SUPERCRITICAL CO₂-BRINE-ROCK INTERACTIONS AT A PROPOSED CARBON SEQUESTRATION/ ENHANCED OIL RECOVERY SITE, VEDDER FORMATION, RIO BRAVO OIL FIELD, SOUTHERN SAN JOAQUIN VALLEY, CALIFORNIA

A potential approach to mitigate CO₂ emissions into the atmosphere is Carbon Capture and Sequestration (CCS). CCS is the process of capturing CO₂ emitted from high CO₂ emission facilities, compressing it into a supercritical state, and injecting it underground into a suitable formation. Previous work has determined that several San Joaquin Valley formations meet the USGS criteria for CCS. One potential CCS candidate, is the Vedder Formation (Rio Bravo oil field). This site is of particular interest because it is a depleted oil reservoir and CO₂ injection can help mobilize residual oil and increase oil production; a process known as enhanced oil recovery (EOR). Potential gas-rock-brine interactions that occur with the injection of CO₂ need to be evaluated to help determine long term CO₂ storage quality for the Vedder Formation.

This study used an experimental and geochemical modeling approach to quantify the changes in aqueous geochemistry and formation mineralogy resulting from the injection of supercritical CO₂ into formation brine. Sample from the Vedder Formation was characterized before exposure to CO₂ by X-ray diffraction (XRD), petrographic microscopy, and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). A high-pressure, high-temperature autoclave experiment was then conducted exposing the sample to supercritical CO₂ for 10 days under reservoir conditions (270 bar, 120°C). The changes observed in the brine chemistry were used to infer mineral dissolution/precipitation and sorption/desorption reactions driven by brine-supercritical CO₂ interactions. Results showed an increase in alkalinity by a factor of 4 and significant increases in K, Ca, and Mg; signifying carbonate and feldspar dissolution. Calcite reaction rates were 7 times faster than inferred feldspar dissolution reaction rates. Changes in trace elements and Sr isotopes were also observed. Geochemical changes were modeled using the PHREEQ-C interactive geochemical modeling program. The reacted sample was examined by SEM-EDS to identify changes in mineralogy and texture.