Real-Time Monitoring of Secondary Mineral Precipitation During CO2–H2O–Rock Interactions Under High Temperature and Pressure Using Fiber Optic Scale Sensors

This study successfully monitored the formation of secondary minerals resulting from CO2–H2O–rock reactions under high-temperature, high-pressure conditions (approximately 250 °C and 6 MPa, respectively) in real time using a sensor based on the attenuated total reflection (ATR) detection principle. First, a verification experiment was conducted using a saturated calcium carbonate solution. This experiment quantitatively confirmed an increase in precipitation and a decrease in transmittance as the temperature increased from 25 °C to 250 °C. Next, CO2–H2O–rock reaction tests were conducted within a batch-type apparatus. Under neutral conditions (pH 7.3), the transmittance rapidly decreased to approximately 20% within five days of initiating the reaction. Combined with our previous results from separate batch-based rock reaction tests conducted under identical conditions, it was revealed that the rapid precipitation of secondary minerals, primarily smectite, was the dominant process. Conventional methods estimate precipitation amounts by analyzing rock surface morphology after reaction tests, which leaves the reaction mechanism unclear. The primary innovation of this study lies in directly capturing precipitation dynamics during the initial reaction stage, which could not be achieved using conventional post reaction analysis methods. By employing this monitoring technique to measure the precipitation rates and quantities of secondary minerals under various test conditions, this study is expected to make significant contributions to the understanding and controlling of precipitation phenomena and changes in formation permeability in CO2 geological storage and carbon-recycling geothermal power generation projects.