| Title | Preliminary Simulations of Carbon Dioxide Injection and Geophysical Monitoring to Improve Imaging and Characterization of Faults and Fractures at EGS Sites |
|---|---|
| Authors | Curtis M. OLDENBURG, Thomas M. DALEY, Andrea BORGIA, Rui ZHANG, Christine DOUGHTY, T.S. RAMAKRISHNAN, Bilgin ALTUNDAS, Nikita CHUGUNOV |
| Year | 2016 |
| Conference | Stanford Geothermal Workshop |
| Keywords | EGS, CO2, Faults, Fractures, Characterization, Active seismic monitoring |
| Abstract | Faults and fractures filled with hot brine are difficult to image and characterize at EGS sites because they are practically indistinguishable from surrounding matrix heterogeneity using traditional seismic and well-logging tools. We are investigating the use of CO2 injection and production (push-pull) for contrast enhancement of faults and fractures for better characterization by active seismic and wireline well-logging approaches. In addition, we are modeling the pressure- and flow-rate-transient response of the system during push-pull to augment geophysical fault and fracture characterization. The approach consists of numerical simulation and feasibility assessment using conceptual models typical of EGS sites. Faults and fractures in the deep subsurface tend to occur in zones and sets with associated damage and gouge regions that are parallel to the slip plane(s) and that provide a larger volume for uptake of CO2 than the volume provided by the slip plane(s) alone. CO2 injected for push-pull well testing has a preference for flowing in the fault and fracture zone rather than entering the matrix because supercritical CO2 is non-wetting relative to water and the permeability of open fractures and fault gouge is much higher than matrix. Numerical flow simulations using TOUGH2/ECO2N show that the CO2 is driven upward by buoyancy in the slip plane during the push cycle over day-long time scales, but also flows into the gouge and damage zones on either side of the slip plane where upward flow due to buoyancy is smaller because of lower permeability. Recovery of CO2 from the slip plane region during the pull cycle is limited because of buoyancy effects. Numerical modeling of elastic wave propagation in a system with discrete fractures was carried out using anisotropic finite difference codes from SPICE with modifications for fracture compliance. We modeled surface and vertical seismic profile (VSP) configurations of time-lapse active-source seismic monitoring with 5% added noise. Results suggest that for a 10 m-thick zone consisting of ten fractures each with 1 mm aperture slip plane, CO2 can be best imaged using time-lapse differencing of the P-wave and P-to-S-wave scattering in a VSP configuration. Wireline well-logging tools that measure electrical conductivity show some promise as another means to detect and image the CO2-filled fracture near the injection well and potential monitoring well(s), especially if a saline-water pre-flush is carried out to enhance conductivity contrast. These multiple complementary characterization approaches will be integrated in iTOUGH2 to carry out joint inversion to develop working models of fault and fracture zone characteristics relevant to EGS energy recovery at prototypical sites. |