Jean Sulem

The study of the mechanical behaviour of carbonate rocks has benefited from the evolution of experimental tools, due to their complex multi-scale microstructure. Triaxial cells transparent to X-rays have emerged and allowed in situ investigation of progressive failure developments and their link with microstructure. We conducted a series of 4D X-ray computed tomography mechanical tests on porous Saint-Maximin limestone at the PSICHE beamline of Synchrotron SOLEIL. Tests were performed using an in-house-developed triaxial press, Modulo. Volumes were scanned using a helical acquisition and reconstructed at 2.83 µm. They were then analysed using digital volume correlation. Volumetric strain and shear strain magnitude maps were computed using a 100 voxels gauge length with Paraview . Additionally, we computed porosity maps based on mean grey levels, at the same gauge length. Analysis of total and normalized incremental strain maps was carried out. We also compared volumetric strain maps with the initial porosity maps. Each step of the procedure is described herein. The errors on strain and porosity are discussed.

Underground hydrogen storage in porous formations (UHSP) subjects reservoir rocks to long‐term cyclic effective stress variations, potentially altering storage capacity and injectivity. In this study, we conduct hydrostatic loading tests on 18 Saint‐Maximin limestone (SML) samples (connected porosity 31.9%–37.2%) under various loading paths, complemented by measurements of permeability and acoustic wave velocities. A specific creep‐fatigue interaction test, combining stabilized creep and subsequent cyclic loading at identical peak stress, is conducted to distinguish cycle‐dependent ratcheting deformation from time‐dependent creep. Experimental results reveal that cyclic behavior of SML is governed by a dual stress‐threshold mechanism. The peak stress ratio governs initiation and evolution of damage, while the valley stress ratio describes the reopening of generated fractures during unloading. > 1 leads to enhanced creep and onset of ratcheting deformation and < 1 induces 1.2% additional porosity reduction within 50 cycles after creep stabilization. Across all loading paths, porosity change serves as a robust metric for mechanical hardening and permeability evolution within the experimental range of up to a 12.8% reduction in porosity and an order‐of‐magnitude decrease in permeability. These findings establish a reference testing approach for quantitatively partitioning different deformation mechanisms, providing an essential basis for more reliable predictions of long‐term UHSP performance.

During an earthquake, slip occurs in a localized shear zone that features a heavily granulated fault core that can be characterized as a shear band. We study the formation of this fault core in a granular rock such as sandstone by developing a model of crushable granular media within the framework of Breakage Mechanics. This model accounts for the evolution of the grain size distribution, while also accounting for the co‐evolution of the solid fraction. An enrichment with the Cosserat continuum allows for the model to predict finite‐width shear bands. The model is then calibrated against experimental data taken from tests on Bentheim sandstone, and a parametric study of the mechanical parameters is conducted using linear stability analysis. We find that for deeply‐buried rocks the shear bands have a compactive component, and the initial value of the solid fraction does not play a strong role in the initial band thickness, but can influence the rate of delocalization of the band. Post‐localization behavior is studied with the finite element method, which shows the formation of zones of dilation outside the band in addition to the compaction within the band. Using a modified Kozeny–Carman permeability law, it is shown that within the band the permeability reduces by several orders of magnitude, but can increase outside the band. Our results highlight the importance of modeling grain size and solid fraction evolution as they exert a controlling influence on hydromechanical properties that play an important role in fault formation and seismic slip.

Underground hydrogen storage in porous formations (UHSP) provides a large-scale and long-term solution to balance the seasonal supply-demand mismatch of renewable energy. Assessing the long-term safety and performance of UHSP requires understanding how reservoir rocks respond to cyclic pressure variations. In this study, we conduct hydrostatic loading tests on Saint-Maximin limestone (SML) samples under various loading paths, complemented by measurements of permeability and acoustic wave velocities. The resulting evolutions of porosity, permeability and the damage are systematically investigated. Specific loading paths are applied to distinguish cycle-dependent ratcheting deformation from time-dependent creep. Experimental results reveal that for cyclic loading, mechanical behavior of SML is governed by a dual stress-threshold mechanism. The peak stress ratio (PR) governs initiation and evolution of damage, as supported by ultrasonic velocity measurements and a post-loading microstructural observation, and leads to enhanced creep and onset of ratcheting deformation when PR > 1. The valley stress ratio (VR) describes the reopening of existing fractures during unloading and induces an amplified ratcheting rate when VR < 1. Across all loading paths, porosity change serves as a robust metric for mechanical hardening and permeability evolution. These findings offer new insights into the coupled hydro-mechanical processes governing reservoir rocks under long-term cyclic operation and contribute to more reliable predictions of UHSP performance.

This study investigates the relationship between deformation mechanisms and heterogeneity of porosity of a porous carbonate rock across several scales. The Saint Maximin limestone (SML), characterized by centimeter‐scale alternations of dense and porous zones, serves as a model rock to examine the evolution of deformation modes. Cylindrical samples of 8 mm in diameter, extracted from both the dense and porous zones of the SML, were tested in axisymmetric triaxial conditions with in situ synchrotron imaging. High‐resolution 3D strain fields and porosity maps were derived from synchrotron images to assess local‐scale correlations. The results demonstrate that initial porosity exerts a primary control on strain patterns, spanning from the microscale to the centimeter scale. Under brittle conditions, deformation localizes into bands, predominantly accommodated within the more porous regions, while the denser zones remain largely unaffected. At intermediate confining pressures, heterogeneity induces a transitional behavior where diffuse and localized deformation coexist. In ductile conditions, multiple small compactant bands progressively saturate the sample, leading to porosity homogenization.