Micro-and Nano-scale Fluid Flow Simulation And Flow Mechanisms Analysis In Shale/tight-sand Reservoirs

With the depletion of conventional resources and with the development of advanced production techniques such as horizontal drilling and multi-stage hydraulic fracturing,unconventional oil and gas resources are drawing more and more attention all over the world.Shale and tight-sand reservoirs are important unconventional resources.The pores in shale and tight-sand rocks are very small,mostly in nanoscale.Oil and gas flow mechanisms in such confined pores are different with those in conventional reservoir rocks.Pore-scale numerical simulation method is an effective way to explore the oil and gas flow mechanisms in shale and tight-sand reservoirs.The lattice Boltzmann method and digital rock technology are adopted to investigate the micro-and nano-scale oil and gas flow mechanisms in shale and tight-sand rocks in this thesis.First,a lattice Boltzmann model for shale and tight-sand gas flow simulation is proposed by introducing the microscale effect,the effects of Knudsen layer and high pressure.The effects of pressure,temperature and pore size on microscale gas flow are studied based on this model and their influencing mechanisms are analyzed in detail.Second,by introducing the regularization procedure and modifying the diffuse reflection boundary condition,the above lattice Boltzmann model is used to simulate gas flow in real tight rocks.It is found with the increase of Knudsen number,the effect of heterogeneity of the pore structure on gas velocity distribution decreases and the gas flow velocity in the tight porous media becomes more even.In addition,a modified apparent permeability calculation model considering the entrance/end effect for shale and tight-sand gas reservoirs is proposed based on the simulation results.Third,the adsorption effect of organic matter is incorporated into above lattice Boltzmann model by introducing the interaction force between the gas particles and solid walls.In addition,the magnitude of the interaction force is determined by molecular simulation.The effect of adsorption/desorption on shale gas production is studied based on this model and it is found adsorption/desorption can either increase or decrease shale gas production,depending on the excess adsorption curve and the initial and finial pressures.Fourth,a lattice Boltzmann model for nanoscale liquid flow simulation is proposed by introducing the effective viscosity and slip length into the model,which are obtained by molecular dynamics simulations.Liquid flow mechanisms in nanoscale porous media are investigated based on this model.The results show that the entrance/end effect can greatly influence nanoscale liquid flow,especially when the fluid-solid interaction force is very small.Finally,the color-gradient lattice Boltzmann model is adopted to simulate oil-water two-phase flow in fractionally wet porous media and the effect of wettability heterogeneity on relative permeability of two-phase flow in porous media is analyzed in detail.Additional flow resistance for two-phase flow in fractionally wet porous media under moderate fluid saturation is observed and it is caused the wettability related microscale fluid distribution.The quantatitive relationship between relative permeability and microscale fluid distribution is explored based on the simulation results.Summing up,the oil and gas flow mechanisms in shale and tight-sand rocks are investigated based on lattice Boltzmann method and digital rock technology in this thesis.The simulation results can provide the theoretical basis for effective development of shale and tight-sand oil and gas reservoirs.