An Enhanced Geothermal System (EGS) is an artificially created geothermal reservoir formed by hydrofracturing hot dry rock. Thermal shock occurs when the cold water contacts the hot rock near the injection borehole, creating a network of small, disorganized, closely spaced micro cracks. As the cold-water injection continues, the hot rock cools down and the micro cracks coalesce, becoming a better-defined network of thermal fractures. Thermal fractures in an EGS reservoir are believed to improve reservoir performance by increasing the surface area for heat exchange and lowering flow impedance; however, it is difficult to precisely predict how they grow and affect the permeability of the reservoir. The goal of this paper is to provide an insight into the transport mechanisms within the thin, permeable, thermally shocked region of an EGS reservoir. COMSOL Multiphysics® is used to set up an indealized porous region with identical geometrical features at different domain scales to show the scale dependence of heat and mass transport in the initial microscale crack network and in the later coalesced thermal fractures. This research shows the importance of EGS maturity in determining how heat and mass are transferred and how to select appropriate analytical tools for different stages of development.
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