Fault and fracture networks with permeability are essential in any geothermal field to provide surfaces for heat transfer and sufficient fluid production rates. Yet characterization of the geometry and hydrogeologic properties of natural faults, fractures, and stimulated fracture networks remains an outstanding problem. We propose to inject and produce supercritical CO2 (scCO2) in and out of faults and fracture networks, using push-pull well experiments coupled with active-source geophysical monitoring and well logging to characterize the fault and fracture characteristics relevant to Enhanced Geothermal Systems (EGS). Replacing formation brine with scCO2 effectively increases the contrast in geophysical properties between fault/fractures and matrix. We present modeling results that explore the technical feasibility of the approach. Using TOUGH2-ECO2N, we simulate the injection and production of CO2 into a normal fault based on faults at the Brady’s geothermal field and model pressure and saturation conditions in the fault zone. Because of the combination of a dipping fault and scCO2 buoyancy, the injected CO2 plume grows upward in the fault gouge against the hanging wall. It does not enter the damage zone because of the non-wetting characteristics of scCO2 relative to the liquid phase, which keeps the gas in the gouge excluding it from the damage zone and matrix. The simulated CO2 injection results are used in the project to model seismic and well-logging approaches to fault characterization. Separate pressure-transient and coupled well-fault injection simulations are also being carried out. We model the response of the system to active seismic monitoring in time-lapse mode using anisotropic finite difference codes from SPICE with modifications for fracture compliance. Results to date show that even narrow fault and fracture zones filled with CO2 can be detected using Vertical Seismic Profiling (VSP). The active seismic simulations are complemented by modeling of well logging, with neutron capture and induction tools showing the most promise for high-temperature systems, especially if saline water pre-flushes are carried out.

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