EVALUATION OF CO2 TRAPPING MECHANISMS AT THE SACROC NORTHERN PLATFORM: SITE OF 35 YEARS OF CO2 INJECTION By

The main purpose of this study is to assess the potential fate of CO2 when it is injected in the deep subsurface for storage. In the coming year (2008), over 20 field geologic sequestration tests are being designed and scheduled for deployment in the United States. An additional 23 are ongoing or slated for deployment soon in other countries. For large-scale geologic sequestration to be deployed and sustainable over the long-term, a meaningful assessment of CO2 trapping mechanisms is essential. Therefore, the chapters in this dissertation are each part of a detailed study of the physical and chemical processes associated with CO2 trapping mechanisms. First, two integrated equations of state (EOS) algorithms were assembled to evaluate multiphase transport of carbon dioxide (CO2). One integrated EOS is based on Redlich and Kwong’s original algorithm developed in 1949, and the other is based on a more recent algorithm developed by Span and Wagner in 1996. Both algorithms calculate solubility, compressibility factor, density, viscosity, fugacity, and enthalpy of CO2 in gaseous and supercritical phases, and mixtures or solutions of CO2 in water, as functions of pressure and temperature. In general, the predictions of thermophysical properties for both algorithms are close, except for a contrast in the predicted fugacity coefficient of CO2, which subsequently propagates to a contrast in predicted solubility in water/brine. To facilitate a general comparison of these EOS algorithms, one-dimensional flow models were developed. Simulation results suggest that dissolution rates of separate-phase CO2 predicted from the two EOS algorithms are significantly different, resulting in markedly different CO2 migration patterns. Finally, a specific case study of CO2 trapping mechanisms and related processes was carried out using numerical simulation. The site of the case study was the SACROC (Scurry Area Canyon Reef Operators Committee) Unit in the Permian Basin of Texas, the oldest continuous CO2 enhanced oil recovery (EOR) site in the United States. SACROC has been subjected to CO2 injection for 35 years, and thus provides an excellent field laboratory for this analysis. Not all CO2 injected for EOR has been recovered, and thus some is trapped in the subsurface. A comprehensive numerical model was developed for analyzing the range of possible trapping mechanisms and other related physical and chemical effects of the injected CO2. Data used to parameterize the three-dimensional model includes spatial distributions of seismicand core-derived porosity and permeability that were estimated by the Texas Bureau of Economic Geology. Two separate models were developed for the analysis of CO2 trapping mechanism. The first model was designed for simulating CO2 trapping mechanisms in a reservoir saturated with brine. The other model was designed for simulating CO2 trapping mechanisms in a reservoir saturated with both brine and oil. CO2 trapping mechanisms in the brine-only model show a distinctive set of stages. In Stage I (1972~2002), the same as the original injection period, hydrostratigraphic trapping is dominant. In Stage II (2002~2017), residual trapping dramatically increases. In Stage III (2017 to several hundreds years), solubility trapping becomes important because over time both the residual and mobile CO2 dissolve in the brine. Finally, at stage IV (~after several thousand years), mineral trapping is predicted to be greater than any other mechanisms. In sum, the major CO2 trapping mechanisms were hydrostratigraphic (mobile), residual, and solubility trapping during 200 years. However, in the brine (28%) plus oil (72%) model, the CO2 trapping mechanisms do not vary much over time. Separate-phase CO2 is stored as a free (mobile) form due to its high saturation, but it behaves like residual (immobile) CO2 because of the smaller contrast between fluid densities of oil and CO2. Further, CO2 mobility is hindered by other fluids such as brine and oil in model due to the reduction of relative permeability. In sum, both oil trapping and hydrostratigraphic (mobile) trapping were dominant mechanisms during 200 years. The combined results of these two model analyses suggest that injecting CO2 into brine formations below oil reservoirs will provide several advantages in terms of CO2 storage capacity and protection from potential leakage. Results of this dissertation are intended to provide insight regarding effective approaches for geologic CO2 sequestration. Specifically, understanding the potential range of CO2 trapping mechanisms in typical reservoirs may help scientists and engineers evaluate sequestration operations to maximize trapping and minimize risks.