Influence of H⁺ on CO₂ solubility during carbonate acidizing

Carbonate acidizing is a matrix stimulation technique used to enhance well productivity and injectivity by reacting carbonate formations with hydrochloric acid (HCl), generating carbon dioxide (CO₂) in aqueous, liquid, gaseous, or supercritical phases depending on pressure, temperature, and fluid composition. Among the ions commonly present in carbonate-acid systems, the effects of calcium, magnesium, and chloride ions on CO₂ solubility have been extensively studied. However, the specific impact of H⁺ remains poorly understood. In this study, acid flooding experiments were conducted at backpressures of 0.1, 2.7, and 8.2 MPa and injection rates of 1, 2, 4, and 8 mL/min at ambient temperature using 1 wt% HCl to investigate H⁺'s influence on CO₂ solubility. Observed differential pressures between the core inlet and outlet suggest that CO₂ solubility is influenced not only by calcium and chloride ions but also by the presence of H⁺. The findings also indicate that the formation of non-aqueous CO₂ can contribute to increases in differential pressure, whereas the absence of such increases does not necessarily imply the absence of non-aqueous CO₂, as it may be masked by other competing mechanisms. Furthermore, the results challenge the common assumption that maintaining backpressures above 6.9 MPa ensures complete CO₂ dissolution. These insights underscore the need for further investigation into CO₂ solubility in acidic, salt-containing systems using specialized measurement setups and support the development of improved predictive models that incorporate the role of H⁺ alongside pressure, temperature, and salt concentration.