水岩作用下裂隙面力学性质、几何形态及水力渗透特征的演变机制研究

Fracture flow tends to congregate toward a few preferential flow channels when the average fracture aperture decreases. Preferential flow channels collect most of the flowing fluid and contribute most to the fracture hydraulic conductivity. Meanwhile, due to water-rock interactions, rock surfaces lose their strengths and led to compaction, which results in decreases in hydraulic conductivity due to less flowing spaces. On the other hand, the fluid is always trying to wash out the rock debris, and it has a greater chance of success because the rock is no longer of the original strength under compaction. Thus, the competition between rock expansion and fluid erosion would lead preferential channels and their conductivity to two totally opposite directions: channels grow wider and conductivity gets higher if the fluid succeeds in washing out the debris and enhances the erosion; or channels get narrower or even disappeared and conductivity becomes lower if otherwise. This mechanism for fracture evolution and its conductivity variation is extremely important for geological and morphological evolution, groundwater and contaminant transport, reservoir hydraulic fracturing, and stability of engineering structures, among other things. ..This study will focus on the variation of fracture conductivity, utilize the widely distributed red-bed soft rock as the object of study. We will bring in the conceptual ideas of morphology and sediment sciences, focus on the morphological evolution and variation of rock strength and conductivity, employ such methodologies as spiral CT scanning and local mini-invasive hardness test to develop mathematical and numerical models for channel conductivity variation due to channel geometric evolution, select new statistics to quantify more detailed distributed characteristics of fluid flow, improve Local Cubic Law based on analysis of Navier Stokes equation, build up corrected numerical model for fracture flow, and conduct sensitivity analysis between flow characteristics and geometric parameters to quantitatively evaluate the evolution of fracture conductivity and its geometric sensitivity.

岩体破裂时极易发生横向错动，使裂隙宽度不规则分布。裂隙宽度减小时流动逐渐汇聚于若干优先沟槽通道，并在很大程度上决定了裂隙渗透性的大小。水岩作用中，裂隙面软化膨胀，使裂隙接触面积增大而通道空间减小；同时流体不断冲蚀，可使通道空间增大。若冲蚀强烈则渗透性加强，裂隙不断发育；反之则渗透性减弱，裂隙闭合甚至消失。可称此为裂隙发育和渗透性变化的分岔现象，其过程由裂隙力学性质、几何形态、水力作用共同决定。认识和控制渗透性变化，在地下施工、灾害防治、环境保护等工作中都受到关注。本项目拟以西南典型软岩为例，着眼于力学性质、几何形态和流动特征的空间分布，将几何形变概化为膨胀、应变和冲蚀形变三类，利用表面硬度构建力学性质分布，利用CT扫描和弹性模型描述几何形变分布，同时从NS方程出发，对LCL方法进行改进并建立流动模型，利用敏感性分析研究流动特征与几何形变的关系，探索描述流动特征变化规律和分岔现象的模型方法。

水岩作用中，岩石裂隙的导水性会受到裂隙空间形状及岩石力学性质变化的影响。早在1980年Witherspoon等就发现粗糙裂隙的导水性曲线随着裂隙宽度减小会逐渐偏离立方定律，并且水的作用可能使岩石表面发生软化，并使得裂隙表面弹性和裂隙内部空间形状随之变化，并最终导致裂隙导水性的变化。这个裂隙渗流变化过程是与很多工程应用都有较强相关性的基础科学问题。为探寻以上变化过程中的规律，本研究完成了以下工作：1）岩石软化及力学性质试验。考察水岩作用中，水的化学性质及作用条件对岩石表面弹性的影响，并由此评估岩石软化对裂隙空间形状的影响；2）含沙裂隙流内泥沙沉降分布试验。考察裂隙内泥沙颗粒的运移和沉降对裂隙空间的影响；3）针对粗糙裂隙导水性曲线对立方定律的三阶段偏离现象的模型研究。该研究只需对裂隙空间形状和裂隙流场做比较简单的假设，然后利用统计方法即可对这条经典的试验曲线进行较好地拟合；4）对LCL模型的改进研究。经典的LCL模型对裂隙导水性只应用局部的立方定律，即忽略裂隙空间形状变化和流体惯性项对流场的影响。而本研究从N-S方程出发，把以上二者对裂隙导水性的影响也包括到改进的LCL公式中，获得了更高的准确度；5）裂隙流动的不规则和不均匀性特征研究。重点考察了裂隙流场的曲折性和集中性两大特征，提出了这两大特征各自的量化度量方式，并考察了在裂隙宽度变化过程中以上特征的变化规律，发现以上特征的变化在裂隙平均宽度近似为裂隙表面起伏度时出现拐点；6）粗糙裂隙流及溶质运移的数值模型研究。讨论了裂隙流场的优先沟槽通道对溶质运移过程的影响；7）裂隙流动和岩石软化过程的耦合模型研究。建立了能够模拟水岩作用过程中岩石软化、溶解和沉积过程中，裂隙空间形状变化以及裂隙水流场变化耦合过程的数值模型。这个模型考虑了裂隙水流、矿物溶质运移、岩石弹性软化、矿物化学溶解、矿物压力溶解以及矿物饱和沉降等多种物理化学过程。研究结果经整理共发表论文5篇，其中SCI论文4篇。