模拟水压致裂的另一种二维FDEM-flow方法

A new two-dimensional FDEM-flow method for simulating hydraulic fracturing

可考虑岩石本身渗透性的FDEM-flow方法充分利用原FEMDEM中节理单元和三角形单元独特的拓扑连接关系,将节理单元作为流体流动的天然通道,基于立方定律来表征流体在岩石本身及裂隙中的流动,而岩石本身渗透性则通过标定合适节理单元张开度来表征。用一个含解析解自由面稳态渗流算例初步验证了本文渗流算法及标定岩石本身渗透率方法的正确性。最后计算了一个水力压裂算例,验证了该FDEM-flow方法可同时考虑岩石本身的渗透性和裂隙的渗透性,用简单的纯裂隙渗流完成了对复杂问题的处理。研究结果表明:修改后的FDEM-flow方法能够很好地再现压裂过程中流体压力的分布及裂隙的扩展,可简洁地求解水力压裂这一复杂力学的问题,适用范围更广。为模拟页岩气开采、干热岩地热获取中的水力压裂问题提供新的求解工具。

In this study, a new FDEM-flow method is developed by considering the permeability of rock matrix, and it fully utilises the unique topological connection between joint elements and triangular elements in the original combined finite-discrete element method （FDEM）. First, the joint elements were used as a natural channel for fluid flow, and the fluid flow in rock matrix and fissures were characterized by fluid flow in joint elements based on the cubic law. While the permeability of rock matrix was determined by calibrating the appropriate aperture of the joint element. Then, a numerical example of the free surface steady flow with the analytical solution was employed to validate the proposed seepage algorithm and the calibration method for rock matrix permeability. Finally, a hydraulic fracturing example was conducted to demonstrate that the permeability of rock matrix and fracture was considered simultaneously in the modified FDEM-flow method. Moreover, by using simple pure fracture seepage this method solved the complex issues of hydraulic fracturing. Therefore, the application scope of the new FDEM-flow method is broader than that of the original FDEM-flow method. This method provides a new solve tool for simulating hydraulic fracturing in shale gas exploration and geothermal exploration in hot dry rock.