A significant rise in the world’s mean temperature (1.1ºC in 100 years) is produced mainly due to the emission of greenhouse effect gases (GHG), particularly carbon dioxide (CO₂) emission (IPCC, 2022). Carbon capture, storage, and utilization (CCSU) is being developed as an alternative to curbing the direct emission of CO₂ into the atmosphere. This process involves capturing CO₂ directly from emitting industries and injecting it underground in supercritical state (31.48ºC and 7.38 MPa). The primary goal is to identify a suitable geological reservoir capable of storing CO₂ for extended periods without the risks of migration. Among the various reservoir alternatives, a depleted oil and gas reservoirs stand out as a viable option. These rocks formations own high porosities, offering great void space for CO₂ storage, and their high permeability facilitates the injection of CO₂.

This paper explores the potential of using glauconitic sandstone extracted from a depleted oil and gas reservoir as a viable CO₂ storage option. The experimental investigation focused on observing the effects of subjecting samples to supercritical CO₂ (scCO₂) at 10.5 MPa and 60ºC for a duration of 30 days within the pressure cell at the UNPSJB's geomechanics laboratory (Argentina). Subsequently, a detailed analysis was conducted to evaluate the changes in petrophysical properties and mechanical behavior of the samples before and after the CO₂ ageing process. To achieve this, various mechanical tests, including direct tensile, uniaxial compressive, and triaxial compressive tests, were carried out at the geotechnical laboratory of CEDEX (Spain).

Based on the mechanical results obtained, it became evident that the mechanical strength of the sandstone decreased after undergoing the carbonation process, as assessed using the Hoek-Brown failure criterion. Regarding porosity, two primary modifications were observed. Firstly, there was a slight reduction in the sandstone's porosity after its exposure to carbon dioxide. Secondly, the pore size distribution exhibited variations, with an increased percentage of pores having smaller diameters. These findings suggest a preliminary hypothesis that chemical precipitation may have taken place within the larger diameter pores during the scCO₂-aging of the samples. To validate this hypothesis, additional analysis of the SEM, XRD, and XRF results will be conducted.

The study of salt structures has important guiding significance for oil and gas exploration and production in salt-bearing sedimentary basins. Previous studies have mainly focused on exploring the influencing factors such as syntectonic sedimentation, preexisting faults and preexisting salt diapirs of salt tectonic deformation. However, there is still a lack of systematic research on the effect of geometric properties of preexisting salt diapirs on the later stage shortening deformation. In this paper, particle flow method is used to investigate the geometric properties of preexisting salt diapirs such as its geometry, depth, width and the combination styles of preexisting salt diapirs on the geomechanics of salt-related structures during shortening. Model results show that the preexisting salt diapirs play an important role in geomechanics of salt-related structures. During shortening, salt bearing structures are prone to plastic flow, local accumulation and thickening, which affect the structural style of the overlying layers. The structural styles mainly include reverse faults and fault-related folds, box-folds, salt nappe structures and salt welding structures. The depth of preexisting salt diapirs is important for controlling the growth of diapirs. The width affects significantly the amount of shortening, which can be accommodated by extruded diapirs. The combination style of preexisting salt diapirs has a certain impact on the deformation of overlying salt tectonics. In addition, the reliability of the simulation is further verified by comparing the model results with the typical seismic profiles of salt tectonics in the Tarim Basin in China.

The storage of CO₂ in reservoir rocks is a crucial method for mitigating greenhouse gas emissions, but its application in Argentina, especially in the San Jorge Gulf basin, remains unexplored. Our study focuses on understanding how CO₂ affects the mechanical, petro-physical, and chemical properties of the Castillo and Pozo D-129 formations, which con-tain varying amounts of pyroclastic material. Rock samples were collected, core-drilled, and subjected to supercritical CO₂ conditions. Tests included uniaxial compression, air permeability, mercury porosimeter, XRF and XRD. Findings revealed that rocks after supercritical CO₂ aged treatment reduce the mechanical strength between 14 to 28%. Chemical analyses find variations in the mineral composition between carbonated and non-carbonated samples. Additionally, a reduction in porosity was observed in the treated specimens.

