Study On Micro Scale Model Of Shale Gas Flow Based On Lattice Boltzmann Method

China has very rich shale gas resources and has become one of the three largest shale gas producers in the world.Shale gas reservoirs have some typical characteristics,which are rich pore types and pore-throat radius ranging several scales.The multi-scale pore structure leads to a multi-scale flow mechanism,which brings huge challenges to the economic and efficient development of shale gas.In this dissertation,the multi-scale flow mechanism of shale gas are studies.With the organic-rich shale core plugs collected from Sichuan Basin,experimental tests including scanning electron microscope and CT imagery were conducted to obtain the pore structure.Based on the experimental results,digital cores that representing the characteristics of shale were reconstructed by Quartet Structure Generation Set?QSGS?algorithm.By using the reconstructed digital cores as the input porous media,shale gas flow is simulated on the nano-scale,pore-scale and REV-scale by lattice Boltzmann method and parameters affecting the apparent permeability were discussed.After the study of the microscopic flow mechanisms,the analytical model for multi-fractured horizontal wells considering the micro-scale effect is established,and parameters affecting the pressure dynamic curves were discussed.Following conclusions have been drawn:The predicted Knudsen diffusivity ranges from 10^-910^-6 m²/s.Bruggeman equation greatly underestimates the tortuosity of the organic matter.Following the relationship D_eff=D?^?,based on the pore-scale simulation results in the present study,?in the range of 1.25¹.72 is recommended for calculating the effective diffusivity in the organic matter.The lower the porosity and the overlap tolerance are,the higher the?is,indicating more tortuous void pathways in the organic matter.Total mass flux can be either enhanced or reduced depending on variations in adsorbed gas coverage and surface diffusivity.Surface diffusion plays a significant role in determining the apparent permeability.The microscopic flow mechanisms have been analyzed profoundly.Effects of roughness properties,temperature,pressure,channel width and surface diffusivity on apparent permeability have been evaluated.Coupling effect of rough surface,real gas and multiscale mechanism has a remarkable impact on micro-gaseous flow.Rough surface changes molecule-wall collisions and obstructs gas transportation in the region adjacent to the wall,which affects the bulk flow region in return.Real gas effect is enhanced with an increasing pressure or a decreasing temperature and gas flow velocity has different variation trend at different pressure/temperature range.Based on representative elementary volume?REV?-scale structure of shale matrix,shale gas flow is simulated using the REV-scale lattice Boltzmann method,and the effects of organic content,clay content,interparticle pores on the apparent of the REV-scale matrix are investigated.Results indicate that the apparent permeability is positively correlated with the porosity and volumetric percentage of the organic,and is not related with reasonable volumetric percentage of the interparticle pores.The Knudsen diffusivity and apparent permeability are introduced into the macro seepage model to establish a dynamic pressure analysis model for multiple fractured horizontal wells in dual-porosity formations,by considering microscale percolation behavior and the stress sensitivity effect.The Laplace transformation and the regular perturbation method are used to obtain an analytical solution.This dissertation improves the flow mechanisms in unconventional gas reservoir.The study simulates the microscopic percolation of shale gas at the nano-scale,pore-scale,and REV-scale,reveals the effects of structure parameters and fluids parameters on the apparent permeability,and establishes a mathematical pressure transient analysis model.The conclusions have important academic values and practical applications to the successful development of shale gas reservoirs.