Comparative Analysis of Porosity and Compressive Strength of Sandstone Under Varying Co₂ Concentrations and Ageing Periods

Carbon Capture, Utilisation and Storage (CCUS) in deep saline aquifers is a critical strategy for mitigating global greenhouse gas emissions. However, the long-term mechanical interaction between CO ₂ -saturated brine and reservoir rock, particularly in the far-field zones, remains a key uncertainty. This study investigates the time-dependent degradation of sandstone porosity and compressive strength under varying CO ₂ concentrations. Homogeneous sandstone cores were subjected to static ageing in CO ₂ :brine with volumetric ratios of 30:100 (CB30) and 60:100 (CB60) at 60 ºC for 1, 15, and 30 days. The changes in rock integrity were evaluated using Uniaxial Compressive Strength (UCS) testing, liquid-saturation porosity measurements, and microstructural analysis (SEM-EDX). Baseline EDX analysis identified the primary cementing mineral as calcite (CaCO ₃ ), confirmed by the elemental presence of Calcium (Ca), Carbon (C), and Oxygen (O). Post-ageing results revealed that this primary cement underwent extensive dissolution in high-concentration samples, while secondary carbonate precipitates formed in low-concentration environments. The results revealed that the effective porosity for the CB30 series decreased consistently from 16.4% to 12.3%. In contrast, the CB60 series decreased from 15.6% (day 1) to 11.5% (day 15), then increased slightly to 12.0% (day 30). Mechanical testing showed that the CB30 series fluctuated from 37.1 MPa (day 1) to 33.1 MPa (day 15), then recovered to 37.2 MPa (day 30). Conversely, the CB60 series maintained stability at 36.2 MPa (days 1 and 15) before collapsing to 22.6 MPa (day 30). These findings suggest that in far-field zones, thermodynamic equilibrium favours secondary mineral precipitation and physical clogging, as supported by post-ageing SEM-EDX analysis confirming the accumulation of calcium-rich precipitates within the pore network, leading to porosity reduction. However, the rock's strength also depends on the CO ₂ concentration. At low concentration, this precipitate clogging was favoured and filled the voids, successfully helping the rock regain its strength. In contrast, at high concentration, the carbonic acid eventually dissolved the calcite cement extensively, causing the rock to lose its structural support and collapse.