页岩储层黏土矿物水化膨胀致裂数值模拟研究

Research on numerical simulation of hydration swelling and cracking of clay minerals in shale reservoir

为了深入研究内应力场的变化对诱导微裂缝的影响,针对黏土矿物水化膨胀这一问题,运用复合材料理论的等应力和等应变理论估算黏土矿物和非黏土矿物的力学参数,采用数值模拟的方法分析黏土矿物吸水膨胀后诱导微裂缝的起裂和扩展规律。结果表明1)内应力最大处产生在黏土矿物与非黏土矿物的界面处,最大Mises应力可达40.86 MPa,并随着与黏土矿物中心距离的增加而减小;2)2个黏土矿物膨胀诱导对称分布的内应力场,黏土矿物膨胀挤压导致其与非黏土矿物界面处的应力放大,当黏土矿物之间的距离由2d增加到4d时,最大Mises应力由47.45 MPa减小到40.81 MPa;3)黏土矿物水化膨胀对压后裂缝网络的形态具有显著的影响,黏土矿物间距离越近,黏土矿物和非黏土矿物界面处的应力越大,越容易形成复杂缝网。当黏土矿物间距大于4d后,这种放大效应消失;4)页岩孔隙中的水向前推进形成含水量梯度,导致微裂缝沿自吸端面不断向前扩展。为页岩储层水力压裂的优化及压裂液的返排措施的制定提供了理论基础,促进我国页岩气压裂水平的进步。

In order to further study the effect of the change of internal stress field on induced microfractures,to solve the problem of hydration and expansion of clay minerals,the mechanical parameters of clay minerals and non-clay minerals are estimated by using the equal stress and equal strain theory of composite material theory.Numerical simulation was used to analyze the initiation and propagation of micro-cracks induced by clay minerals after water absorption and expansion.The results show that:1)The maximum Mises stress occurs at the interface between clay minerals and non-clay minerals,and decreases with the increase of distance from the center of clay minerals.The maximum Mises stress is 40.86 MPa.2)The internal stress field produced by the two clay minerals is symmetrically distributed,stress amplification at the interface between clay minerals and non-clay minerals due to expansion compressing each other.The maximum Mises stress decreases from 47.45 MPa to 40.81 MPa when the distance between clay minerals increases from 2d to 4d.3)The hydration and expansion of clay minerals have a significant effect on the morphology of the post-compression fracture network.The closer the distance between clay minerals,the greater the stress at the interface between clay minerals and non-clay minerals,and the easier it is to form a complex fracture network.When the clay mineral spacing is longer than 4d,the amplification effect disappears.4)Water in shale pores advances forward to form a water content gradient,leading to continuous expansion of micro-fractures along the self-imbibition face.It provides a theoretical basis for the optimization of shale reservoir hydraulic fracturing and the formulation of fracturing fluid flowback measures,and promotes the progress of shale gas fracturing level in China.