重力条件下粗糙裂隙溶蚀过程的可视化试验研究

Visualization experimental investigation into the dissolution processes in rough fracture under gravity conditions

岩体裂隙的溶蚀现象广泛存在于自然过程和工程实践,重力对溶蚀过程具有重要作用。可视化观测技术是研究粗糙裂隙溶蚀机理的关键手段,然而传统的可视化技术存在裂隙粗糙壁面难以复制、溶蚀过程难以实时观测等问题。通过自主研发的粗糙裂隙溶蚀过程可视化试验平台,开展了垂直裂隙和水平裂隙在4种流量条件(0.05,0.1,0.3,1 mL/min)下的溶蚀可视化实验,研究了重力效应对溶蚀模式和溶蚀形态的影响,采用分形维数量化了不同溶蚀模式的形态学特征,最终确定了不同佩克莱数(Pe)条件下的突破时注液量。试验结果表明:在Pe≤62.1范围内,重力效应对溶蚀模式具有重要影响,垂直裂隙中的溶蚀发育为浮力主导模式和通道模式,重力效应将诱发单一、集中的溶蚀通道;而水平裂隙则统一发育为开度演变均匀、宽度较大的通道,即经典的虫洞溶蚀模式;在Pe数较大时(Pe=207.0)时,垂直裂隙和水平裂隙中的溶蚀均发育为均匀溶蚀。试验结果还证实了垂直裂隙更易发育为贯通的溶蚀通道,从而加速溶蚀突破;Pe=20.7时为最优注入条件,垂直裂隙的突破时注液量最小。在此条件下,垂直裂隙的突破时注液量仅为水平裂隙的1/4。建议重点关注重力效应对溶蚀过程的影响,研究结果对CO_(2)地质封存等工程实践具有重要意义。

Dissolution of rock fractures is common in natural processes and engineering practices, in which gravity plays an important role. Visual observation technology is a key means to study the mechanism of dissolution in rough fractures. However, the traditional visualization technologies have some problems, such as difficult to reproduce the rough wall of fractures and difficult to observe the dissolution process in real time. In this work, a flow-visualization system for dissolution processes in rough fractures is developed, on which flow-through experiments are conducted on four flow rates(0.05, 0.1, 0.3, and 1 mL/min) for vertical and horizontal fractures,and the gravity effect is evaluated on dissolution patterns and dissolution morphologies, which are characterized by fractal dimension and other morphological parameters. The pore volumes at breakthrough are calculated for each Peclet number(Pe). The experimental results show that the gravity effect significantly influences the dissolution patterns for Pe≤62.1. The dissolution morphologies exhibit buoyancy-dominated patterns and channeling patterns for vertical fractures, and the gravity effect will induce a single and concentrated channel. For horizontal fractures, the dissolution forms dissolution channels with relatively uniform aperture evolution and large width, which are classical wormhole patterns. When the Peclet number is large(Pe=207.0), dissolution in both the vertical and horizontal fractures develops into uniform dissolution. The experimental results also confirm that the dissolution in vertical fractures is more likely to develop channels through the inlet to the outlet to accelerate the breakthrough;Pe=20.7 is the optimal injection condition, which means that the amount of injected liquid is the minimum when breakthrough takes place in vertical fractures. Under this condition, the pore volumes at breakthrough in vertical fractures are only 1/4 of the horizontal fractures. The results in this paper are of great significance to engineering practice such as CO_(2)geological storage, indicating that the effect of gravity on the dissolution process should be greatly taken into account.