Investigation of the pH effect of a typical host rock and buffer solution on CO2 sequestration in synthetic brines

Carbon dioxide sequestration in deep saline aquifers is a critical component of long-term storage options. It is suggested that the precipitation of mineral carbonates is mostly dependent on brine pH and is favoured above a basic pH of 9.0. However, brine pH will drop to acidic values once CO2 is injected into the brine. Therefore, there is a need to raise brine pH and maintain it stable. Synthetic brines were used here instead of natural brines because of the difficulty in obtaining and storing natural brines. Therefore, experiments were conducted to prepare a series of synthetic brines and to compare their suitability to natural brines for carbon sequestration firstly. A typical host rock (Oriskany rock) and a buffer solution (NaCl/NaHCO3) were selected to buffer brine pH. In a subsequent step, studies were conducted to correlate how brine samples respond in the presence of the host rock or the buffer solution at realistic reservoir temperatures (40 and 100 °C) and pressures (1160 and 1500 psi) for CO2 storage. The results show that synthetic brines prepared can be used as analogues as natural brines for carbon sequestration studies in terms of chemical composition and pH response. Both XRD and SEM/EDS analyses confirmed the presence of mineral carbonates in the CO2–rock–brine and the CO2–buffer–brine experiments. However, the amount of carbonates precipitated from the CO2–buffer–brine reactions is nearly 18 times larger than that formed from the CO2–rock–brine experiments. ICP-MS studies also verified that there was only 4% reduction in Ca concentration in solution after the CO2–rock–brine studies, while the concentrations of Ca and Sr decreased by 90% during the CO2–buffer–brine experiments.