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Underground energy storage (UES) in porous and cavity reservoirs can be used to balance the mismatch between the production and demand of renewable energy. Understanding the geomechanical behaviour of these reservoirs under different storage conditions, i.e., storage frequency and fluid pressure, is key in defining their capacity and effective lifetime. This work presents an analysis performed on sandstones to unravel their geomechanical response under cyclic loading. It includes, importantly, both experimental and numerical investigations under deviatoric stress conditions below the rock dilatant cracking threshold. From the experimental point of view, axial strains and acoustic emissions indicated that inelastic strains accumulated cycle after cycle, following a decreasing rate per cycle. Four types of deformations were interpreted: elastic, viscoelastic, plastic, and cyclic-plastic. Based on these experimental results and observations, the Modified Cam-clay model was extended to account for cyclic plastic deformations and the Kelvin-Voigt model was used to model visco-elasticity. This approach can be used to study cyclic sandstone deformation's implications on subsidence, fault reactivation, and cap rock flexure, among other physical phenomena impacting a reservoir's storage capacity.

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<p>With human activities in the subsurface increasing, so does the risk of induced seismicity. For mitigation of the seismic hazard and limiting the risk, monitoring and forecasting are essential. A laboratory study was performed to find precursors to fault failure. In this study, Red Pfaelzer sandstones samples were used, which are analog to the Groningen gas reservoir sandstones. A saw-cut fault was cut at 35 degrees, and the samples were saturated. Fault slip was induced by loading the sample at a constant strain rate, and simultaneously active acoustic transmission measurements were performed. Every 3 seconds 512 S-waves were sent, recorded, stacked to reduce the signal-to-noise ratio, and analyzed. The direct seismic wave velocity, coda wave velocity, and transmissivity were monitored before and during the reactivation of the faulted samples. Different loading patterns and confining pressures were investigated in combination with active acoustic monitoring. Velocity and amplitude variations were observed before the induced fault slip and can be used as precursory signals. Two methods to determine changing velocities were used. Direct S-wave velocities are compared to velocity change obtained by coda wave interferometry. Both analyses gave similar precursory signals, showing a clear change in slope, from increase to decreasing velocities and amplitudes prior to fault reactivation. Fault reactivation is preceded by fault creep and the destroying of some of the asperities on the fault plane, causing the seismic wave amplitude and velocity to decrease. Combining all precursors, the onset of fault slip can be determined and therefore upcoming slip can be forecasted in a laboratory setting. Our results show precursory changes in seismic properties under different loading situations and show a clear variation to the onset of fault reactivation. These results show the potential of continuous acoustic monitoring for detection and forecasting seismicity and help the mitigation of earthquakes.</p>

<p>Over the last few decades, it has become apparent that different human activities in the subsurface, such as water waste injection, hydraulic fracturing, and geothermal energy production can lead to induced seismicity. Understanding the effects of fluid injection-related parameters on seismic response or evolution of it is essential for finding a method to manage and minimize the induced seismicity risk. Experimental and numerical studies indicate that varying injection patterns and rates can be used to effect and/or mitigate seismicity. However, most of the studies are for intact rock medium, and the mechanism of injection-induced seismicity of faulted rock medium is not clear yet. In this study, we performed fault reactivation experiments on faulted (saw-cut) Red Pfaelzer sandstones to provide new insight into the effect of stress/pressure cycling and rate on fault slip behavior and seismicity evolution. The saw-cut samples were subjected to two different reactivation mechanisms: 1) stress-driven and 2) injection-driven fault reactivation. Three different reactivation scenarios were performed during the stress-driven fault reactivation experiments: continuous sliding, cyclic sliding, and under-threshold cycling sliding. Ten AE transducers were used to detect microseismicity during the fault reactivation experiments, and consequently, different microseismic parameters, such as frequency-magnitude distribution (b-value), AE energy, and AE rate were estimated. Stress-driven fault reactivation experiments showed that (i) a below-threshold cycling scenario prevents seismicity and pure shear slip; however if the shear stress exceeds the previous maximum shear stress, seismicity risk increases drastically in terms of b-value, maximum AE energy, and magnitude. (ii) Compared to continuous sliding, cyclic sliding triggers less seismicity in terms of total b-value and large AE events due to the uniform reduction in roughness and asperity on the fault plane. (iii) By increasing the number of cycles, in general, the number of generated events and AE energy per cycle is reduced. Nevertheless, there is a risk of generating large AE events during the first cycles. In addition, results from the injection-driven fault reactivation experiments demonstrated that high injection rate results in higher peak slip velocity. Compared to the stepwise injection pattern, the cyclic recursive injection scenario showed higher peak slip velocity, due to the high hydraulic energy budget and fault compaction. A proper injection strategy needs to consider various factors, such as fault drainage, critical shear stress, injection rate, and injection pattern (frequency and amplitude). Our results demonstrate that selecting proper stress/pressure amplitude, and pressurization rate for the injection design strategy can help to reduce seismicity risk.  </p>