EGS Collab is a series of meso-scale experiments and associated numerical simulation activities being funded by the United States Department of Energy, Geothermal Technologies Office (GTO) to investigate enhanced geothermal system processes under in-situ stress and slightly elevated temperature conditions. This project is designed to provide scientists and engineers with immediate access to impermeable rock at scales larger than possible in the laboratory, but generally smaller than those for commercial production. Immediate access to rock is provided via the existing drifts of the former Homestake Gold Mine, now operated as the Sanford Underground Research Facility in Lead, South Dakota. Experiment 1 involved the creation of a fracture network hydraulically connecting two boreholes within a volume of predominately phyllite of the Precambrian Poorman formation off the western side of the West Access drift on the 4850 Level, at approximately 1500 mbgs. The second stage of the experiment involved the injection of chilled water (i.e., around 11˚C) into hydraulic fractures created during the stimulation stage via a packed interval around 50 m from the borehole collar at the drift rib, over a period of 195 days. Flow rates and temperatures of Experiment 1 were significantly lower than those anticipated for commercial EGS reservoirs, however, one of the objectives of the EGS Collab project is to investigate and understand the fundamental processes observed during the experiment, and to translate those learnings into numerical simulators that can then be extended to commercial-scale EGS. An immediate beneficiary of these analyses is the EGS reservoir being developed for the Frontier Observatory for Research in Geothermal Energy (FORGE), the GTO’s flagship EGS research effort. This paper is focused on understanding the dynamic nature of the flow resistance across the fracture network of EGS Collab Experiment 1. The fracture network of Experiment 1 included hydraulic fractures induced from the stimulation borehole, natural fractures, weep zones, splay hydraulic fractures, and damaged, but grouted monitoring boreholes. The production borehole was intersected via an hydraulically active natural fracture and the dominate hydraulic fracture. Over the course of the chilled-water injection test, the injection rate and temperature of the injected water remained nearly constant, with the exception of occasional outages, however, volumetric recoveries and network flow resistance both increased in a nominally steady fashion over time. More interestingly the network flow resistance dropped sharply with injection pumping halts, regardless of the halt duration, followed by rapid recovery of the injection pressure. The embedded borehole and fracture modeling approach couples flow and transport in the rock matrix, boreholes and fractures via three distinct but analogous discretizations. This approach was successful in modeling the seemingly disparate rapid tracer and delayed thermal recoveries of Experiment 1. For this study, the embedded borehole and fracture modeling approach is coupled with geomechanics (i.e., thermal-hydraulic-mechanical (THM) coupling) to investigate the dynamic behavior of the fracture aperture and thus flow resistance in response to changes in matrix rock temperature and pore pressure for EGS Collab Experiment 1. The principal objective of the study is understand the observed increased flow resistance across the fracture network, but additionally to provide some insight to observed sharp drops in flow resistance with injection pumping halts.

Press the Back button in your browser, or search again.

Copyright 2021, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.