Influence of CO₂–Brine Co-injection on CO₂ Storage Capacity Enhancement in Deep Saline Aquifers: An Experimental Study on Hawkesbury Sandstone Formation

This study was conducted to identify a potential CO 2 -storage-capacity-enhancement technique: enhancement of CO 2 storage through co-injection or the simultaneous injection of CO 2 and brine into saline aquifers. A series of triaxial permeability tests on brine-saturated sandstone samples for a range of injection pressures (4–10 MPa) under different confining pressures (20–35 MPa) was performed at 35 °C constant temperature. To clearly identify the sequestration-enhancement capability of the proposed novel method, conventional CO 2 injection (injection of only CO 2 into brine-saturated rock) was also carried out under the same test conditions. Both tests were subjected to comprehensive chemical analyses to evaluate the alteration of CO 2 solubility in brine caused by the proposed method. CO 2 flow behavior under conventional CO 2 injection may convert from two- to single-phase flow over time around the injection point, as a result of forced brine migration. This may create a suddenly increased advective flux sometime after injection, which negatively affects the carbonation efficiency and eventually the mineral trapping process. Such issues can be eliminated by the proposed CO 2 –brine co-injection technique, because it offers significantly slower flow rates under the continuously available two-phase condition in the system. According to the results, the resulting slower flow rates under the co-injection process further enhance the mineral trapping process, allowing for additional time to activate the dissolution reaction compared to the conventional injection process. This will enable field projects to reduce the extensive time required for CO 2 solubility in brine. Importantly, the solubility enhancement resulting from the proposed co-injection is much greater for supercritical CO 2 . This demonstrates the better performance expected of the proposed novel co-injection process under field conditions, because CO 2 generally exists in its supercritical state in deep saline aquifers. In addition, the results show that the resulting H + ion concentration increases with an increasing confining pressure, confirming the positive influence of aquifer depth on solubility under any injection conditions. Interestingly, this depth effect on solubility enhancement appears to be greater for co-injection than conventional injection. The proposed co-injection technique is also favorable for the long-term safety of the process, because the associated reduced CO 2 relative permeability in saline aquifers also offers more opportunity for hydrodynamic trapping mechanisms.