Effect of Salinity on Effective CO₂ Permeability in Reservoir Rock Determined by Pressure Transient Methods: an Experimental Study on Hawkesbury Sandstone

The determination of effective carbon dioxide (CO₂) permeability in reservoir rock and its variation is of great interest in the process of CO₂ sequestration in deep saline aquifers, as CO₂ sequestration-induced permeability alternations appear to create major problems during the CO₂ injection process. The main objective of this study is to investigate the effect of salinity on the effective CO₂ permeability of reservoir rock under different injection pressures. A series of high-pressure tri-axial experiments was, therefore, performed to investigate the effect of salinity on effective CO₂ permeability in Hawkesbury sandstone under various brine concentrations. The selected brine concentrations were 0, 10, 20, and 30 % sodium chloride (NaCl) by weight and the experiments were conducted for a range of CO₂ injection pressures (2, 4, 6, 8, 10, and 12 MPa) at a constant confinement of 20 MPa and a temperature of 35 °C, respectively. According to the results, the degree of salinity of the aquifer’s pore fluid plays a vital role in the effective CO₂ permeability variation which occurs with CO₂ injection, and the effective permeability decreases with increasing salinity in the range of 0–30 % of NaCl. Interestingly, in dry reservoir rock samples, the phase transition of the injection of CO₂ from gas to super-critical condition caused a sudden reduction of CO₂ permeability, related to the slip flow effect which occurs in gas CO₂. Transfer into vapor or super-critical CO₂ causes this slip flow to be largely reduced, reducing the reservoir permeability for CO₂ movement in dry reservoir rock samples. However, this behavior was not observed for water- and brine-saturated samples, and an increasing trend of effective CO₂ permeability was observed with increasing injection pressure. A detailed chemical analysis was then conducted to understand the physical phenomenon causing the salinity effect on effective CO₂ permeability using scanning electron microscopy analyses. Such analyses explain the reason for the observed permeability variations by giving detailed images of the rock sample’s microstructure. There were clear depositions of NaCl crystals in the rock’s pore space, and the amount increased with increasing brine concentration.