Abstract
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.
Highlights
Global demand for electricity generation from alternative energy sources is increasing
This paper investigates the transport mechanisms within a thermally shocked region to provide a better understanding of the overall heat and mass transport in an Enhanced Geothermal System (EGS) reservoir
Relative Mass Concentration left to right Relative Temperature (RT) right to left
Summary
Global demand for electricity generation from alternative energy sources is increasing. For the last several decades, starting with the Fenton Hill project, several EGS projects have been developed with the hope of understanding the complex nature of man-made geothermal reservoirs. This type of man-made geothermal reservoir requires a cold-water injection to a hot but dry granitic rock at several kilometers deep to create an artificial reservoir and recover the injected water as hot steam through production wells. One of the main engineering problems has been the inability to connect the injection well with production well that are several hundred meters apart This is partially due to the lack of our understanding of how the state of stresses from the weight of the overlaying strata and lockedin stresses of the tectonic origin combine to interact with the induced thermal changes. It is important to investigate how heat and mass are transferred in the thermally shocked region and how they will influence the development of reservoir permeability
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