| Title | Modeling Studies of CO2 Injection for Imaging and Characterizing Faults in Geothermal Systems |
|---|---|
| Authors | Curtis M. OLDENBURG, Andrea BORGIA, Rui ZHANG, Yoojin JUNG, Kyung Jae LEE, Christine DOUGHTY, Thomas M. DALEY, Nikita CHUGUNOV, Bilgin ALTUNDAS |
| Year | 2018 |
| Conference | Stanford Geothermal Workshop |
| Keywords | EGS, CO2, faults, fractures, characterization, active seismic monitoring |
| Abstract | We have carried out a modeling project aimed at assessing the utility of using supercritical carbon dioxide (scCO2) injection to enhance fault characterization in geothermal systems. The methods we used in the study included numerical simulations of push-pull CO2 injection using TOUGH2/ECO2N, including inversion, sensitivity, and data-worth analyses using iTOUGH2, dynamic range assessment of well-logging tools for high-temperature systems, and simulation of seismic monitoring using a finite difference code based on the SPICE codes. The prototypical enhanced geothermal system (EGS) site we focused on is the single-fault (Brady’s-type) system at Desert Peak, Nevada, but we also investigated data-worth at a conjugate fault system based on the Dixie Valley geothermal system. Results of simulations of CO2 push-pull injection into a single dipping fault modeled after the Desert Peak site show that CO2 migrates upward in the fault gouge against the hanging wall with limited entry into the damage zone because scCO2 is non-wetting relative to the liquid phase. During the pull phase, mostly water is produced because upward buoyancy puts the CO2 out of reach of fluid drawdown in the well. Using the simulated pressures and saturations of CO2 and brine in the fault gouge, we analyzed the feasibility of well logging and active seismic monitoring to detect the CO2 and contribute to characterizing the fault. Dynamic range effective medium modeling of various high-temperature well-logging tools suggests that neutron capture is the most promising approach in the cased-hole environment provided there is enough salinity contrast, e.g., as could be facilitated by pre-flush with high-salinity brine. As for active seismic monitoring, the time-lapse crosswell geometry produces the strongest signal with time-lapse differences of 1-10% resulting from CO2 migration in the fault gouge. The pressure transient of CO2 injection into a single fault shows unique traits due to the multiphase flow conditions developed by CO2 injection. Fault gouge permeability can be estimated from pressure transient data. CO2 injection into a dual fault system (conjugate fault) such as that at Dixie Valley results in CO2 entering both limbs of the fault, with CO2 migration and pressure dissipation in the faults controlled by the permeability of surrounding damage and matrix components of the fault zone. We carried out data-worth analysis for hydraulic data from a dual fault system such as that at Dixie Valley, and we determined that pressures in the gouge at approximately one-half the depth of the injection point are the most valuable observation data to forecast CO2 distribution in the faults. In summary, modeling and simulation of CO2 push-pull hydraulic well testing with sensitivity and data-worth analysis, crosswell active seismic monitoring, and well logging suggest that these approaches are complementary and capable of providing useful characterization information for fault zones in EGS systems. |