水力压裂停泵泄压条件下页岩水力裂缝网络闭合理论模型研究

Multi-stage fracturing in the horizontal well is a key technology of shale gas development. However, after the pumping is stopped during the process of hydraulic fracturing, the hydraulic fracture network (HFN) will close at some degree that directly affects the conductivity and shale gas production. Thus, a study on the closure behavior of HFN in the shale gas reservoir is extremely important. This project will consider the expansion effect of shale hydration and deploy the deformation equation of HFN and the flow equation of fracturing fluid, aiming at building the closure equations for the HFN under the pressure of the fracturing fluid. This project will consider the proppant migration inside the HFN and propose the proppant reflux equation after the pump’s shutoff. The closure equation of HFN under the action of proppant is established after the tri-axial hydraulic fracturing tests and closure tests of propped fracture on the shale outcrops. Coupling these equations creates a closure model of HFN after the pump’s shutoff. The closure law of HFN in the shale gas reservoir will be revealed. It will provide the theoretical basis for the proppant optimization and fracturing effect prediction in the shale gas development.

水平井分段压裂技术是页岩气开发的关键技术。然而，在水力压裂停泵泄压后，水力裂缝网络会发生一定程度的闭合，这直接影响了导流能力和页岩气产能。因此，针对页岩气储层水力裂缝网络闭合行为的研究至关重要。本项目拟考虑页岩水化膨胀效应，根据复杂裂缝网络的变形方程和压裂液流动方程，建立水力裂缝网络在压裂液作用下的闭合力学方程；考虑支撑剂在复杂裂缝网络中的运移过程，建立支撑剂在停泵泄压后的回流方程；开展页岩露头真三轴水力压裂试验和支撑缝闭合试验，建立水力裂缝网络在支撑剂作用下的闭合力学方程；采用牛顿迭代法，将上述方程组耦合以建立停泵泄压后水力裂缝网络闭合力学模型，揭示页岩气储层水力裂缝网络的闭合规律。本模型可为页岩气开发中支撑剂优选和压裂效果预测提供理论依据。

页岩气储层具有低孔低渗特征，且储层中通常发育有复杂的天然裂缝系统。水平井钻井技术和水力压裂技术是开发页岩气储层的两大核心技术。通过水平井水力压裂技术在储层中形成复杂的裂缝网络，是增大页岩气储层渗透性的关键所在。本项目采用边界元法，构建了流固耦合的水力压裂力学模型，模拟了水平井分段压裂时水力裂缝的空间形态，揭示了水平井中多条水力裂缝同步扩展过程时裂缝竞争规律，解释了多条裂缝之间的应力干扰作用；研究了近井壁水力裂缝起裂特征和裂缝弯曲特征，揭示了含裂缝的井壁水力裂缝扩展形态，以及裂缝形态与近井壁裂缝关键参数、压裂水力参数之间的关系；针对复杂的天然裂缝系统和弯曲的水力裂缝形态，构建了I/II复合型水力裂缝穿透天然裂缝的判别准则，并得到了真三轴水力压裂实验的验证，采用该判别准则和水力压裂力学模型，模拟了复杂天然裂缝性储层中水平井水力压裂裂缝扩展形态及其与地应力、净压力之间的关系；在完成水力压裂裂缝扩展模拟之后，根据支撑剂的压缩变形和喷嘴尺寸条件，构建了储层改造压裂液返排力学模型，模拟了压降曲线、返排速率、喷嘴尺寸、支撑剂沉降、返排时间等多个因素之间的关系。本项目的研究结果有助于深入认识页岩气储层水力压裂裂缝空间形态和压裂作业之后的压裂液返排规律，有助于提升水力压裂技术，从而提高页岩气储层改造效率。