A Coupled Numerical Simulation For Oil Shale In-situ Mining

In recent years,with the development of China's economy,the demand for conventional energy has continued to grow,and the dependence on foreign oil has increased year by year.Finding alternative energy sources for oil has become one of the key issues to be solved urgently.Oil shale,as one of the important fossil fuels,has a great deal of resources.It is the important replacement resource of oil and natural gas.Its resource in China is about 1.5 times that of existing oil and gas resources.The effective development of oil shale can greatly alleviate the problem of oil and gas energy shortage in China and realize the diversification of oil and gas supply,which is of paramount importance to guaranteeing the national energy security and sustainable development.At present,oil shale is mainly utilized by ex-situ retorting.But some of the problems were existed such as high cost,destroyed mining ecological environment and energy waste.Therefore,the development and application of in-situ mining technology has attracted widely attention of the world,which also points out direction for development and utilization of oil shale in large scale.This paper mainly aims at a kind of in-situ mining technology that use the method of the convection heating up oil-shale(abbreviated as IFCD),which is being tested in the Fuyu in-situ mining test field in Sonyuan,Jilin province.Experimental study and numerical simulation were used to summarize the changes of physical properties of oil shale during heating process.In addition,the variation of temperature field,stress field and strain field in oil shale layer during in-situ mining process are also discussed systematically.According to coupling simulation results,this paper provides some optimum suggestions of in-situ mining technology.With theoretical research and practical example,this paper establishes a feasible dynamic coupled simulation method of in-situ heating temperature and thermal stress,which provides theoretical basis and guidance for in-situ mining engineering simulation.The simulation method put forward in this paper can have some guidance and reference effect on the development of oil shale in-situ mining correlation technical research.The main research results of this paper include:(1)The thermal physical properties of the Fuyu oil shale were tested by the Hot Disk thermal constant analyzer.Experimental studies show that with the increase of temperature,the physical properties such as thermal conductivity,specific heat capacity,elastic modulus,Poisson's ratio,compressive strength and density show a downward trend,while physical quantities such as porosity,permeability and thermaldiffusivity are on the rise.The thermal physical and mechanical properties of oil shale are generally anisotropic.Except for thermal diffusion coefficient,the fundamental physical parallel on the direction vertical to the bedding plane is larger than that on the direction parallel to the bedding plane.Based on these laws,this paper established a dynamic parameter model of the oil shale physical properties changes with temperature.The model contained various physical and chemical changes experienced by oil shale during in-situ mining in the form of numerical changes,so as to analyze the numerical response of the temperature and stress field in the case of dynamic properties.(2)A nonlinear flow-solid-heat multi-field coupling simulation scheme is designed for IFCD in-situ mining technology.The coupled simulation mathematical model,solution ideas,implementation methods and data storage management methods are discussed in detail.In order to improve the calculation accuracy,based on the finite element method and finite volume method,which are general approach to physical problems,this paper proposes a simulation strategy that combining FEM and FVM.The temperature field and stress field in the decoupling model are decomposed into two parts: fluid-solid coupling and thermo-solid coupling,which fully exploits the advantages of FEM in structural analysis and FVM in fluid mechanics analysis.This strategy improves the existing oil shale engineering simulation method.(3)Within 3 years of in-situ heating,the average temperature of the oil shale layer can reach 600 K.At this time,more than 50% regions of the physical model exceed the threshold temperature of oil generation.Hereafter,the hot steam is mainly used to maintain the temperature of the formation,and the heating efficiency decreases rapidly with the increase of heating time.The average temperature of steady oil shale layer remains at around 743 K.At the end of heating,the average stress of the oil shale layer is 2.43 MPa.The area exceeding 5 MPa only accounts for 3% of the total volume of the physical model.It is mainly distributed near the fractured space and the sidewall.The instantaneous stress at the outer edge of the fracture zone can exceed 13 MPa.The simulation results,including stress field and temperature filed,were used to judge the plastic deformation and failure area of underground formation,which mainly based on the failure criterion that coupling effects of confining pressure and underground temperature.Under the simulated conditions(with injection rate of10m/s,fluid temperature of 900K),when the oil shale layer is completely heated,the plastic deformation zone is mainly distributed at the edge of the fracture gap,the average displacement is about 0.7cm,and the maximum displacement can exceed2.9cm.The plastic dissipation energy of the underground formation increases due to the transition and fluid recirculation inside the fracture,and the oil shale layer could be destroyed at the outer edge of the fracture zone.This paper proposes some suggestions for improving and optimizing the IFCD,which has certain guiding significance for the theoretical research and practical application of oil shale in-situ development.