In Situ Spectrophotometric Determination of pH under Geologic CO₂ Sequestration Conditions: Method Development and Application

CO₂ injection into deep geologic formations for long-term storage will cause a decrease in aqueous pH due to CO₂ dissolution into reservoir water/brine. Current studies seeking to assess chemical changes under geological CO₂ sequestration (GCS) conditions rely largely on thermodynamic modeling due to the lack of reliable experimental methods. In this work, a spectrophotometric method utilizing bromophenol blue to measure pH in laboratory experiments under GCS-relevant conditions was developed. The method was tested in simulated reservoir fluids (CO₂–NaCl–H₂O) at different temperatures, pressures, and ionic strengths, and the results were compared with those from other experimental studies and geochemical models. Measured pH values were generally in agreement with the models, but inconsistencies were present between the models. In situ pH measurements for a basalt rock–CO₂–brine system were conducted under GCS conditions. The pH increased to 3.52 during a 10-day period due to rock dissolution, compared to pH 2.95 for the CO₂–brine system without rock. The calculated pH values from geochemical models were 0.22–0.25 units higher than the measured values (assuming all iron in the system was in the form of Fe²⁺). This work demonstrates the use of in situ spectrophotometry for pH measurement under GCS-relevant conditions.