The estimation of the pH of CO₂-saturated water/brine binary systems under CO₂ saline aquifer storage conditions

Carbon capture and storage (CCS) is a method of reducing carbon dioxide (CO₂) emissions, which could be crucial in addressing global warming. Accurately estimating the pH of CO₂-saturated water/brine binary systems is an important physical–chemical property of reservoir fluids at CO₂ injection sites. Although there has been substantial research on the pH of CO₂-saturated brines, it mainly focuses on brine systems containing monovalent cations. Research on the pH of brine systems containing bivalent cations is lacking. The aim of this paper is to develop a chemical equilibrium model based on pressure–volume–temperature (PVT) data around the mechanism of CO₂ dissolution–acidification and the CO₂ chemical equilibrium process in brine aquifers, which can be used to predict the pH value of the CO₂-saturated brine system (0–4 mol kg⁻¹) under high-temperature (273–473 K) and high-pressure (up to 100 MPa) conditions. The model employs phase equilibrium equations to describe the CO₂-water/brine dissolution–acidification process, with corrections to the CO₂ activity coefficient part of the Spycher and Pruess model. The results showed that the predicted pH values were in good agreement with the experimental data. The pH of the CO₂-saturated CaCl₂(aq) and MgCl₂(aq) systems was predicted using both a chemical equilibrium model and the PHREEQC geochemical simulator based on the Pitzer model. The differences in the predicted values of the two models were analyzed. Based on the process of CO₂ dissolution acidification, a reliable empirical equation was established to calculate the pH of CO₂-saturated water/brine system, and it was applied to the storage of CO₂ brine layer.