CFD modelling of the velocity profile within a single horizontal fracture in an enhanced geothermal system

Abstract

Accurate prediction of geothermal fluid velocity profiles in the fractures is essential in determining the mass flow rate and hence energy extraction in Enhanced Geothermal Systems (EGS) which embrace Hot Dry Rock (HDR) systems and Hot Sedimentary Aquifer (HAS) systems. Previous studies have addressed flows in fractures assuming fracture walls as impermeable boundaries. However this assumption is unrealistic since the channel walls may contain cracks and fissures arising from the initial hydraulic fracturing process. The channel walls thus exhibit permeable characteristics at the boundary which will affect the velocity profiles in the fracture. There has been recent development of mathematical models to predict velocity profiles for low Reynolds number flows in HDR fractures by considering the effects of slip boundary conditions at the walls. In this paper, computational fluid dynamics (CFD) model based on the Finite Volume approach is used to predict the fluid velocity profile in a single fracture in an EGS system. A fluid-porous interface model based on an analytical equation has been implemented in the commercial CFD code ANSYS/CFX. One advantage of this model is that it can take into consideration of different values of the slip coefficient, α, which is a dimensionless quantity characterising the structure of permeable material at the fluid-porous interface wall. This interface velocity model is used to investigate the effects of values of α on the channel flow. It is found when α increases from 0.1 to 4, there is an increase in pressure drop in the flow. The fluid-porous interface velocity decreases and the maximum velocity at the center line of the channel increases as α increases from 0.1 to 4.