The EGS Collab Hydrofracture Experiment at the Sanford Underground Research Facility - Campaign Cross-Borehole Seismic Characterization

摘要

The enhanced geothermal system (EGS) multi-laboratory and multi-university collaborative (Collab) project brings together skilled and experienced scientists and engineers in the areas of subsurface process modeling, monitoring, and a series of experiments to focus on intermediate-scale EGS reservoir generation processes and related model validation at crystalline rock sites. Cooperative research under the EGS Collab project will provide a foundation of knowledge and modeling capability that form a bridge to meeting the challenges of EGS development and proliferation. The EGS Collab project is being performed within the re-purposed mine workings (drifts) of Sanford Underground Research Facility (SURF), located in Lead, South Dakota, USA. For the first experiment, a suite of sub-horizontal boreholes was drilled from within one of the SURF mine drifts directly into the surrounding crystalline rock formation. The suite is comprised of one stimulation well, one production well, and six monitoring wells. The goal of this experiment is to generate fractures radiating from the stimulation well that intersect the production well, and then perform flow testing of this inter-well/hydrofracture system. Stimulation and flow is monitored with micro-earthquake (MEQ) and acoustic emission (AE) instrumentation that is grouted into the monitoring wells. A fundamental component of MEQ/AE monitoring requires campaign-style cross-borehole seismic characterization. Cross-borehole techniques include compressional (P-) and Shear (S-) wave tomography. The results provide baseline P- and S-wave velocity models that are critical in calibrating hypocenter locations from MEQ/AE monitoring, and also yield elastic moduli data/constraints that will be utilized for stimulation modeling. A supplemental goal is to perform velocity change detection analysis by collecting cross-borehole P- and S-wave data between the stimulation and production wells prior and subsequent to fracture stimulation.