Fault Reactivation And Rock Deformation Mechanisms Under Stress/Pressure Cycling

Abstract

Addressing climate change and transitioning to renewable energy will involve subsurface activities like carbon storage, geothermal exploitation, and underground energy storage. However, fluid injection and extraction in the subsurface can alter the pressure, temperature, stress, and rock geochemistry, potentially leading to seismicity and subsidence. Understanding the mechanisms of fault reactivation and the geomechanical response of intact reservoir rock to variations in pore fluid pressure from injection and depletion operations is thus crucial. In this thesis, we integrate our findings from multiple studies to explore the impact of parameters related to injection and depletion, such as pattern (monotonic, cyclic) and rate, on the deformation of intact reservoir rock, slip behaviour in faulted reservoir rock, and the evolution of microseismicity, with a focus on how we can mitigate induced seismicity.

Our experimental investigations employ uniaxial compressive tests on intact Red Felser sandstone samples, subjecting them to cyclic recursive (CR), cyclic progressive (CP), and monotonic stress patterns at varying stress rates. The recording of Acoustic Emission (AE) waveforms revealed that cyclic stress patterns, especially CP, are characterized by lower maximum AE amplitudes compared to the monotonic pattern. By reducing the stress rate, the maximum AE energy and final mechanical strength both decrease significantly. Moreover, high-stress rates were found to alter the AE signature of events, suggesting that cyclic stress patterns combined with low-stress rates may mitigate induced seismicity in subsurface injection operations.

For underground energy storage, we investigate the geomechanical response of Red Felser sandstone to cyclic loading, crucial for safe and efficient underground porous reservoir operations. Experimental results, complemented by constitutive modeling, revealed various deformation mechanisms, including linear elastic, viscoelastic, and inelastic responses. Our study shows that the magnitude of inelastic deformations is influenced by mean stress, amplitude, and frequency of the stress waveform, with our models closely fitting the experimental data.

As part of our investigation into mitigating induced seismicity, we examine how stress and sliding patterns affect fault slip behaviour and seismicity evolution. To achieve this we carry out displacement-driven fault reactivation experiments on saw-cut Red Felser sandstones. Our results indicated that cyclic sliding, compared to continuous sliding, reduces seismicity but can accelerate slip velocity during the reloading phase due to the healing of gouge material on the fault plane. Additionally, under-threshold cycling effectively prevents seismicity and shear slip but poses a risk of increased seismicity if shear stress exceeds critical levels.

Furthermore, we explore the influence of injection pattern and rate on fault reactivation in porous Red Felser sandstone. High injection rates were linked to increased slip velocity and seismicity. Furthermore, our results from samples subjected to various injection patterns demonstrate that the cyclic recursive pattern exhibits a higher maximum slip velocity, more episodes of slow slip, and greater radiated AE energy than a monotonic pattern. A proper injection strategy must consider fault drainage, critical shear stress, injection rate, and injection pattern. Our results demonstrate that a monotonic injection pattern and low pressurization rate may mitigate seismicity on pre-existing faults in a highly permeable porous reservoir.

Finally, we investigate the fault slip nucleation within a displaced fault system. Our triaxial experiments on displaced faults reveal that differential compaction intensifies from the top of the sample towards the internal corner at the centre of the fault, indicating a variation in the stress field surrounding the fault plane. Our direct measurements near the displaced fault plane confirm the anomalies and peaks in stress observed in previous numerical and analytical studies.

This thesis offers new insights into the mechanical behaviour and seismicity evolution of intact and faulted reservoir rocks under variations in stress patterns and rates. These findings may contribute to mitigating injection-induced seismicity in intact and porous faulted rock settings. Furthermore, they enhance our understanding of the behaviour of deep geo-reservoirs subjected to diverse injection strategies, thereby expanding our knowledge of reservoir-related phenomena.