基于零厚度内聚力单元的水力裂缝随机扩展方法研究

Research on random propagation method of hydraulic fracture based on zero-thickness cohesive element

为有效模拟裂缝性页岩储层中水力裂缝随机扩展过程,基于单元节点的拓扑数据结构,利用网格节点分裂方式,建立了一种基于有限元网格嵌入零厚度内聚力单元的水力裂缝随机扩展新方法。利用KGD模型解析解和2种室内试验,验证了新方法的准确性和有效性。同时,通过数值算例研究了水平地应力差和储层非均质性对水力裂缝随机扩展过程的影响。研究表明:(1)该方法弥补了ABAQUS平台内置的内聚力单元无法有效模拟水力裂缝随机扩展的不足;(2)在较高水平地应力差下页岩储层非均质性越强,与水力裂缝相交的高角度天然裂缝越容易开启。所建方法能准确地描述复杂水力裂缝的随机扩展行为,可为裂缝性页岩储层的数值模拟提供新手段。

In order to effectively simulate the process of random propagation of hydraulic fractures in fractured shale reservoirs, a new method of random propagation of hydraulic fractures based on the finite element mesh embedded with zero-thickness cohesive elements is proposed. This new method is based on the topological data structure of element nodes and the splitting mode of mesh nodes. The accuracy and effectiveness of the random propagation method are verified by comparing with the analytical solution of KGD model and two kinds of laboratory experiments. Meanwhile, the influences of horizontal in-situ stress difference and reservoir heterogeneity on the process of random propagation of hydraulic fractures are evaluated by running numerical examples. The results show that:(1) the new method makes up for the deficiency that the cohesive element built-in ABAQUS platform can not effectively simulate the random propagation of hydraulic fractures;(2) under a higher horizontal in-situ stress difference condition, the stronger the heterogeneity of a shale reservoir is, the easier it is to reopen a high-angle natural fracture intersecting with hydraulic fractures. The proposed method can accurately describe the random propagation behavior of complex hydraulic fractures, and thus provide a novel means for numerical simulation of naturally fractured shale reservoirs.