AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Coupled Reactive Flow and Transport Modeling of CO2 Sequestration in the Mount Simon Sandstone Formation, Midwest USA
(1) Geological Sciences, Indiana University, Bloomington, IN.
(2) ExxonMobil Upstream Research Company, Houston, TX.
Multi-phase reactive flow and transport modeling is an effective tool for monitoring, verification, and accounting of CO2 sequestration in deep geological formations. In the current study, modeling is performed to simulate large scale CO2 injection (a million tons per year for 100 years) into Mt. Simon sandstone, a major candidate saline reservoir in the Midwest of USA. The long term fate of CO2 was simulated by extending the modeling period to 10,000 years. The results indicate that most of the injected CO2 remains within a radius of 3300 m lateral distribution. Four major trapping mechanisms and their spatial and temporal variations are evaluated in our simulations: hydrodynamic, solubility, residual, and mineral trapping. A strongly acidified zone (pH 3-5) forms in the areas affected by the injected CO2 (0 - 3300 m), and consequently causes extensive mineral precipitation and dissolution. The predicted long-term fate of CO2 is closely linked to the geochemical reactions conceptualized in the models. In our model, the replenishing upstream water continues to dissolve CO2 long after the injection, which results in total dissolution of hydrodynamically trapped CO2 at the end of 10,000 years. In contrast, most previous models neglected the regional flow after injection and hence artificially limited the extent of geochemical reactions as if in a batch system. Consequently, a supercritical CO2 plume (hydrodynamic trapping) would persist after 10,000 years. The continued supply of acidified water from interaction between replenishing water and CO2 also results in extensive dissolution of feldspars and precipitation of secondary clay minerals, to a much more extent than what predicted in models without including regional flow. However, the prediction of complete dissolution of feldspars in 10,000 years can also result from the artifact that the linear rate laws are used in our model (as well as all previous work), which overestimates the rates of feldspar dissolution near equilibrium. Nevertheless, our simulations indicate the prolonged existence of an acidic brine plume, which suggests long-term risk assessment should transfer from the primary risk of CO2 leakage to secondary risk of acidic plume leakage after all CO2 is dissolved.