CO2/Water Interfacial Tension under Induced Acidic Conditions Employing Dissipative Particle Dynamics Simulations

Evaluating the interfacial tension (IFT) between CO2 and water is necessary for assessing the structural integrity of the caprock during geological carbon storage. While prior studies have examined the IFT of the CO2-water system, there are limited studies on how the solubility of CO2 in aquifer water, that leads to acidic conditions, influences the IFT of CO2-water system. This study employed dissipative particle dynamics (DPD) simulations to investigate the IFT of a CO2-water system under situations commonly found in deep saline aquifers. The interaction parameters associated with the DPD force-field were determined through molecular dynamic simulation. The IFT is calculated through the Irving-Kirkwood equation, which considers both normal and tangential forces interacting on the surface of the interface to calculate IFT. Furthermore, the IFT of CO2-water system has also been analyzed using qualitative methods such as radius of gyration, interfacial thickness, and mean square displacement (MSD). The outcomes of the present investigation indicate that the IFT of the CO2-water system decreases when exposed to acidic surroundings, irrespective of the conditions of the aquifer. For instance, the IFT experienced a significant decrease from 49.52 mN/m under nonacidic situations to 47.71 mN/m under acidic conditions at a pressure of 15 MPa and a temperature of 353 K. The qualitative investigation demonstrated that acidic conditions cause a reduction in the radius of gyration and MSD of both CO2 and water, while causing an overall increase in their interfacial width. Furthermore, the IFT of the CO2 and water system decreased considerably as the pressure increased. However, the IFT exhibits a direct relation to both the temperature and the amount of salts in the brine. This study illustrates that the effectiveness of caprock in holding CO2 is reduced by induced acidic conditions. Consequently, the probability of CO2 being released from aquifers is increased.