针对页岩压裂过程中,压裂液返排效率普遍很低的现象,基于高分辨率页岩岩样SEM(scanning electron microscope)电子显微镜扫描图像,利用马尔科夫链蒙特卡罗(Markov Chain Monte Carlo)方法重构得到页岩的三维数字岩心,采用具有高密度比的格子Boltzmann模型,从孔隙尺度来模拟页岩中气水两相驱替过程.首先通过计算表面张力和相对渗透率来验证模型的准确性,然后模拟页岩数字岩心中的油水两相流动,页岩中首先饱和水(气)相,然后从一端注入气(水)相,模拟终止条件为驱替相在出口端发生突破,气水运动黏度比和密度比分别设置为10:1和1:1000.水驱气过程中发生突破时,水相的饱和度为70%,而气驱水过程中发生突破时,气相的饱和度只有4.5%,给出了三维数字岩心中驱替相分布,在气驱水的过程中发生突破时,大部分水被滞留在页岩孔隙中,从而解释了页岩水力压裂中,压裂液返排效率低于10%的现象.
Within the low recovery of fracturing fluid during hydraulic fracturing in shale, the displacement between water and gas is simulated in pore-scale. The lattice Boltzmann model with high-density ratio is employed, and the geometry is three-dimensional shale digital core,which is reconstructed by Markov Chain Monte Carlo method with a high resolution SEM scanning image as input. The model is verified by calculating the surface tension and relative permeability. Water(gas) is saturated at first, and then, gas(water) is injected at inlet. The simulation is terminated when breakthrough of displacement phase occurs at outlet. The kinetic viscosity ratio and density ratio of gas and water are set to 10:1 and 1:1000, respectively. The water saturation is 70% at breakthrough when water displaces gas,while the gas saturation is only 4.5% at breakthrough when gas is advancing phase. The distribution of displacement phase is given in three-dimensional digital core. Most water is trapped in the pore at breakthrough, when water is displaced by gas. Hence, the phenomenon of recovery of fracturing fluid is less than 10% in shale is explained.