The Study On Simulation Of Shale Gas Flow In Micro/Nano Pores By Lattice Boltzmann Method

Shale gas reservoir is different from conventional reservoir in the mechanism of occurrence and seepage and mining method which people know less about the problem.Firstly the permeability of shale gas reservoir is extremely low which is generally less than 1 micro darcy,and the pore size is micro/nano orders of magnitude.The gas flow is slip flow or transition flow in the micro/nano pores and the Darcy's law is no longer applied.Secondly the shale gas content in the adsorbed state is between 20%and 85%of the total gas content,and the adsorbed gas has an important influence on its seepage.Thirdly the gas in the adsorbed layer will diffuse due to the concentration gradient.At present the study about shale gas flow in micro/nano pores is less,therefore researching the shale gas seepage mechanism in micro/nano pores from the micro perspective has theoretical and practical significant to develop the seepage theory of shale gas and mine the shale gas.In this paper,considering the influence of high Knudsen number in the micro/nano pores,the relaxation time is modified based on lattice Boltzmann method.The gas slip in the boundary of micro/nano pores is simulated on the combination of reflection boundary and rebound boundary.The relationship between adsorption layer thickness of shale gas and temperature,pressure is established using Langmuir adsorption model.The shale gas flow in the micro/nano singular tube and porous media is simulated by coupling the adsorption and diffusion in the adsorbed layer with lattice Boltzmann method,and the influence of pore size,pressure,temperature,slip fraction,adsorption and diffusion coefficient on shale gas flow is analyzed.The main conclusions are as follows:Because the flow of shale gas in micro/nano pores belongs to the slip flow and transition flow,the gas slip has great influence on the shale gas flow.With the decreasing of rb,the effect of gas slip is more and more obvious and the flow rate of fluid in the pores increases,and rb=0.7is selected as the most suitable slip fraction.The adsorption of shale gas on the surface of matrix is the main factor that affects shale gas seepage in the micro/nano pores.With the pore pressure increasing,the thickness of adsorbed layer increasing,and the flow velocity of shale gas in the pores decreases and the permeability decreases significantly as the size of capillary pore is less than 10nm.When the temperature rises,the flow velocity of shale gas increases and the permeability increases accordingly.The diffusion of shale gas on the surface of adsorbed layer is the main factor that affects shale gas flow in the micro/nano pores,especially in the nano pores.With the increase of diffusion coefficient,the flow velocity of shale gas in the pores increases and the permeability increasing.The range of surface diffusion coefficient is 1.0^-8m²/s¹.0×10^-7m²/s.This paper analyzes the shale gas flow characteristics in the micro/nano single tube model and porous media model from the qualitative and quantitative angle through numerical simulation.It turns out that the shale gas flow is affected greatly by gas slip,gas adsorption and surface diffusion in the micro/nano scale.Therefore it is necessary to consider the influencing factors to study the shale gas flow in the micro/nano pores.