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<p>Dissolution of halite rock can significantly impact underground constructions (e.g., caverns for energy storage and abandoned caverns) and above ground constructions (e.g., highways and buildings) potentially causing a threat to human life from land subsidence and sinkhole hazards, instability to underground construction and pollutant release. In this work, we explore and quantify changes in elastic and hydromechanical properties during dissolution of halite rock by migration of water.</p><p>We evaluated the impact of dissolution on the geophysical properties of pristine (non-fractured) and fractured halite samples (with ~2.7% dolomite), using a synthetic (seawater-like) brine solution (3.5wt% NaCl). The dissolution test commenced by setting an initial effective pressure of 15 MPa (with minimum pore pressure of 0.1 MPa), equivalent to a depth of ~720 m below ground level. This confining pressure of 14.9 MPa ensured the adequate contact between sample and the ultrasonic instrumentation (P- and S-wave sensors), and the set of electrodes for electrical resistivity. The test procedure was set to investigate the effect of increasing pore pressure from 0.1 to 14 MPa on dissolution. This procedure was only successful for the non-fractured sample, as dissolution rapidly occurred in the fractured sample during the initial stage of the test.</p><p>The non-fractured halite shows that P-wave velocity increases with increasing inlet pore pressure initially, followed by a lower pore fluid sensitivity stage. After this stage, the P-wave and the Vp/Vs ratio reduce and then ultrasonic velocities tend to their original values when effective pressure tends to zero. These results suggest that capillary pressure effects are initially increasing the bulk properties of the rock by filling the micro-pores, while dissolution is occurring locally, nearby the inlet-flow port, and therefore invisible to our geophysical tools. The small porosity fraction of 1.1% allows the saturating fluid to rapidly equilibrate with the surrounding halite within the pores, slowing down the dissolution process. In a close halite system with a local and continuous brine supply source, local dissolution may allow pressure increase up to the overburden stress and affect the geomechanical integrity of the reservoir by a combined fracturing-dissolution process.</p>

<p>Carbon Capture Utilization and Storage (CCUS) is an essential technology to meet net-zero carbon emission targets. Due to CO<sub>2</sub> injection, original reservoir properties are altered. Early warning of potential CO<sub>2</sub> injection-induced reservoir instability depends on our correct interpretation of the geophysical remote sensing data, particularly seismic and electromagnetic datasets. Using joint elastic-electrical datasets is a proven effective approach to simultaneously characterize mineral skeleton properties and pore fluid distribution, and therefore a powerful reservoir monitoring tool for CCUS.</p><p>To interpret large-scale elastic-electrical datasets, original rock properties and fundamental CO<sub>2</sub>-fluid-rock interactions are  preliminarily investigated by combining available well-logging data, lab-controlled experiments using rock samples, and rock physics theories; the latter two are inevitably dependent on one another. Despite we can mimic changing reservoir conditions in the lab and generate datasets that provide essential information to understand specific processes at the micro- and meso-scales (and serve as inputs for large-scale reservoir simulations), every experiment carries limitations inherent to the particular lab capabilities, together with the obvious time- and space-scale related uncertainties (i.e., core-scale experiments only partially describe the events occurring in the field). Then, we need theoretical rock physics to make experimental assumptions, and reciprocally we use the experimental data to validate models.</p><p>Physical and petrographic properties of reservoir rocks condition the degree of heterogeneity and anisotropy of the CO<sub>2</sub> storage unit that, in turn, influence the total storage capacity and fluid migration. Original clay content, grain size distribution, mineralogy, porosity and permeability are among the most influencing parameters, particularly for low reactive siliciclastic formations (i.e., desired CO<sub>2</sub> storage reservoirs). But these properties randomly change to some extent within any reservoir formation.</p><p>Here, we investigate how reservoir heterogeneity influences our geophysical interpretation of the potential CO<sub>2</sub> storage site Aurora, offshore Norway. Recent studies suggest high clay content and porosity variability within the Johansen Formation sandstone, Aurora’s primary reservoir. Due to lack of Johansen Fm. samples, we selected three sandstone samples from the Central Graben, Offsore UK (Forties Formation), formed in a similar depositional environment, with similar mineralogical composition, and porosity (20 to 28%), clay content (10 to 26%) and permeability (1 to 8 mD) ranges. The elastic (from P- and S-wave velocities) and transport (from permeability and resistivity) properties of the tested samples were used to assess the influence of their intrinsic properties on the pore fluid distribution during CO<sub>2</sub> injection and the permanent CO<sub>2</sub>-induced changes in the Aurora reservoir complex. We apply well-known rock physics theories, including effective stress law, Archie’s relationship, and the Biot-Stoll and White and Dutta-Ode models, for both to impose the most similar reservoir conditions according to our lab limitations and to assess the experimental results. We observe (i) elastic and transport properties variations (up to 15% and 30%, respectively) between samples, mainly related to porosity differences, and (ii) more significant permanent alterations post-CO<sub>2</sub> injection in those with higher porosity and clay content. Our results show the importance of accounting for heterogeneity-related changes in sandstone reservoirs during/after subsurface CO<sub>2</sub> storage activities for enhanced geophysical interpretation.  </p>