Temperature-dependent viscosity: Relevance to the numerical simulation of enhanced geothermal systems

Abstract

The primary focus of this paper is to investigate the relevance of temperature-dependent viscosity in the numerical modeling of enhanced geothermal systems (EGS) for thermal performance evaluation and forecasting. The numerical simulation model studied accounts for two situations that may occur during heat extraction from an enhanced geothermal system. First, the viscosity and density of water vary with temperature and pressure. Second is the possibility that the fractures, the main flow conduits, may have asperities that could create channels and alter flow paths, thus affecting the amount and distribution of the surface area available for heat transfer. The study shows that if the fracture aperture is of uniform aperture, there is no significant difference between assuming a constant viscosity in the model over a temperature-dependent viscosity. However, if an enhanced geothermal system is known to be channelized and the temperature difference between the reservoir and the injected fluid is large, then the temperature-dependent viscosity would be necessary for modeling the system to accurately simulate its thermal performance. On the other hand, if the aperture distribution of the enhanced geothermal system is evenly distributed, a constant viscosity may suffice in simulating the process. Moreover, if the temperature difference between the reservoir and the injected fluid is small, a constant viscosity can be used in the model without significantly impacting computational accuracy. Overall a large correlation length leading to an increased area of preferential flow paths presents the most significant effect contributing to the differences seen between modeling with a constant viscosity or a temperature-dependent viscosity.