| Abstract |
Enhanced Geothermal Systems (EGS) make use of natural and developed fracture connectivity in otherwise low permeability rock to circulate fluid in the subsurface for geothermal power production. Previous work has suggested that the degree of fracture connectivity as well as fracture geometry, can play an outsized role in thermal drawdown and the lifespan of the geothermal plant. Contraction of the rock matrix as cool water is circulated can affect the width of the fracture aperture and therefore the connectivity of the fracture network. Thermo-hydro-mechanical (THM) models are a useful tool to explore these processes. However, fracture networks, as occur in nature, are complex and difficult to model, and thus fracture networks in THM models are often simplified and/or represented as one of many statistically potential interpretations. Whether these statistically derived fracture networks are representative of the subsurface, and how this can impact analysis and understanding of these systems, is a matter of interest. PFLOTRAN is a parallelized multiphase reactive transport code that is used to simulate subsurface flow and transport, including geothermal processes and fracture flow. A PFLOTRAN fracture process model has been further developed to generate fracture families, defined by key variables describing their orientation, spatial extent, and aperture width, with multiple fractures fulfilling these descrptions that are produced stochastically using a standard deviation from the defined key variables and a random seed number. An initial 2-D modelled enhanced geothermal system (EGS) simulation featuring a complex background fracture network (100 fractures) separating two wells was constructed and run to demonstrate the potential for preferential flow channeling when thermo-mechanical processes are considered in the model. Five sets of stochastically generated fracture families connecting between injection and production wells were subsequently modelled in a domain with negligible natural background fracture network and low permeability. The random seed numbers defining final fracture location and orientation were varied in six simulations, along with thermally driven changes to fracture aperture (THM). Six counterpart simulations were then run without the thermal changes (TH). Changes in thermal drawdown at the production well between TH and THM simulations, as well as between the different fracture networks, were examined to explore how variation in fracture network extent and fracture orientation may affect these processes and how this may influence our understanding of subsurface fracture flow in the context of EGS. |