ALBERT

Description: Fluid-induced seismicity in tectonically inactive regions has been attributed to fluid overpressurization of rock fractures during natural resource extraction and storage. We conducted a series of triaxial shear-flow experiments on sawcut fractures in granite and showed that the fracture responses can be dissimilar under various fluid pressurization conditions. For pressure-controlled fluid pressurization, a uniform fluid pressure distribution can be promoted by lowering pressurization rate and enhancing fracture permeability. However, during volume-controlled fluid pressurization, a high pressurization rate causes a drastic increase in fluid pressure before fracture failure. In this case, our analytical model reveals that the fracture area and normal stiffness also influence fluid pressure variations. The maximum seismic moment predicted by this model is well validated by the experimental data for the cases with low pressurization rates. The discrepancy between the analytical and experimental data increases with higher fluid overpressure ratio owing to the assumption of uniform fluid pressure distribution in the model. The sensitivity analysis demonstrates the importance of fracture size estimation in the maximum seismic moment prediction. Our model can potentially be applied to control the fluid overpressurization of rock fractures and to mitigate the risks of fluid-induced seismicity.

Description: The transitional normal stress of a rock joint subject to shear refers to the critical normal stress under which the normal dilation is completely suppressed. The transitional normal stress is involved in many shear strength/constitutive models of joints as a key material constant; however, this parameter is poorly constrained and its determination is mostly empirical. Here we propose a simple formulation to predict the transitional normal stress based on the systematic, well-calibrated PFC2D (two-dimensional particle flow code) simulation of the shear characteristics of both sawtooth and JRC-profiled rock joints. In the PFC2D modelling, rock joints are confined by low to high normal stresses approximating the uniaxial compressive strength (joint wall compressive strength of fresh, dry and closely matched joints) of the simulated rock. The formulation can satisfactorily quantify the transitional normal stress of regular and irregular joint surfaces as a function of the rock strength and the joint surface roughness, as validated by the laboratory data. Therefore, it can be readily introduced to the shear strength criteria and constitutive models of rock joints, which could significantly promote the accurate quantification of rock joint shear behaviour.

Description: The accurate evaluation of pore pressure and injected volume is crucial for the laboratory characterization of hydromechanical responses of rock fractures. This study reports a series of laboratory experiments to systematically demonstrate the effects of external temperature and dead volume on laboratory measurements of pore pressure and injected volume in a rock fracture. We characterize the hydraulic aperture of the fracture as a function of effective normal stress using the exponential aperture model. This model is then employed to predict the pore pressure change and injected volume in the fracture without the influences of external temperature and dead volume. The external temperature changes in the cyclic loading test due to the Joule-Thompson effect for fluids. The effect of external temperature on pore pressure change in the fracture can be well explained by thermal pressurization of fluids. Our results also show that the external dead volume can significantly lower the pore pressure change in the fracture during the cyclic loading test under undrained conditions. The injected volume can also be substantially enlarged due to the external dead volume in a typical pore pressure system. Internal measurement of the pore pressure in the fracture using a fiber optic sensor cannot exclude the influences of external temperature and dead volume, primarily because of the good hydraulic communication between the fracture and pore pressure system. This study suggests that the effects of external temperature and dead volume on pore pressure response and injected volume should be evaluated for accurate laboratory characterization and inter-laboratory comparison.

Description: Induced seismicity associated with fluid injection into underground formations jeopardizes the sustainable utilization of the subsurface. Understanding the fault behavior is the key to successful management and mitigation of injection-induced seismic risks. As a fundamental approach, laboratory experiments have been extensively conducted to assist constraining the processes that lead to and sustain various injection-induced fault slip modes. Here, we present a state-of-the-art review on the emerging topic of injection-induced seismicity from the laboratory perspective. The basics of fault behavior, including fault strength and instability, are first briefly summarized, followed by the paradoxical stability analysis arising from the current theoretical framework. After the description of common laboratory methods and auxiliary techniques, we then comprehensively review the effects of fault properties, stress state, temperature, fluid physics, fluid chemistry and injection protocol on fault behavior with particular focus on the implications for injection-induced seismicity. We find that most of the shear tests are conducted under displacement-driven conditions, while the number of injection-driven shear tests is comparatively limited. The review shows that the previous work on displacement-driven rock friction and fault slip modes partially unravel the mystery of injection-induced fault behavior, and recent experimental studies on the injection-driven response of critically stressed faults provide complementary insights. Overall, laboratory experiments have substantially advanced especially our understanding of the roles of fault roughness, fault mineralogy, stress state, fluid viscosity, fluid induced mineral dissolution, and injection rate in injection-induced seismicity, which has been successfully used to interpret many field observations. However, there are still outstanding questions in this area, which could be addressed by future experimental studies, such as the feasibility of seismic-informed adaptive injection strategy for mitigating seismic risks, cold fluid injection into critically stressed faults under hydrothermal conditions, and fault friction evolution during cyclic injection spanning from undrained to drained conditions.