A study of the geomechanical and petrophysical impact of CO₂-saturated brine injection in limestones with various low permeability

Deep saline aquifers are promising locations for carbon sequestration, but the impact of CO₂ injection on rock properties requires careful investigation. This study examines the effects of CO₂-saturated brine injection on limestone, specifically analyzing how rock permeability influences wormhole generation and subsequent changes in petrophysical and geomechanical properties. Four Indiana limestone samples, each with dimensions of 1.5 inches in diameter and 3 inches in length, were utilized. Four different permeability values were tested: 3, 10, 15, and 22 mD. During the experiments, the samples were subjected to a pore pressure of 2000 psi and a temperature of 60 °C, with a brine concentration of 120,000 ppm mixed with CO₂ at a ratio of 70:30 (700 mL brine and 300 mL CO₂ at 1200 psi), maintained at 1200 psi initially. An injection rate of 1 cm³/min was employed for the coreflooding experiments. Before and after coreflooding, the samples' dynamic Young's modulus (YM) and Poisson's ratio were measured at various confining pressures. Additionally, the treated samples' porosity, permeability, and surface hardness were assessed. To visualize the generated wormholes and quantify their volumes, a micro-CT scan was employed. Our study demonstrated that among the four tested permeability values, the experiment with a 3 mD core yielded the lowest pore volume to breakthrough (PVBT). A clear trend was observed between PVBT and the permeability of carbonate samples, as the PVBT increased significantly with increasing rock permeability. Additionally, a general correlation was observed between the mechanical properties of treated samples and rock permeability: increased permeability resulted in less significant deterioration of the core samples due to the lower dissolution. The investigation established that low permeability samples result in greater damage to the core, as evidenced by reductions in both surface hardness and dynamic Young's modulus.