基于格子Boltzmann方法的页岩纳米多孔介质流体流动模拟

Simulation on liquid flow in shale nanoporous media based on lattice Boltzmann method

纳米多孔介质中受限流体流动机制对提高页岩油采收率、水净化等科学和工程应用至关重要。以理论方程和分子动力学模拟(MDS)结果为基础,基于格子Boltzmann方法(LBM),建立了局部表观黏度-LBM(LAV-LBM)孔隙尺度模拟模型来研究纳米多孔介质中流体流动机制,该模型通过局部表观黏度和密度分布将纳米尺度效应(滑移边界、非均质黏度/密度)代入LBM,使LBM模拟过程中无法准确表达的复杂滑移边界条件可以退化成简单易实现的无滑移边界。通过理论方程对比验证、分子模拟-LAV-LBM密度分布验证、理论方程-LAV-LBM速度分布验证结果表明,基于LAV-LBM模型可以有效模拟纳米多孔介质中流体流动规律。基于该模型,研究了纳米尺度效应、多孔介质尺寸、表面润湿性对表观渗透率和增强系数的影响规律。研究结果表明:当接触角约小于70°时,由于高/低近壁面相黏度、低/高滑移速度导致表观渗透率和增强系数减小;表观渗透率和增强系数随着接触角增大而增大,原因是边界滑移速度逐渐增大,近壁面水相黏度逐渐减小;非均质密度导致相同时间内,近壁面相流体体积流量与体相区域不同,非均质密度越强,对流动能力影响越大;孔径越大,纳米尺度效应越小,增强系数趋近于1。通过实际应用研究了如何通过分子模拟结果和LAV-LBM模型将单孔隙纳米尺度流动扩展到多孔介质纳米尺度流动,为MDS到LBM的尺度升级提供了研究思路和基础模型。

To clarify the flow mechanism of confined liquids in nanoporous media is crucial for the scientific and engineering applications such as enhanced shale oil recovery and water purification.In this study,based on molecular dynamic simulation(MDS)results,theoretical equation and the lattice Boltzmann method(LBM),a local apparent viscosity-LBM(LAV-LBM)pore-scale simulation model was established to study the flow mechanism of liquids in nanoporous media.The model can incorporate the nanoscale effects(slip boundary,and heterogeneous viscosity/density)into LBM through local apparent viscosity and density distribution,and thus the complex slip boundary that cannot be accurately expressed in LBM simulation process can degrade into a simple non-slip boundary easy to be implemented.Based on the results of theoretical equation comparison verification,molecular simulation-LAV-LBM density distribution verification,and theoretical equation-LAV-LBM velocity distribution verification,it is indicated that the LAV-LBM model can help effectively simulate the flow law of liquids in nanoporous media.Based on the LAV-LBM model,the paper explores the influence law of nanoscale effect,porous media size and surface wettability on the apparent permeability and enhancement factor.The results show that when the contact angle is smaller/greater than 70o,the large/small near-wall viscosity and small/large slip velocity result in a decrease/increase of apparent permeability and enhancement factor;the apparent permeability and enhancement factor is increased with the increase of contact angle,which is as a result of the increasing slip velocity and decreasing near-wall viscosity;due to the heterogeneous density,the volume flow rate of liquids in the near-wall phase area is different from that in the bulk phase area within the same time period;the stronger the heterogeneous density is,the greater the influence on the flow capacity is;the larger the pore size is,the smaller the nanoscale effect is,and the closer the enhancement factor approaches 1.Finally,based on practical application,the paper introduces the method for extending the single-pore nanoscale flow to the porous media nanoscale flow based on the results of molecular simulation and the LAV-LBM model,providing a new research idea and basic model for the scale upgrade from MDS to LBM.