基于数字岩心和LBM的页岩气流固耦合数值模拟

The numerical simulation of the shale gas fluid-structure interaction based on the digital rock and LBM

页岩具有很强的压力敏感性,围压和孔压的变化会改变页岩孔隙的大小,从而对页岩气的流动规律产生影响,利用数字岩心结合格子Boltzmann方法(lattice Boltzmann method,LBM)来研究页岩气微观渗流规律得到越来越多学者的重视.本文建立了应力条件下的数字岩心应力应变模型和页岩气渗流LBM模型,研究了应力对页岩气渗流的影响规律.研究结果表明:有机质中的纳米孔隙对应力更加敏感,随应力变化的程度相比矿物骨架孔隙更大,从而影响页岩气在纳米孔隙中的解吸和扩散;孔压对各渗流机理的影响要比围压的影响大,是因为孔压的变化不仅影响了孔隙的尺寸还影响了气体的平均分子自由程;当平均孔压从17 MPa降低到5 MPa时,解吸的气体量和通过扩散流动的气体量占总气体流量的比例不断增加,分别增加了2%和1.9%,而通过滑脱流动的气体量占总气体流量的比例不断减少,减少了3.8%.利用应力条件下的数字岩心和格子Boltzmann方法可以更精确地模拟页岩气在储层中的流动规律,更好地理解页岩气的产出机理.

The shale has strong sensitivity to pressure, so the changing of confining pressure and pore pressure will affect the shale pore size, and have an influence on the flow of shale gas. Using digital rock and the lattice Boltzmann method to study the microscopic seepage mechanism of shale gas obtained more and more attention of scholars. The stress-strain model of digital rock under stress condition and the shale seepage LBM model were established to study the influence of stress on shale gas seepage. The results show that the nano-pores in the organic matter are more sensitive to the stress, and its variation is bigger than mineral matrix pores with the changing of stress, as a result the diffusion and desorption of shale gas in the nano-pores will be affected; The influence of pore pressure on seepage flow mechanism is bigger than the confining pressure. It＇s because the change of pore pressure can affect the pore size and the average free path of molecular movement; when the average pore pressure decreases from 17 to 5 MPa, the proportion of gas obtained by desorption and diffusion effect of the total gas amount increase, 2% and 1.9% respectively, while the proportion of gas obtained by slippage effect decrease 3.8%. Using the digital core under stress condition, and the lattice Boltzmann method can simulate the shale gas flow in the reservoir more accurately and understand the production mechanism of shale gas better.