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.
Due to the nanometer scale pore size and extremely low permeability of a shale matrix,traditional Darcy＇s law can not exactly describe the combined gas transport mechanisms of viscous flow and Knudsen diffusion.Three transport models modified by the Darcy equation with apparent permeability are used to describe the combined gas transport mechanisms in ultra-tight porous media,the result shows that Knudsen diffusion has a great impact on the gas transport and Darcy＇s law cannot be used in a shale matrix with a pore diameter less than 1 μm.A single porosity model and a double porosity model with consideration of the combined gas transport mechanisms are developed to evaluate the influence of gas transport mechanisms and fracture parameters respectively on shale gas production.The numerical results show that the gas production predicted by Darcy＇s law is lower than that predicted with consideration of Knudsen diffusion and the tighter the shale matrix,the greater difference of the gas production estimates.In addition,the numerical simulation results indicate that shale fractures have a great impact on shale gas production.Shale gas cannot be produced economically without fractures.
Yao JunSun HaiFan Dong-yanWang Chen-chenSun Zhi-xue
