Numerical Simulation Of In Situ Combustion Of Oil Shale

With the rapid advancement of industry,oil plays an important role in the development of social economy.However,the production of crude oil in China is difficult to meet the requirements.As an important unconventional energy,oil shale has attracted people's attention because of its huge reserves.It is urgent to find a high-efficiency and low-cost cleaning method to the exploitation of oil shale,due to the environmental pollution and the high-cost of the traditional mining methods.As a clean mining method,in situ combustion method has the advantages of low cost and high oil production rate.So it has been applied in the exploitation of heavy oil.In this study,we apply the in situ combustion method to the exploitation of oil shale.With high cost,time-consuming and low repeatability of the laboratory experiment,the in situ combustion process is studied by numerical simulation in this paper.In this study,both one dimensional and two dimensional models of in situ combustion process of oil shale have been simulated.The mechanism of in situ combustion of oil shale is discussed,and the sensitivity analysis of several parameters is carried out.The mathematical model established in this paper can reflect the in situ combustion process of oil shale correctly,and the results can provide theoretical guidance for field test,which has a great practical significance for the development of oil shale.It is a complicated physicochemical process of in situ combustion of oil shale,which involving complex mechanisms of multiphase,multicomponent flow,energy transfer and chemical reactions.Therefore,a mathematical model is developed based on the correct description of the physical mechanism.Assuming that the fluid flow in the reservoir conforms to Darcy's law,while the oil shale and each phase are in the local thermal equilibrium?i.e.the local temperature is equal?.The changes of mass and energy caused by chemical reactions in the process of combustion are expressed in the form of the source term in the governing equation.The equation set of in situ combustion process of oil shale is:The mediums in the equation set consist of N phases and the rock matrix,while the phase l contains Nl components.Where ? is the porosity of the porous media,?l,al,Dl and ul are the density,the volume fraction,diffusion coefficient and the velocity of phase l,respectively.Yl,k is the mass fraction of component k within phase l.Si,and Sl,k are the source term of phase/and component k within phase l,respectively.cl and cr are the specific heat capacity of phase l and the oil shale,respectively.??kab?krl??l and hl are the thermal conductivity,absolute permeability,relative permeability of phase l,viscosity of phase l,and the height of phase l relative to the reference plane,respectively.As to equation set?1?,the first two equations are the mass conservation equations for phase l and component k within phase l.The third equation in the equation set is the energy conservation equation,in which the source term Se is determined by the chemical reactions.It is obviously that the temperature is determined by the heat conduction,the heat convection and the chemical reactions.A complete mathematical model of in situ combustion of oil shale is established by giving suitable initial conditions and boundary conditions to equation set?1?.The variables in the equation set are ?1,Yl,k,T and P,and they are the functions of time t,and position?x,y?.So,it is a system of nonlinear partial differential equations.It is very difficult to find the exact solution of the problem.Firstly,a one-dimensional model of in situ combustion of oil shale is simulated by using the finite difference method in this study.A mathematical model of two phases and nine components is developed based on equation set?1?.In order to solve the problem,the discrete difference scheme are given.In the process of in situ combustion of oil shale,the changes of energy and mass are mostly caused by the complex chemical reactions in the porous media.In this paper,the chemical reactions in the in situ combustion process are simplified into three chemical reactions,which based on the consistent amount of energy and the substance with the experimental results:OM?VM+C?2?C+[?fr?/2+?1-fr?]O2?frCO+?1-fr?CO2?3?CaCO₃?0.56CaO+0.A4CO2?4?where OM represents the organic material in oil shale?kerogen?,VM is the mixture of oil and gas produced by oil shale pyrolysis,and C represents coke.Based on the simplified model,the source terms in the equation set?1?are built according to Arrhenius Law.The mass source terms of kerogen and calcium carbonate are respectively:where nOM and nC?CO₃ are the concentration of OM and CaCO₃,respectively.AOM,AC?CO₃,EOM,and EC?CO₃ are the frequency factor and activation energy of OM and CaCO₃,respectively.Based on the mass source terms of OM and CaCO₃,the expressions of the source terms in equation set?1?can be calculated according to the chemical reaction coefficients.Based on the mathematical model and the discrete method,the simplified one-dimensional model for in situ combustion of oil shale is simulated.The numerical results are highly match the experiment data,and the numerical model can reflect the in situ combustion process of oil shale well.It is also observed that the pyrolysis zone of oil shale is located downstream of the coke combustion zone.This structure ensures that the produced oil and gas do not participate in the oxidation reaction and can be recovered successfully.There is no residue of coke in the reaction zone and the oxidation reactions of coke provides a large amount of energy for the combustion process.Secondly,the in situ combustion process of oil shale in the two-dimensional case is simulated by using the computational fluid dynamic software Fluent.According to the properties of the oil shale sample,the chemical reaction model is further simplified in this study.In other words,the decomposition reactions of inorganic compounds are neglected.By embedding user-defined subroutines into Fluent,the in situ combustion process of oil shale is simulated.The numerical results are compared with the relevant experimental results.Which shows that both the temperature distribution curves of oil shale and the frontal moving velocity agree with the results of the experiment.In order to find the optimum mining method,the inlet injection rate,oxygen content at inlet,initial oil content and the permeability of the porous medium are chosen as the research variables,and the sensitivity of them are analyzed.The results show that increasing the gas injection rate and the oxygen content can improve the average temperature and the production rate of the oil shale.The in situ combustion method is more suitable for highly initial oil rate of the oil shale layer.At the same time,improve the permeability of oil can improve the production rate of oil shale.According to the above results,we can provide some theoretical guidance for field test of in situ combustion method.