document preview

Abstract
A theoretical aquifer model predicts time-lapse mineral carbonation and isotope fractionation of injected CO2 in sediment. Geologic sequestration of CO2 has become one of the promising ways to reduce atmospheric emission of CO2 from human activity. However, the current and future effects of geologic storage after injecting CO2 are not known well. We developed a simple mathematical model based on a transport-reaction equation and calculated the abundance of carbon stable isotope in the reservoir with respect to time which allows us to predict CO2 saturation in sediment or CO2 flume distribution by ground reservoir water. These results indicate significant potential of the theoretical aquifer model for monitoring and verification of CO2 sequestration into the sediment.

Introduction
Geologic sequestration of carbon dioxide (CO2) is a promising solution for not only reducing global atmospheric emissions of CO2 but also decreasing changes in the global climate system (Houghton et al. 1996). Currently, seven Regional Carbon Sequestration Partnerships (RCSPs) created by DOE have performed large-scale CO2 injection in different regions: the Jurassic Age sandstone formation throughout from Wyoming to New Mexico (SWP), the Tuscaloosa Massive sandstone (SECARB), the deep saline sandstone formation in the Alberta Basin in Northwest British Columbia (PCOR), the Illinois Basin (MGSC), the Midwest region (MRCSP), the San Joaquin Basin in Central California (WESTCARB), and the Nugget sandstone formation in Southwest Wyoming (Big Sky). Each partnership is presently injecting or will be injecting over one million tons of CO2 per year1.

Once injected into an aquifer, dissolved CO2 may react with metallic cations in pore water. This chemical reaction produces inorganic carbonates such as calcium carbonate (CaCO3), aragonite (CaCO3), siderite (FeCO3), and magnesium carbonate (MgCO3) that are stable over long time scales.

The precipitation of inorganic carbonates derived from injected CO2 is important because it sequesters enormous quantities of carbon as part of the global carbon cycle. Thus, designing research program to sequester CO2 as carbonate rock in geological formation has been an outstanding issue for carbon sequestration (Aresta 1987; Seifritz 1992; Dunsmore 1992).

As CO2 is injected into geologic formations, the chemical reaction between dissolved CO2 and metallic cations in pore water results in carbon mineralization accompanied by large fractionations of carbon isotopes between 12C and 13C. The fractionation of carbon isotopes may be controlled by reservoir conditions of pressure, temperature, mineral compositions in pore water, and an injection rate of CO2.

Since large-scale CO2 is injected, concerns about leakage of CO2 from a geologic storage reservoir have been arisen for safe long-term storage. Stable isotopes can verify long-term storage of injected CO2 in geologic formation (Johnson et al. 2009). The carbon isotope ratio is a sensitive diagnostic technology to distinguish between ground water samples within CO2 reservoir zone and ambient water samples.

In order to monitor leakage of CO2 from a geologic storage reservoir, we need to predict time-lapse carbonation between water samples with and without isotope fractionation of injected CO2 in aquifer. In this study, we injected CO2 for a certain amount of time period and shut in the injection well and generated CO2 flume and properties distribution in the reservoir with respect to time. After pressure driven CO2 transport stops we assumed that the transport of CO2 is controlled by diffusion and reactions. Therefore, we developed a simple mathematical model based on a transport-reaction equation and estimated the abundance of carbon stable isotope in the reservoir with respect to time. This paper concludes with results for significant potential of the theoretical aquifer model for monitoring and verification of CO2 sequestration into the sediment.

1www.fossil.energy.gov