| Abstract |
This study focuses on evaluating the correlation between heat production and field and operational parameters in Enhanced Geothermal Systems (EGS) using a thermo-hydro-mechanically coupled numerical modeling approach. The numerical modeling approach is based on the discrete element technique in which the pre-existing discrete fracture network is generated stochastically and explicitly represented. In the adopted technique, the transient response of laminar flow through the pre-existing fracture network is simulated. The advective heat transfer by fluid flow, the convection at the boundary between moving fluid and rock, and the conduction of heat through surrounding rock are taken into account. The mechanical solution is fully coupled with thermal and hydraulic responses. The objective of this study is to evaluate how field characteristics and operational parameters affect the response of EGS. Therefore, a series of sensitivity studies with respect to various reservoir characteristics, such as fracture size distribution, density and orientation, fracture properties, in-situ stress condition and operational parameters (i.e., injection rate) are carried out. The studies are performed in a two-dimensional framework, and both stimulation and production phases are modeled. The purpose of the stimulation phase is to enhance the overall permeability of the reservoir by inducing hydro-shearing, i.e., the irreversible aperture increase due to slip. In the numerical modeling studies, duration of stimulation typically varies between hours to a few days. Subsequent to the stimulation phase, the production phase for a typical period of five to ten years is simulated. In order to compare different cases, a series of metrics that quantify the response of the reservoir during stimulation and production phases are monitored through time. The stimulation indices include total area of pressurized fractures and area of stimulated fractures. The production indices include temperature draw down at the production wells, rate of production, rate of generated power and total energy produced. The results of this study provide good insight into the complex mechanisms that control the response of EGS reservoirs to injection, and further emphasize the role that numerical modeling can play in better addressing the issue of data uncertainty. Through sensitivity studies, numerical modeling tools can provide a viable approximation of the expected range of reservoir responses. Furthermore, they can shed light on the role and relative importance of in-situ properties, and help direct efforts toward characterizing and understanding properties that are deemed to be more important. |