基于颗粒流流-固耦合改进算法的岩石渗透性演化数值模拟研究

摘要

岩石渗透性的意义重大,构造应力和人类工程活动会对渗透性产生巨大影响.岩石内部孔压与流速分布情况的全面监测对阐明渗透性的演化机制至关重要,其在试验中难以实施,但在数值模拟中可容易实现.因此,本文采用一种离散元方法—颗粒流程序(Particle Flow Code,简称PFC)开展岩石材料渗透性行为的模拟研究.针对PFC中原流-固耦合算法的不足,对其进行改进,使改进后的算法更能体现流体在岩石裂隙中的流动优势.分别采用原算法与改进算法对三轴压缩过程中的渗透性演化进行数值模拟,对比结果表明改进算法能更好地反映试验现象.本文利用改进流-固耦合算法,进一步分析了渗流过程中的孔压和流速分布的演化情况.结果表明,孔压和流速都优先通过裂隙传递而非岩石基质.根据运移流体的能力将裂隙划分为三类: I)贯穿进口端和出口端的裂隙;II)仅与进口端连接的裂隙;III)仅与出口端连接的裂隙.I)类裂隙始终是最主要的流速和孔压的传递通道.II )类裂隙中孔压首先增大并逐渐趋于稳定,而流速首先增大,当流体运移到裂隙终端后流速减小甚至降低为零.在III)类裂隙中,无明显的流体运移或孔压集中.%Permeability is a vital property of rock mass, which is highly affected by tectonic stress and human engineering activities. A comprehensive monitoring of pore pressure and flow rate distributions inside the rock mass is very important to elucidate the permeability evolution mechanisms, which is difficult to realize in laboratory, but easy to be achieved in numerical simulations. Therefore, the particle flow code (PFC), a discrete element method, is used to simulate permeability behaviors of rock materials in this study. Owe to the limitation of the existed solid-fluid coupling algorithm in PFC, an improved flow-coupling algorithm is presented to better reflect the preferential flow in rock fractures. The comparative analysis is conducted between original and improved algorithm when simulating rock permeability evolution during triaxial compression, showing that the improved algorithm can better describe the experimental phenomenon. Furthermore, the evolution of pore pressure and flow rate distribution during the flow process are analyzed by using the improved algorithm. It is concluded that during the steady flow process in the fractured specimen, the pore pressure and flow rate both prefer transmitting through the fractures rather than rock matrix. Based on the results, fractures are divided into the following three types: I) fractures link to both the inlet and outlet, II) fractures only link to the inlet, and III) fractures only link to the outlet. The type I fracture is always the preferential propagating path for both the pore pressure and flow rate. For type II fractures, the pore pressure increases and then becomes steady. However, the flow rate increases first and begins to decrease after the flow reaches the stop end of the fracture and finally vanishes. There is no obvious pore pressure or flow rate concentration within type III fractures.