Performance Evaluation of Engineered Geothermal Systems Using Discrete Fracture Network Simulations

Abstract

The US Department of Energy (DOE) undertook a study of EGS behavior using DFN models and the DFN approaches. Sandia National Laboratories (Sandia) and Lawrence Berkeley National Laboratory (LBNL) directed the study, and they in turn selected the Itasca Consulting Group (Itasca), Golder Associates Inc. (Golder), Lawrence Livermore National Laboratory (LLNL), and LBNL to develop numerical experiments using DFN models to determine under what conditions EGS may or may not be viable and to understand critical parameters and sensitivities. The results of this study have recently been published in electronic form (Kennedy et al, 2021)*. The major conclusions included the following. (1) a successful EGS system will likely involve multiple fractures and multiple zones. These may be achieved either by multi-staged stimulations, like unconventional oil and gas developments, or by using stimulation methods that develop critical shearing stresses on natural fractures. (2) Optimizing well layouts to develop EGS by stimulating natural fractures needs to consider the geometry of the natural fractures and the in-situ stress field. (3) A multiple fracture or stimulation system will require either separate management of each stimulation to avoid premature breakthrough on the most transmissive pathways or will require an active management approach that shuts down the more permeable pathways as they experience thermal breakthrough at the production well. (4) Chemical effects are important particularly to reservoir longevity and demonstrations of a hybrid DFN-continuum approach provide a potential way forward. (5) Most of the simulations in this report were two dimensional; however, three- dimensional effects may be important, with respect to depth gradients of temperature and stress, which vary significantly with regional tectonic settings and influence the extent of EGS applicability within the USA. *https://escholarship.org/uc/item/4168d73x