弹塑性储层水力压裂裂缝扩展研究

Study on Hydraulic Fracture Propagation in Elastoplastic Reservoir

随着水力压裂技术在非常规油气资源开发中的大量应用,越来越多的学者发现传统的线弹性断裂力学不能对弹塑性储层中水力裂缝扩展进行准确预测。鉴于此,基于Mohr-Coulomb塑性理论,在考虑储层岩石塑性变形、压裂液流动与水力裂缝扩展非线性耦合的基础上,通过插入Cohesive单元的方法,建立三维多孔介质弹塑性土体模型,以此来研究弹塑性地层不同工况下水力压裂裂缝扩展规律。研究结果表明:水力裂缝在扩展过程中会在尖端产生明显的塑性应变,阻碍裂缝扩展;地层的抗拉强度越大,裂缝尖端的塑性应变越大,岩石的破裂压力越高,裂缝呈现出"短宽缝"的形态;增大压裂液的排量,会明显改变水力裂缝的形态,容易形成"长宽缝",与此同时会使裂缝的扩展压力显著升高,给压裂作业带来困难。研究成果可为该地区现场压裂施工提供参考。

With the extensive application of hydraulic fracturing technology in the development of unconventional oil and gas resources,more and more scholars have found that the traditional linear elastic fracture mechanics cannot accurately predict the propagation of hydraulic fractures in elastoplastic reservoirs.Therefore,in this paper,based on the Mohr-Coulomb plastic theory,after having considered the nonlinear coupling of plastic deformation of reservoir rock,fracturing fluid flow and hydraulic fracture propagation,by means of inserting the Cohesive element,a 3 D porous medium elastoplastic soil model was built to study the propagation law of hydraulic fractures in elastoplastic strata under different working conditions.The study results show that the hydraulic fracture produces obvious plastic strain at the tip in the propagation process,which hinders the fracture propagation.The greater the tensile strength of the formation,the greater the plastic strain at the fracture tip,the higher the breakdown pressure of the rock,and the fracture presents a“short and wide fracture”form;increasing the flow rate of fracturing fluid can obviously change the shape of hydraulic fractures,but at the same time,it can obviously increase the propagation pressure of fractures,which brings difficulties to fracturing operation.The study results provide reference for fracturing operation in the region.