Evaluation of experimentally measured and model-calculated pH for rock-brine-CO₂ systems under geologic CO₂ sequestration conditions

Reliable pH estimation is essential for understanding the geochemical reactions that occur in rock-brine-CO₂ systems when CO₂ is injected into deep geologic formations for long-term storage. Due to a lack of reliable experimental methods, most laboratory studies of CO₂-rock-brine interactions conducted under geologic CO₂ sequestration (GCS) conditions have relied on thermodynamic modeling to estimate pH; however, the accuracy of these model predictions is typically uncertain. In this study, we expanded the measurement range of a spectrophotometric method for pH determination, and we applied the method to measure the pH in batch-reactor experiments at 75°C and 100atm utilizing rock samples from five ongoing GCS demonstration projects. A combination of color-changing pH indicators, bromophenol blue and bromocresol green, was shown to enable measurements over the pH range of 2.5-5.2. In-situ pH measurements were compared with pH values calculated using geochemical models. Calculations with four different thermodynamic databases resulted in a maximum difference of 0.16pH units. Among these databases, the Phrqpitz database generally provided the most accurate pH predictions for rocks comprised of carbonate, siltstone, and sandstone. With Phrqpitz, the differences between measured and calculated pH values were within 0.03pH units for these three rocks. However, for basalt, significant differences (0.10-0.25pH units) were observed even with Phrqpitz. These discrepancies may be due to the models' failure to fully account for certain proton consuming and producing reactions that occur between the basalt minerals and CO₂-saturated brine solutions.