Determination of Organic Partitioning Coefficients in Water-Supercritical CO₂ Systems by Simultaneous in Situ UV and Near-Infrared Spectroscopies

CO₂ injected into depleted oil or gas reservoirs for long-term storage has the potential to mobilize organic compounds and distribute them between sediments and reservoir brines. Understanding this process is important when considering health and environmental risks, but little quantitative data currently exists on the partitioning of organics between supercritical CO₂ and water. In this work, a high-pressure, in situ measurement capability was developed to assess the distribution of organics between CO₂ and water at conditions relevant to deep underground storage of CO₂. The apparatus consists of a titanium reactor with quartz windows, near-infrared and UV spectroscopic detectors, and switching valves that facilitate quantitative injection of organic reagents into the pressurized reactor. To demonstrate the utility of the system, partitioning coefficients were determined for benzene in water/supercritical CO₂ over the range 35-65 °C and approximately 25-150 bar. Density changes in the CO₂ phase with increasing pressure were shown to have dramatic impacts on benzene's partitioning behavior. Our partitioning coefficients were approximately 5-15 times lower than values previously determined by ex situ techniques that are prone to sampling losses. The in situ methodology reported here could be applied to quantify the distribution behavior of a wide range of organic compounds that may be present in geologic CO₂ storage scenarios.