考虑层理效应的页岩压裂模拟全耦合黏结单元法

Modeling Shale with Consideration of Bedding Plane by Hydraulic-mechanical Coupled Cohesive Finite Element Method

页岩水力压裂技术是目前最常用的储层改造技术。页岩中存在具有方向性的不连续结构面,使其具有较为明显的层理效应,主要体现为页岩的横观各向同性。为了在水力压裂中反映层理效应,在考虑层理效应的黏结单元方法(cohesive finite element method,CFEM)基础上,考虑力学场和渗流场的相互作用,推导界面单元全流固耦合方程。最后,通过水力压裂试验结果与模拟计算结果的对比,展现在不同地应力条件下页岩层理性对水力裂纹扩展的影响作用。在应用该方法时可以根据给定层理方向,模型自动将与层理平行(或近于平行)的黏结面转化为层面界面,而且在整个模拟过程中,不需要引入额外断裂准则,也不需要网格修正,为页岩水压致裂问题的数值模拟提供了新途径。

Shale hydraulic fracturing is the main technology in unconventional resource rupture.Shale contains distributed directional bedding planes,which make the shale transverse isotropic.This bedding effect has considerable influence on the hydraulic fracture propagation.To simulate the hydraulic fracture in shale with consideration of bedding effect,the fully hydro-mechanical coupled equation of interface element was derived for the bedding plane-embedded cohesive finite element method(CFEM).With this CFEM method,the hydraulic fracturing process of shale was simulated.It was shown that the simulated results well agreed with the experimental ones.The results show that the bedding planes have significant influence on the fracture trajectory.When using the method,the cohesive elements that paralleling or almost paralleling to the bedding plane are automatically transformed into bedding elements.Moreover,no external fracture criterion is needed and no mesh modification is involved.So,the method provides an alternative approach to hydraulic fracture simulation in shale.