Geochemical Monitoring of Fluid-Rock Interaction and CO2 Storage at the Weyburn CO2-Injection Enhanced Oil Recovery Site, Saskatchewan, Canada

Abstract

The Weyburn field is one of a number of large oilfields that lie along the Mississippian subcrop belt on the northern extent of the Williston Basin. An opportunity to study the potential for geological storage of CO2 was established through the multidisciplinary IEA Weyburn CO2–EOR (enhanced oil recovery) Monitoring program in which geochemical monitoring and modeling plays an important role. Pre- (Baseline) and post-injection (Monitor-I) samples of produced fluids from approximately 45 wells were taken and chemically analyzed to determine changes in the fluid chemistry and isotope composition over time. After CO2 injection began, the general spatial distribution patterns were similar, but pH has dropped, HCO3, Ca, Mg concentrations and δ13CO2 values increased, pointing towards carbonate dissolution. Isotope data require a heavy source of carbon for CO2, most easily attributed to dissolution of Mississippian carbonate minerals. Addition of CO2 causes dissolution of carbonate minerals and production of bicarbonate until equilibrium with the carbonate minerals is reached. However, larger amounts of CO2 can be trapped if basic silicate minerals are present, which react with the CO2 to buffer pH, increasing the bicarbonate in the formation water or precipitating the CO2 as carbonate minerals until the basic silicate minerals are consumed. Modeling of water-rock reactions suggests that clay minerals may be present and capable of acting as pH buffers, allowing injected CO2 to be stored as freshly formed bicarbonate in the formation water or newly precipitated carbonate minerals, if the kinetics of reaction are favorable.