| Title | Supercritical CO2 in a Granite-Hosted Geothermal System: Experimental Insights into Multiphase Fluid-Rock Interactions |
|---|---|
| Authors | LO RÉ, Caroline; KASZUBA, John; MOORE, Joseph; MCPHERSON, Brian |
| Year | 2012 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Enhanced Geothermal Systems, experimental, CO2, granite, Roosevelt, EGS |
| Abstract | In commercial geothermal operations, it is imperative that permeability and porosity not be reduced by chemical reactions instigated by working fluids. Supercritical CO2 has recently been proposed as a working fluid in enhanced geothermal systems (EGS) due to its low viscosity, large expansivity, and reduced reactivity with rock as compared to water. However, the interaction of supercritical CO2 with groundwater and host rock may induce dissolution/precipitation reactions as CO2-brine mixtures migrate through the reservoir; unfavorable reductions of permeability and porosity may result. Geochemical modeling and hydrothermal experiments are underway to evaluate associated geochemical and mineralogical relationships and to determine how geothermal systems may respond given ‘spontaneous’ injection of CO2 into a granitic reservoir. Simulations and experiments emulate geothermal conditions and geochemistry of granitic reservoirs such as the Roosevelt Geothermal Field, Utah. Geochemical calculations were performed using Geochemist’s Workbench, the b-dot ion association model, and the resident thermodynamic database thermo.com.V8.R6+.dat. In addition, the Duan et al. (2006) equations of state for CO2 are utilized in calculations simulating CO2-brine-rock interactions. Initial model calculations were conducted to determine a brine chemistry that would be as close to equilibrium as possible with the granite. This was done to minimize water-rock interaction in the hydrothermal experiments prior to CO2 injection. The model was then used to simulate experimental results, pre- and post-CO2 injection. Preliminary modeling indicates that pre-injection reactions will likely include feldspar alteration to clay minerals and/or low-temperature zeolites. Post-CO2 injection calculations suggest carbonates may precipitate at the expense of Ca, Mg, and Fe silicates. Hydrothermal experiments are conducted in rocking autoclaves (rocker bombs) and flexible Au-Ti reaction cells (Dickson cells) using established methods (Seyfried et al., 1987). The granite consists of 75% ground (28 days. The initial water:rock:supercritical CO2 ratio is ~83:6:17 by mass. Excess CO2 is injected to produce a separate supercritical CO2 phase, ensuring aqueous CO2 saturation for the duration of each experiment. Results from this study will inform our understanding of fluid-rock interactions in CO2-brine-granite systems, especially with respect to possible dissolution/precipitation reactions. |