Fluid effects on the interaction of waves with rock joints

Abstract

Rock joints are ubiquitous in the Earth's crust, providing main conduits and spaces for flow, transportation, migration, and storage of fluids. The existence of fluids could have important influences on mechanical, hydraulic, thermodynamic, and seismic behaviours of rock joints. Understanding the role of fluids in the interaction of waves with rock joints is of great interest to seismologists, geoscientists, mining engineers and so on. This thesis research aims to determine the fluid effects on the interaction of waves with individual rock joints at the laboratory scale under different test conditions. To this end, an ultrasonic test system in the laboratory was developed and applied for the investigation of the low-intensity wave behaviours across individual rock joints containing fluids. On the other hand, a steel SHPB test configuration was established for the study of the high-intensity stress wave responses of single fluid-filled rock joints. The main contents and results of this thesis research are summarised as follows. Substantial ultrasonic tests were conducted on various water-saturated clay-rich rock joints using the self-designed ultrasonic test configuration, aiming to determine the effects of water on low-intensity wave attributes of individual clay-rich rock joints. The test results show that the water saturation dependent wave responses of single clay-rich rock joints are strongly affected by the wave mode and the clay mineralogical components. Particularly, for P-wave propagation across single rock joints filled with the kaolinite-dominant gouge, the increasing water saturation enhances wave velocity and attenuation while an opposite trend is found as P-wave passing through the joint filled with the bentonite-dominant gouge. On the other hand, for the propagation of S-waves, an approximately nonlinear decrease trend is observed for the wave velocity while a concave trend (down first and then up) is revealed for wave energy attenuation with the water saturation degree regardless of the clay mineralogical components. Moreover, it is observed that, for both P-and S-waves, the wave velocity and attenuation across the clay-rich rock joint increase with the dominant frequency of incident waves. The above-mentioned findings could be interpreted through a combination of the local flow mechanism, the clay hydration and the wave theories relating to individual rock joints. The self-developed ultrasonic test system was adopted to carry out acoustic measurements on single fluid-filled rock joints to investigate low-intensity compressional wave propagation and attenuation through individual fluid-filled rock joints. The test results indicate that wave behaviours across single fluid-filled rock joints are strongly affected by the composition and volume fraction of filling fluids, the joint orientation, and the thermal condition. Specifically, filling water results in faster wave propagation and more wave transmission compared to light oils. In most circumstances, increasing liquid content in a single-liquid filled joint enhances wave velocity and wave energy transmission. For dual-liquid filled rock joints, both wave velocity and wave transmission increase with increasing water content. Regarding the effects of joint orientation, for vertical single-liquid filled joints, wave velocity and wave transmission drastically increase with rising liquid content. By comparison, for horizontal single-liquid filled joints, the liquid content could only dominate wave propagation and attenuation when it exceeds a critical value. With respect to the thermal effect, it is found that a higher temperature results in faster wave velocity and more wave energy transmission through the water-filled rock joint. By contrast, the increasing temperature causes decreases in the wave velocity and wave energy transmission for the light oil-filled joint. The lab-scale results could be attributed to the fluid stiffening and viscous friction of the joint induced by the discrepancies in compressibility and viscosity of various fluids. The self-established SHPB test system was employed to perform dynamic impacting tests on single fluid-filled rock joints, with the aim of understanding the interaction of high-intensity waves with individual fluid-filled rock joints. Based on the laboratory data, the dynamic mechanical properties, energy evolution characteristics and wave responses of rock specimens with single fluid-filled rock joints were analysed and elaborated. The test results reveal that the smaller joint thickness and the larger joint contact area result in the stiffer joint regardless of the fluid filling condition, causing less wave attenuation and reflection. With respect to the role of filling fluid, the increasing filling liquid content leads to an increase in the joint stiffness, reducing wave attenuation across the joint for most of the scenarios considered. More specifically, for rock joints filled with low-viscosity liquids, wave energy transmission monotonically increases with the filling liquid content. For rock joints filled with high-viscosity liquids, wave energy transmission firstly decreases as liquid content increases from 0% to 25% while it increases with liquid content rising from 25% to 100%. The experimental findings could be well explained by the wave-induced fluid flow and fluid stiffening effect. More importantly, the high-speed camera images provide the evidence for the role of the wave-induced fluid flow in the wave attenuation caused by the fluid-filled rock joints. The findings in this thesis may not only shed a light on the fluid effects on the interaction of waves and rock joints at a laboratory scale, but also could serve as a guide to interpret seismic wave phenomena relating to fluid-saturated rock discontinuities encountered in many natural and human-made activities. Moreover, these lab-scale results could make contributions to the validation of the existing theories and provide more insights into the development and improvement of theoretical models in the field of the interplay between stress waves and rock discontinuities.