Coupling of thermal-hydraulic-mechanical processes for geothermal reservoir modelling

Abstract

This paper uses a fully coupled framework of thermal-hydraulic-mechanical processes to investigate how the injection and extraction of fluid within a geothermal reservoir impacts on the distributions of temperature, pore pressure, and deformation within the rock formations. Based on this formulation, a numerical model is developed in light of the thermodynamics of porous materials. The proposed procedure relies on the derivation of dissipative flow rules by postulating proper storage and dissipation functions. This approach opens new horizons for several resource engineering applications. Since it allows for full coupling, this formulation can play a key role in predicting risks when used for reservoir simulation. The results indicate that the injection-extraction process and temperature change have a definite impact on altering the in-situ properties of the reservoir. KEY WORDS: poro-mechanics, resource engineering, fluid injection and extraction, temperature change, pore pressure, stress, deformation, uplift, subsidence. INTRODUCTIONAs the population of the world is set to double by the end of this century, sustaining our current lifestyle requires the production of clean energy at affordable prices. While fossil fuels are still expected to be important energy resources for the next decades, the share of renewable energies is expected to increase significantly. The need to increase the supply of renewable energy sources has led to an increasing amount of research on harvesting geothermal energy. The geothermal energy process involves the utilisation of the Earth's natural geothermal gradient to extract heat and transform it into a directly useful energy such as electricity. To maintain recoverability and limit the footprint of geothermal energy, the process involves injecting cold fluids into an underground reservoir of hot permeable rock, allowing the fluid to flow through the rock formation and then extracting the heated fluid. The injection and extraction of fluids and subsequent temperature change throughout the reservoir impacts the physical properties of the permeable formations. The basic mechanism underlying the geo-mechanical response of a reservoir is related to the magnitude of the pore pressure and temperature variations. Deep underground reservoirs that have low values of porosity, permeability and compressibility result in greater pressure