基于FEM-DFN的页岩气储层水力压裂复杂裂缝交错扩展模型

A FEM-DFN-based complex fracture staggered propagation model for hydralic fracturing of shale gas reservoirs

页岩气藏压裂过程中,常遇到因施工压力超限储层压不开、近井地带裂缝形态复杂而引起砂堵的恶性事故,如何有效模拟复杂裂缝的起裂与交错扩展形态是页岩气体积压裂优化设计的关键。为此,基于有限元—离散裂缝网络(FEM-DFN),考虑水力裂缝扩展的非线性损伤破裂力学行为与裂缝面摩擦效应,改进了拉伸—剪切复合破裂模式的本构方程,建立了基于显式时间积分的页岩气储层水力压裂复杂裂缝交错扩展的多场耦合模型,并在实验室和现场微地震监测中进行了验证。研究结果表明:①水力裂缝与天然裂缝交错机制主要为2类,即被天然裂缝诱导并沿天然裂缝扩展、直接穿透天然裂缝,地质与工程因素对交错机制的转变存在阈值,当排量超过26 m^(3)/min时,或压裂液黏度超过40mPa·s时,或应力差超过18MPa时,或裂缝面摩擦系数超过0.8时,水力裂缝将直接穿透天然裂缝;②天然裂缝对水力裂缝的诱导作用随着两者夹角的变化而变化,在夹角较小的情况下,水力裂缝容易在最大水平主应力方向交汇点之前提前被天然裂缝诱导,而当夹角增大时则在交汇点处进入天然裂缝扩展,且表现为一侧拉张裂缝一侧剪切裂缝;③相邻两簇水力裂缝容易在天然裂缝的诱导作用下相互吸引,进而交汇成优势主裂缝。结论认为,天然裂缝的分布状态是决定压裂裂缝复杂度的关键性因素,研究成果可为中深层页岩气压裂裂缝分析和施工参数优化提供理论与技术支撑。

Some serious problems, such as failure of reservoir fracturing due to over-limit operation pressure, and sand plugging caused by complex morphology of fractures near wellbore, often occur during fracturing of shale gas reservoirs. Effective simulation of complex hydraulic fracture initiation and propagation is critical to the design optimization of volume fracturing in shale gas reservoirs. Based on the finite element method and discrete fracture network(FEM-DFN), and considering the mechanical behaviors of nonlinear damages and the frictional effect of fracture surface during hydraulic fracture propagation, a multi-field coupling model of complex fracture staggered propagation during hydraulic fracturing of shale gas reservoirs was built with explicit time integration(ETI) and by modifying the constitutive equation for tension–shear fractures. The model was verified by laboratory test and field microseismic monitoring. The results show that hydraulic fractures(HFs) interact with natural fractures(NFs) mainly in two mechanisms: HFs are induced by and propagate along NFs, and HFs cross NFs. Geological and engineering factors correspond to thresholds for the change of HF-NF interaction mechanism.When the displacement is greater than 26 m^(3)/min, or the viscosity of fracturing fluid is higher than 40 mPa·s, the stress difference is more than 18 MPa, and the fracture surface friction coefficient exceeds 0.8, HFs directly cross NFs. The induction of NFs to HFs changes with the NF-HF angle. Given a small angle, HFs may easily be induced by NFs before they intersect in the direction of the maximum principal horizontal stress. Given a large angle, HFs begin to propagate together with NFs from the intersection, and present as tensile fractures on one side and as shear fractures on the other side. HFs in two neighboring clusters may inter-attract due to the induction of NFs and then converge to become dominant fractures. It is concluded that NF distribution is a key factor controlling the complexity of HFs. The study results provide theoretical and technical support for the fracture analysis and parameter optimization of fracturing of mid-deep shale gas reservoirs.