The increased mobility and buoyancy of CO2, coupled with rising demands for renewable energy production, make it an attractive alternative heat-exchange fluid in lieu of water for use in enhanced geothermal systems (EGS). However, the geochemical impact of elevated CO2 levels on these engineered systems and the consequent effects on reservoir capacity and system permeability are poorly-constrained at present, leading to uncertainty in predictions of longer-term reservoir performance. For example, consistently high levels of aqueous CO2 cycling through the subsurface may result in relatively rapid and extensive dissolution of pH-sensitive minerals, with an increased risk of secondary alteration phase precipitation (e.g., oxides, clays, and/or carbonate minerals) and adverse effects on EGS resource productivity. If, however, injected CO2-rich fluids traverse the system primarily through fracture networks, other factors such as accessibly reactive surface areas, fracture asperity susceptibility, and fracture surface/wallrock exchange may also factor into the ultimate evolution of reservoir permeability. To evaluate the comparative impacts of both geochemistry and geomechanics in sustaining fracture network flow under conditions relevant to CO2-EGS, a 60-day core-flooding experiment was conducted on a naturally fractured and chemically complex greywacke core sample exposed to CO2-acidified brine at 200C and 25MPa. The objective of this study was to investigate the potential effects of using CO2 as a heat-exchange fluid on fracture flow and reaction within a pre-existing well-characterized fracture representative of reactivated fractures targeted for stimulation in many EGS projects. Over the course of the experiment, changes in solution chemistry and pressure/permeability were monitored. In addition, pre- and post-reaction three-dimensional high-resolution X-ray computed tomography (XRCT) imaging was used to determine changes in fracture aperture and geometry as well as compositional variation and extent of reaction. Particle image velocimetry (PIV) analysis is applied to the XRCT data to reveal local deformation in the core and improve calculation of the porosity generated during experimental brine-CO2-core reaction. This experimental dataset will be used to calibrate a coupled numerical geochemical and geomechanical model describing fracture growth and sealing within the fracture network in response to geochemical changes. Experimental and simulation results will be presented and their implications for CO2-EGS discussed. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
|Number of pages||1|
|Publication status||Published - 2012|
|Event||Fall Meeting of the American-Geophysical-Union 2012 - San Francisco, United States of America|
Duration: 3 Dec 2012 → 7 Dec 2012
|Conference||Fall Meeting of the American-Geophysical-Union 2012|
|Country||United States of America|
|Period||3/12/12 → 7/12/12|