The spectral boundary integral method is popular for simulating fault, fracture, and frictional processes at a planar interface. However, the method is less commonly used to simulate off-fault dynamic fields. Here we develop a spectral boundary integral method for poroelastodynamic solid. The method has two steps: first, a numerical approximation of a convolution kernel and second, an efficient temporal convolution of slip speed and the appropriate kernel. The first step is computationally expensive but easily parallelizable and scalable such that the computational time is mostly restricted by computational resources. The kernel is independent of the slip history such that the same kernel can be used to explore a wide range of slip scenarios. We apply the method by exploring the short-time dynamic and static responses: first, with a simple source at intermediate and far-field distances and second, with a complex near-field source. We check if similar results can be attained with dynamic elasticity and undrained pore-pressure response and conclude that such an approach works well in the near-field but not necessarily at an intermediate and far-field distance. We analyze the dynamic pore-pressure response and find that the P-wave arrival carries a significant pore pressure peak that may be observed in high sampling rate pore-pressure measurements. We conclude that a spectral boundary integral method may offer a viable alternative to other approaches where the bulk is discretized, providing a better understanding of the near-field dynamics of the bulk in response to finite fault ruptures.

A novel transient unloading testing system was adopted to simulate the transient excavation of tunnels under different lateral pressure coefficients ( ). The results show that the transient excavation of a tunnel induces significant stress redistributions and concentrations, particle displacements and vibrations to the surrounding rocks. The decrease of enhances the dynamic disturbance of transient tunnel excavation, and especially when  = 0.4 and 0.2, the tensile stress can be observed on the top of the tunnel. The peak particle velocity (PPV) of the measuring points on the top of the tunnel decreases with the increasing distance between the tunnel boundary and measuring point. The transient unloading wave is generally concentrated on lower frequencies in the amplitude-frequency spectrum under the same unloading conditions, especially for lower values. In addition, the dynamic Mohr-Coulomb criterion was used to reveal the failure mechanism of a transient excavated tunnel by involving the loading rate effect. It is found that the excavation damaged zone (EDZ) of the tunnel is dominated by the shear failure, and the number of the shear failure zones increases with the decrease of . The EDZ of tunnels after transient excavations varies from ring-shape to egg-shape and X-type shear with the decrease of . The evolution of the EDZ induced by the transient unloading is associated with , i.e., the shear failure of surrounding rocks mainly occurs in the stress redistribution stage under high (1.0-0.7), while the dramatic destruction of surrounding rocks is more prone to occur after the transient unloading process when  ≤ 0.6.

Mudstones and shales serve as natural barrier rocks in various geoenergy applications. Although many studies have investigated their mechanical properties, characterizing these parameters at the microscale remains challenging due to their fine-grained nature and susceptibility to microstructural damage introduced during sample preparation. This study aims to investigate the micromechanical properties of clay matrix composite in mudstones by combining high-speed nanoindentation mapping and machine learning data analysis. The nanoindentation approach effectively captured the heterogeneity in high-resolution mechanical property maps. Utilizing machine learning-based -means clustering, the mechanical characteristics of matrix clay, brittle minerals, as well as measurements on grain boundaries and structural discontinuities (e.g., cracks) were successfully distinguished. The classification results were validated through correlation with broad ion beam-scanning electron microscopy images. The resulting average reduced elastic modulus ( ) and hardness () values for the clay matrix were determined to be 16.2 ± 6.2 and 0.5 ± 0.5 GPa, respectively, showing consistency across different test settings and indenter tips. Furthermore, the sensitivity of indentation measurements to various factors was investigated, revealing limited sensitivity to indentation depth and tip geometry (when comparing Cube corner and Berkovich tip in a small range of indentation depth variations), but decreased stability at lower loading rates. Box counting and bootstrapping methods were applied to assess the representativeness of parameters determined for the clay matrix. A relatively small dataset (indentation number = 60) is needed to achieve representativeness, while the main challenges is to cover a representative mapping area for clay matrix characterization. Overall, this study demonstrates the feasibility of high-speed nanoindentation mapping combined with data analysis for micromechanical characterization of the clay matrix in mudstones, paving the way for efficient analysis of similar fine-grained sedimentary rocks.

The interaction of natural and hydraulic fractures may facilitate lateral fluid propagation in an unconventional reservoir resulting in fast fluid pressure transmission from treatment wells to a fault zone and potential fault shear slip reactivation and associated induced seismicity. Several induced earthquakes (up to 4.1 Mw) occurred since 2013 during hydraulic fracturing of the Upper Devonian Duvernay Formation in the Western Canada Sedimentary Basin. The mechanism of lateral fluid migration in the unconventional reservoir is not well understood. The current study aims to investigate the interaction of natural fractures and hydraulic fractures for the case study in the area south of Fox Creek, where a linear zone of induced earthquakes (up to 3.9 Mw) occurred along a fault in 2015 during hydraulic fracturing of horizontal wells. We analyze the growth of hydraulic fractures in presence of natural fractures, the impact of resulting complex fracture network on fluid transmission and fluid pressure buildup around the treatment wells. Hydraulic fracture modeling (HFM), reservoir simulations and 3D coupled reservoir-geomechanical modeling are applied to match the timing of hydraulic fracture propagation and transmitted fluid pressure increase in the fault zone versus induced earthquake occurrence. HFM results are verified by microseismic clouds distribution. Reservoir simulations are validated by a history matching of fluid injection volume and bottomhole pressure data. Additional HFM simulations are carried out to optimize the pumping schedule in the studied well pad that would help to prevent hydraulic fractures reaching the fault and minimize the risk of induced seismicity.

Hydrocarbon production from sandstone reservoirs causes elastic and inelastic reservoir compaction, potentially leading to surface subsidence and even seismicity, such as observed in the Groningen gas field, Netherlands. Inelastic compaction can partly be instantaneous, though rate-/time-dependent processes may play a role on the longer term. Therefore, compaction may continue even if production is stopped. To reliably evaluate the impact of post-abandonment behaviour, mechanism-based rate-/time-dependent compaction laws are needed. We performed triaxial compression experiments on Slochteren sandstone (reservoir of the Groningen field) samples, with porosity 14.6-18.9%, to investigate the effect of strain rate (rates of ) under conventional triaxial and uniaxial strain (i.e. zero-lateral strain) boundary conditions. Under triaxial conditions, lowering of stress-strain curves was observed with decreasing strain rate at all differential stresses, the effect being enhanced at higher temperature and pore fluid pH. By contrast, strain rate had limited effect on axial stress vs. strain behaviour under uniaxial strain conditions, though decreasing strain rate, as well as increasing fluid pH, resulted in a smaller increase in confining pressure required to maintain a zero-displacement lateral boundary condition. The mechanical data, complemented by microstructural analysis, suggest that subcritical cracking, coupled with grain rearrangement, was the dominant mechanism causing inelastic deformation under triaxial and uniaxial strain conditions. Our results suggest that the amount of reservoir compaction will be limited after production stops. However, time-dependent deformation will lead to changes in the in-situ state of stress, which should be included in models assessing reservoir compaction and induced seismicity in the Groningen field.