Modeling Thermal-Hydrologic-Mechanical Processes for EGS Collab Thermal Circulation Tests Using Embedded Discrete Fracture Model

Abstract

Engineered or Enhanced Geothermal Systems (EGS) are being investigated in a 10-20 meter scale at EGS Collab using multiple scientific monitoring, measurement and characterization tools. The research at EGS-Collab will not only facilitate Frontier Observatory for Research in Geothermal Energy (FORGE) project but also improve our understanding of subsurface stimulation and development of geothermal reservoirs. After stimulation of a hydraulic fracture, starting from May 2019, a thermal circulation test was conducted by injecting cold water into the hydraulic fracture. Endpoint temperatures (at the end of 2019) were measured and flow rate from multiple wells were monitored. With this data in hand, reservoir simulation or numerical modeling of fluid/heat flow should be applied to explore its correctness and applicability, and meanwhile better validate the characterization of the fracture system. In this study, a coupled Thermal-Hydrologic-Mechanical (THM) simulator combined with Embedded Discrete Fracture Model (EDFM) is adopted to model the circulation process. Due to the uncertainty of fracture permeability, fracture geometry, boundary conditions, and fracture network, a thorough investigation of different scenarios has been performed. A simplified one-fracture model was first investigated, followed by a full-scale three fracture model. Several conditions of different fracture permeability, boundary conditions, geomechanics and activated natural fracture system were proposed and simulated. The simulations show that these factors all impact the results to some extent, and production temperature, flow rate and injection pressure can be matched if appropriate parameters are input into the model. Further investigation should be performed with long term circulation data and more characterization data, in order to check the possibilities of these model conditions.