Effects of thermally-induced fractures on EGS performance

Abstract

An Enhanced (or Engineered) Geothermal System (EGS) is defined as an engineered reservoir formed by hydro-fracturing hot dry rock to extract heat from low permeability geothermal resources to generate electricity. EGS technology is undergoing stages of advancement, and a substantial amount of research has been conducted to understand different aspects of this technology. It is well known that in the EGS reservoir, circulation of the cold fluid through the stimulated region induces thermo-elastic stresses that triggers thermal fractures. Initially, when the induced temperature gradient is critically high, a thermally-shocked region comprising a network of small, disorganized, closely-spaced thermal cracks is formed adjacent to the primary hydraulic fractures. These small thermal cracks tend to coalesce, and better-defined planar thermal fractures propagate into the rock matrix. Thermally-induced fractures (in both forms of small thermal cracks within the thermally-shocked region, and well-defined planar thermal fractures) are believed to improve EGS reservoir performance by increasing the surface area for heat exchange, and by lowering the flow impedance. However, current investigations exclude thermally-induced fractures from reservoir simulation practices. Due to physically removing thermally-induced fractures from EGS reservoir simulation, research on the mechanisms of fluid flow, heat, and mass transport within thermally-shocked region of an EGS reservoir has drawn less attention. This exclusion also hinders understanding the contribution of the thermally-induced fractures in EGS performance. The main goal of this research is to improve the understanding of the coupled transport mechanisms within the EGS reservoir. In the first part of this research, which is a microscale study, the mechanisms of heat and mass transport within the thin, thermally-shocked region of an EGS reservoir are assessed. A segment of the thermally-shocked region is idealized as a porous medium, and ten models of identical geometrical features but different domain scales are developed. These models cover a wide range of fragmentation, from severely thermally-shocked to lightly thermally-shocked regions. Two methods are utilized to determine the fluid flow and transport processes, the direct pore-scale and the continuum Darcy-scale methods. The COMSOL Multiphysics software is utilized as a Finite Element (FE) framework for the numerical implementation. It is found that for the severely thermally-shocked model, diffusion is a dominant mechanism of heat and mass transport. For a moderately thermally-shocked model, mass transfer is mainly accomplished by advection. However, heat is transferred via both fluid motion and conduction. In the second part of this thesis, the effects of the thermally-induced fractures on EGS performance are assessed by integrating thermally-induced fractures into the field-scale reservoir simulation. An innovative hybrid approach comprised of the explicit discrete-fracture method and the Effective Continuum Method (ECM) is developed for simulating thermo-hydro processes within the EGS reservoir. The proposed hybrid approach allows to integrate thermally-induced fractures into the numerical simulation without additional computational burden. The results of the numerical model showed that the presence of thermally-induced fractures increases the production temperature of the EGS reservoir by 24% on average over 30 years of heat extraction from the reservoir. A sensitivity analysis study shows that the effect of the parameters of the planar thermal fractures on the thermal performance of the EGS was not as significant as that of the presence of these thermal fractures on the reservoir performance. Results of the parametric study show that minute differences in the permeability, length, number and width of thermal fractures are not so significant to the modeling results. But the inclusion of these thermal fractures do have a significant impact on modeling long term reservoir characteristics.