Effects of Realistic Aperture Distributions on the Thermal Performance of Discretely Fractured Geothermal Reservoirs

Abstract:

Forecasting thermal performance over time is critical in the design and sustainable operation of Enhanced Geothermal Systems (EGS). Heterogeneous reservoir permeability often introduces uncertainty in the fluid flow distribution and heat transport in fracture-dominated reservoirs. Whereas previous authors have investigated the effects of random aperture distribution within a single fracture, here we explore the influence of aperture variability constrained by actual reservoir data. Using COMSOL Multiphysics, we create a 3D model that couples porous fluid flow, fracture flow and heat transport in a thermal reservoir consisting of a relatively impermeable porous matrix and a single horizontal fracture. Through our models we can explore the influence of temperature-dependency of water viscosity and density, buoyancy effects, and regional groundwater flow to estimate thermal drawdown. Simulations are run for numerous representative aperture distributions, derived from a Genetic Algorithm calibrated by experimental tracer test and frictional pressure loss data from a meso-scale geothermal analog field site in upstate New York, USA. Preliminary results suggest that the realistic aperture distributions exhibit even more flow-channeling than random aperture fields considered in previous studies. This phenomenon often leads to diminished thermal performance by reducing the total heat transfer area between the rock and the circulating fluid, creating “short-circuits” between the injection and production well pair. However, adequate well positioning in such field sites can also lead to enhanced thermal performance when convoluted pathways through the fracture plane enable the fluid to access a larger portion of its surface area, resulting in more heat extraction but a greater inter-well frictional pressure drop. Our results highlight the importance of characterizing aperture distributions in EGS in order to optimize thermal efficiency and devise appropriate reservoir management strategies.