Carbonate minerals and CO2 are both considerably more soluble at low temperatures than they are at elevated temperatures. This inverse solubility has led a number of researchers to hypothesize that injecting low-temperature (i.e., less than the background reservoir temperature) CO2 into deep, saline reservoirs for CO2 Capture, Utilization, and Storage (CCUS) will dissolve CO2 and carbonate minerals near the injection well and subsequently exsolve and re-precipitate these phases as the fluids flow into the geothermally warm portion of the reservoir. In this study, we utilize high performance computing to examine the coupled effects of cool CO2 injection and background hydraulic head gradients on reservoir-scale mineral volume changes. We employ the fully coupled reactive transport simulator PFLOTRAN with calculations distributed over up to 800 processors to test 21 scenarios designed to represent a range of reservoir depths, hydraulic head gradients, and CO2 injection rates and temperatures. In the default simulations, 50°C CO2 is injected at a rate of 50kg/s into a 200bar, 100°C calcite or dolomite reservoir. By comparing these simulations with others run at varying conditions, we show that the effect of cool CO2 injection on reservoir-scale mineral volume changes tends to be relatively minor. We conclude that the low heat capacity of CO2 effectively prevents low-temperature CO2 injection from decreasing the temperature across large portions of the simulated carbonate reservoirs. This small thermal perturbation, combined with the low relative permeability of brine within the supercritical CO2 plume, yields limited dissolution and precipitation effects directly attributable to cool CO2 injection. Finally, we calculate that relatively high water-to-rock ratios, which may occur over much longer CCUS reservoir lifetimes or in materials with sufficiently high brine relative permeability within the supercritical CO2 plume, would be required to substantially affect injectivity through thermally-induced mineral dissolution and precipitation. Importantly, this study shows the utility of reservoir scale-reactive transport simulators for testing hypotheses and placing laboratory-scale observations into a CCUS reservoir-scale context.
Bibliographical noteFunding Information:
We gratefully acknowledge support from the Department of Energy ( DOE ) Geothermal Technologies Program under Grant Number EE0002764 and from the Initiative for Renewable Energy and the Environment ( IREE ), a signature program of the Institute on the Environment ( IonE ) at the University of Minnesota (UMN) for this contribution and related research. MOS also acknowledges support for this and related research from an NSF Sustainable Energy Pathways (SEP) grant, NSF SEP-1230691 , and is grateful for the support of the Hydrogeology and Geofluids research group by the George and Orpha Gibson Endowment. MOS also thanks the Werner Siemens Foundation for its support of the Geothermal Energy and Geofluids group in the Institute of Geophysics, Department of Earth Sciences, ETH-Zürich, Zürich, Switzerland. BMT acknowledges receipt of the UMN Doctoral Dissertation Fellowship, which provided funding for a portion of this research. This work was carried out using computing resources at the University of Minnesota Supercomputing Institute (MSI). Any opinions, findings, conclusions, or recommendations in this material are those of the authors and do not necessarily reflect the views of the DOE, NSF, IREE, IonE, MSI, ETH-Zürich, or UMN. Glenn Hammond and Peter Lichtner are additionally acknowledged for their assistance with PFLOTRAN input files. Associate Editor Jens Birkholzer and two anonymous reviewers are gratefully thanked for their handling and helpful comments, which helped to improve the quality and impact of this manuscript.
- Reactive transport