Evaluating Fluid Circulation in Enhanced Geothermal Systems at the Forge Site Using Thermo-Hydro-Mechanical Models and Fiber-Optic Distributed Strain Measurements
Authors: Ou Yuhao , Liu Qianjun , Hu Jinchuan , Zhao Zihao , Sharma Mukul 
Published: September 2024

Good inter-well connectivity and high fracture network conductivity are crucial to the success of any enhanced geothermal system. Distributed fiber optic data from a geothermal fluid circulation test provides a direct measure of hydraulic connectivity of the wells through hydraulic and natural fractures. The objectives of this paper are to measure and quantitatively model the thermal and flow connectivity of a well pair in an enhanced geothermal system (EGS).

High temperature fiber optic cables were installed in the production well at the FORGE site in Utah. They provide real-time, induced strain and temperature measurements with high spatial resolution and sensitivity during geothermal fluid circulation. To better understand, interpret and quantify the fiber-optic data, a fully coupled thermo-mechanical model is developed to simulate fluid circulation within the EGS. The model implicitly couples and solves for the temperature, fluid pressure and strain in the entire fracture-reservoir-wellbore system. An implicit fracture contact model is implemented to account for the dynamic changes in fracture width and fracture conductivity during fluid circulation. The fiber optic response is computed and visualized by plotting the strain rates along the producer over time. The simulations are then used to interpret field measurements and evaluate the target geothermal fluid circulation system.

The simulated temperature-strain evolution match well with recorded fiber optic data. Clear tensional strain rate signals are observed, showing the locations of hydraulic fractures intersecting the wellbore. A small initial fracture conductivity leads to high formation resistance to fluid circulation, revealing the reasons for the observed injection pressure build-up. The simulated pressure-rate evolution is consistent with field data recorded at the injector and producer. This allows us to quantitatively explain the results observed in the fluid circulation tests. Fluid storage in the mechanical stimulated reservoir volume (SRV) and gradual dilation of the connected fractures create slow moving pressure fronts towards the producer. Our fiber optic data clearly detects the rock deformation induced by the pressure and thermal fronts. A sensitivity study reveals that fractures with larger normal stiffness (higher overall fracture conductivity) make the tensional strain rate signal occur earlier and last longer on the fiber optic waterfall plot, while larger SRV compressibility delays the signal. Strain values are negatively correlated with SRV Young's modulus. A larger simulated reservoir volume around fractures leads to wider tensional signals. This analysis and interpretation help field engineers to evaluate and better design fracture treatments (better inter-well connectivity) in upcoming fracture stages.

The paper, for the first time, integrates field measured fiber optic, injection rate and pressure data with a comprehensive thermo-mechanical model for simulating the complex strain evolution in EGS wells during geothermal fluid circulation. The new and unique data set together with our new modeling capability that integrate temperature, fluid flow and rock deformation allow us to correctly interpret the data and obtain a much better estimate of the evolution of the fracture network connecting the injection and production wells.