Robust geomechanical models are critical for forecasting and monitoring geomechanical issues associated with hydrocarbon production. Geomechanical responses to carbon utilisation and storage(CUS) have received little attention in the literature. This research proposes to investigate the poromechanical response of faulted depleted gas fields to CUS. The study area is located in the Bredasdorp Basin in South Africa. A one-way coupled geomechanical model is developed to gain insights into the geomechanical challenges related to CUS in faulted depleted gas fields. This model combines a dynamic model derived from earlier studies on the EM gas field with a 3D geomechanical model created using Petrel software. Particular attention is given to modelling the 3D petrophysical Young modulus in the underburden to obtain a more accurate mechanical response. Three geomechanical deformation case scenarios were formulated to assess the behaviour of the E-M gas field. The first scenario, known as the Depletion case, involved simulating natural gas production for nine years. In the second scenario, EGR_CO₂, EGR was simulated in the depleted gas field for 17 years, followed by CO₂ storage for 40 years. The final scenario, STORE_CO2, focused on injecting CO₂ into the depleted gas field over 40 years. The research outcomes unveiled notable observations regarding the geomechanical aspects. One signification revelation was the substantial contrast in the change in stress between the central and peripheral regions of the reservoir. Moreover, during the depletion phase, the reservoir experienced a subsidence of 30 cm, while an uplift of 18 cm was observed during the injection phase. Additionally, it was found that the uplift of the underburden compensated for the loss of fluid support, enabling the reservoir to uphold the overlying overburden despite the liquid phase depletion. However, after 40 years of CO₂ injection, the caprock exhibited failure, whereas the reservoir rocks remained stable. Furthermore, depletion heightened the probability of fault slippage, whereas CO₂ injection significantly reduced deviatoric and shear stresses, indirectly minimising the tendency of fault activation. Finally, the geomechanical issues associated with fluid injection in the reservoir during EGR depend on whether the production of natural gas balanced CO₂ injection. The reservoir is expected to remain stable if injection and production are balanced.

Naturally fractured/cleated coals are likely to develop viscoelastic strain rate-dependency during geological engineering activities such as coal mining, coal seam gas production and gas injection into coal seams. A full investigation of strain rate-dependency of naturally fractured coals is thus crucial to have safe and optimised coal seam operations. The micro-scale mechanisms causing the core-scale rate dependent observations, however, are not yet fully understood. In this study, we conduct a series of systematic multi-scale experiments to shed light on the mechanisms controlling the strain rate dependency of coal. The multi-scale experiments consist of core-scale triaxial testing of coal specimens with different strain rates under isotropic and deviatoric loading on different coal specimens. Every loading is followed with an unloading to analyse the energy dissipation during the tests and ensure that any bulk damage is avoided. A series of micro-scale tests on jointed coal specimen coupled with microscopy and digital image correlation (DIC) analysis is additionally conducted to further study the micro-scale processes controlling the macro-scale strain rate-dependent behaviour of the specimens. The results of triaxial tests on dry specimens show a clear strain rate-dependency under isotropic loading where a higher strain rate causes a stiffening effect on coal specimens. No strain rate-dependency is, however, observed for specimens under isotropic loading when fractures are closed by carbon dioxide (CO₂) saturation at different pressures nor under deviatoric loading for any specimens. Unloading of specimens, on the other hand, shows a considerable strain rate dependent energy dissipation in isotropic loading tests. The results of the micro-scale tests on coal’s fracture interestingly indicates that the asperities damage under normal stress controls the energy dissipation and strain rate-dependency of the bulk of specimens. The combined results reveal that, for the first time, the strain rate-dependency in naturally fractured coal within its elastic limit is attributed to damage of the asperities of pre-existing fractures under fracture closure process.