopts the approach of“field sampling-pyrolysis test-physical simulation-numerical simulation-mine application”.The Fushun oil shale mining area in Liaoning Province was selected as the study area.Through pyrolysis and pollution migration and diffusion experiments,the target pollutants were determined.Based on the geological information of the mining area,the numerical simulation method was used to study the mechanism of disturbance of the stratum during the in-situ pyrolysis process and to determine the migration and diffusion path of the target pollutants.A laboratory-scale migration and transformation model of benzene-series compounds in aquifers was established to determine the parameters of dissolved phase hydrodynamic diffusion and to determine the residual saturation of NAPL phase in porous media of aquifers based on numerical simulation.Using the constructed model,the spatial and temporal evolution of BTEX in the overlying aquifer of oil shale was simulated to explore the migration and diffusion laws of BTEX and the multi-phase transformation mechanism,providing a scientific basis for the protection of groundwater environment during in-situ mining.The main research content and achievements of the paper are as follows:（1）Based on the geological information of the Fushun oil shale mining area,the COMSOL Multiphysics numerical simulation software was used to establish a numerical model of in-situ mining of Fushun oil shale,and to study the changes in the stress field and stratum displacement of the overlying rock layer caused by pyrolysis under the conditions of in-situ mining of oil shale.The simulation study found that:（1）In the late stage of oil shale pyrolysis,the stress field of the stratum underwent significant changes due to the rapid cleavage of organic matter.（2）The stratum subsidence caused by pyrolysis mainly occurred in the fully pyrolyzed oil shale area,and the upper rock layer showed obvious displacement,which gradually increased with the continuous increase of pyrolysis temperature.（3）During the late stage of organic matter pyrolysis and the stage of inorganic matter cleavage,water-conducting fracture zones were formed and gradually developed at the edges of the pyrolyzed oil shale area and its overlying rock layer,and communicated with adjacent aquifers.（2）A soil column experiment was designed to simulate the in-situ pyrolysis stratum system of oil shale.By continuously heating the oil shale layer at the bottom of the soil column,the environment of the stratum after in-situ pyrolysis was simulated,and the organic matter in the overlying media was detected for different durations.The research showed that the release of TOC in the overlying media of fully pyrolyzed oil shale increased with the continuation of insulation time,while the release of TOC in the overlying media of oil shale that was not fully pyrolyzed decreased with the continuation of insulation time,indicating that the stratum still has the ability and conditions to continue to pollute after in-situ mining activities.Volatile organic matter such as toluene,ethylbenzene,and xylene were detected in both fully pyrolyzed and non-fully pyrolyzed oil shale overlying media,so the main target pollutants released from in-situ pyrolysis of oil shale were determined to be benzene-series compounds（BTEX）.（3）A two-dimensional hydrodynamic dispersion experiment device was designed to carry out multi-phase migration experiments of benzene,toluene,xylene,and ethylbenzene in aquifers.By analyzing the characteristics of benzene-series substances in the dissolution phase and their migration process,the temporal and spatial distribution laws in the aquifers were obtained,and the hydrodynamic dispersion parameters of the four benzene-series substances in the dissolution phase were calculated using analytical methods.The longitudinal dispersion coefficients for benzene,toluene,xylene,and ethylbenzene were 0.114m,0.050m,0.046m,and 0.053m,respectively,and the transverse dispersion coefficients were 1.35×10-7m,3.90×10-7m,3.85×10-7m,and 2.96×10-7m,respectively.The monitoring and analysis of NAPL-phase benzene-series substances in aquifers showed that the four benzene-series substances mainly migrated horizontally in the aquifer medium.With the continuation of time,the area of NAPL-phase contamination plume expanded,but the expansion rate gradually decreased and then stabilized.Based on the two-dimensional experiment,a numerical simulation model was established that considered the processes of mass transfer,volatilization,dissolution,and adsorption at the same scale,and the model was solved using the COMSOL Multiphysics software.The residual saturation of the NAPL phase of the four benzene-series substances was determined using the experimental data.The observed and simulated values of the area of NAPL phase contamination plume and the concentration of the dissolution phase were well fitted,and the model can be used to simulate the multi-phase migration and diffusion process of benzene-series substances.（4）Based on the geological and hydrogeological conditions of the study area,and considering the impact of in-situ thermal cracking of oil shale on the aquifers,a numerical model was established to simulate the migration and diffusion of benzene,toluene,xylene,and ethylbenzene（BTEX）in the aquifers under the scenario of in-situ thermal cracking of oil shale through conductive fractures.The model simulated the multi-phase migration and diffusion process of BTEX in the porous medium of the aquifers,and explained the formation,evolution,and distribution patterns of BTEX and the mechanism of contaminating the aquifers.The research found that after the four benzene-series substances migrated to the aquifers,their NAPL phase and dissolved phase underwent vertical and horizontal migration in the aquifers.At 60～80 days,the NAPL phase passed through the aquifers and reached the intersection of the aquifers and the overlying medium,where it converged.With the continuation of time,the area of NAPL phase contamination plume gradually expanded and stabilized at 140～180 days.The area of dissolved phase contamination plume continued to expand with time,and its peak contamination concentration was distributed along with the migration of the NAPL phase.After the formation of the NAPL phase contamination plume,the residual NAPL phase in the porous medium was in a continuous dissolution process.By 500 days,the area of NAPL phase contamination plume no longer expanded and its average saturation gradually decreased,while the area of dissolved phase contamination plume continued to increase and was 3 to 5 times that of the NAPL phase.The multi-phase migration and diffusion study revealed that the dissolved phase contaminants migrated faster and produced a larger contaminated plume area,but the NAPL phase contamination process was more persistent and constituted a new source of dissolved phase contamination.The study on the mechanism of BTEX release and pollution to aquifer during in-situ mining of oil shale has important theoretical significance for understanding and revealing the release law of organic pollutants and the transport mechanism to aquifer during in-situ mining of oil shale,and has important practical significance for the protection of groundwater resources during in-situ mining of oil shale. |
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