A New Numerical Model of Thermal Aperture Changes in an Enhanced Geothermal System Reservoir

Abstract

Extraction of energy from deep crystalline formation requires pressurisation of the rock mass to create new or enhance existing natural fractures. The fractures generate a pathway for transport of the fluid and heat back to the surface for optimum energy production. However, the complexity of fracture aperture opening and closure during exploitation has posed a challenge for both engineers and geologists. The most widely used model for fracture aperture changes is frequently limited to the hydro-mechanical effect only. This paper presents a new model that incorporates a complete cycle of thermal, hydraulic and mechanical aperture changes in an enhanced geothermal system (EGS) reservoir using a coupled thermo-hydro-mechanical (THM) simulator. The goal is to understand the extent to which thermal aperture changes might enhance the performance of EGS reservoirs. Three reservoir cases are developed: a model of hydro-mechanical aperture changes with a constant thermal aperture; a model of thermal aperture changes employing a constant hydro-mechanical aperture; and a combined model of both hydro-mechanical and thermal aperture changes. The outcomes show that the changes are greater with the thermal aperture than with the hydro-mechanical aperture due to the impact of thermal contraction.