The EGS Collab Project – Fracture Stimulation and Flow Experiments for Coupled Process Model Validation at the Sanford Underground Research Facility (SURF), South Dakota, USA

Abstract

Successful widespread deployment of Enhanced Geothermal Systems (EGS) will require accurate predictions of flow rates through induced and natural fractures and changes in produced fluid temperatures over time. Complex, heterogeneous fracture pathways can lead to channeling, short-circuiting, premature thermal breakthrough, and loss of injected fluid, thus complicating EGS heat extraction. Field testing of fracture stimulation methods and the resulting fluid flow and heat transfer through these engineered reservoirs can constrain and validate coupled process models needed to design and monitor the performance of EGS. The objective of the EGS Collab project is to establish intermediate-scale (~10 meter) field test beds coupled with stimulation and interwell flow tests to provide a basis to better understand fracture stimulation methods, resulting fracture geometries, and processes that control heat transfer between rock and stimulated fractures. We have developed the first experimental test bed for conducting these experiments at the Sanford Underground Research Facility (SURF), located in Lead, SD, USA. This test bed is located on the 4850 Level (1478 meters below the ground surface) of the former Homestake gold mine in the Precambrian Poorman Formation phyllite. The test bed consists of a stimulation/injection borehole, a production borehole, and six highly instrumented monitoring boreholes. The axes of the injection and production boreholes were designed to be parallel to Shmin, which should cause hydraulic fractures to be preferentially generated perpendicular to the boreholes. After first characterizing the test bed, we have performed a series of hydraulic stimulation experiments at different notched intervals with the goal of creating a series of hydraulic fractures that connect the injection and production boreholes. For these stimulated zones, we have conducted a series of flow, tracer, and heat exchange experiments, and have incorporated these results, along with study of the core and image log data, to create a discrete fracture network model of our test bed. These experiments have been closely monitored using a comprehensive suite of instrumentation consisting of continuous active-source seismic monitoring (CASSM), passive microseismic (MEQ), electrical resistivity tomography (ERT), borehole pressure monitoring, in situ borehole deformation monitoring using the novel Step-Rate Injection Method for Fracture In-Situ Properties (SIMFIP) tool, and continuous distributed monitoring of temperature, seismicity, and strain using fiber optic cables. These experiments provide a means of testing concepts, tools, and codes that could later be employed under geothermal reservoir conditions at the Frontier Observatory for Research in Geothermal Energy (FORGE) and in future industry-scale EGS projects. Concurrent with the meso-scale experiments, we are advancing our numerical simulation capabilities via novel approaches to applying existing simulators and the implementation of new schemes modifying existing computer codes. Pre- and post-test modeling of each test allows for improved experimental and monitoring design, as well as model prediction and validation. These data are being analyzed and compared with models and field observations to further elucidate the basic relationships between stress, induced seismicity, and permeability enhancement. We will observe and quantify other key governing parameters that impact permeability, and will attempt to understand how these parameters might change throughout the development and operation of an EGS project with the goal of enabling commercial viability of EGS.