Proceedings 42 nd New Zealand Geothermal Workshop 24-26 November 2020 Waitangi, New Zealand ISSN 2703-4275 A NEW NUMERICAL MODEL OF THERMAL APERTURE CHANGES IN AN ENHANCED GEOTHERMAL SYSTEM RESERVOIR Musa D Aliyu 1 , Rosalind A Archer 1 1 Department of Engineering Science, the University of Auckland, Private Bag 92019, Auckland 1142, New Zealand. musa.aliyu@auckland.ac.nz r.archer@auckland.ac.nz Keywords: Thermal aperture, hydro-mechanical aperture, combined aperture, EGS reservoir, THM simulation. 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. 1. INTRODUCTION In hot dry rock (HDR) systems, fractures often control the transportation of fluid and heat because their permeability is usually larger than that of the surrounding rock masses. The permeability of fractures changes as they open or close from the resultant shift in effective stress, pore pressure, chemical equilibrium and temperature (Abé et al., 1985, 1979; Abé and Sekine, 1983). The effect of thermal, hydraulic, mechanical and chemical (THMC) processes on variations in fracture permeability must be observed in tangent with one another because they can work together to produce either a closure or opening of a fracture (Willis-Richards and Wallroth, 1995). For instance, in an enhanced geothermal system (EGS) reservoir, maintaining a fracture network of sufficiently high permeability is crucial to ensure the continuous exchange of heat between the rock matrix and the circulation fluid. The application of coupled THMC processes via modelling or laboratory experiments might increase the chances of extracting heat from EGS reservoirs and improve their operations under economically viable conditions. It is crucial to investigate the effect of THMC processes in fractures by employing well-calibrated models for the accurate prediction of the long-term performance of EGS reservoirs. Considerable efforts have been made towards modelling the effects of hydraulic fracturing on EGS reservoirs using numerous coupled process-based techniques (Taron and Elsworth, 2010, 2009). One of the earliest coupled thermo-hydro-mechanical (THM) models was developed by Harlow and Pracht (1972) using the finite difference (FD) method. They developed several conceptual models of HDR geothermal reservoirs to study the effect of hydraulic fracturing on thermal energy extraction. McFarland (1975) developed a two-dimensional (2D) FD model to represent the operation of an HDR geothermal reservoir with a penny-shaped crack created by hydraulic fracturing. (Abé et al., 1976b) investigated the stable growth of a penny-shaped crack created by hydraulic fracturing in an HDR geothermal reservoir using a coupled THM solution. In another study, they examined the growth of a vertical penny- crack induced because of hydraulic fracturing operation during heat extraction from an HDR reservoir (Abé et al., 1976a). Furthermore, Bažant and Ohtsubo (1978) employed McFarland's geometrical approach to developing a finite element (FE) model of an HDR geothermal reservoir using coupled THM processes to investigate the effect of temperature changes on crack thickness. DuTeaux et al. (1996) developed a fully coupled THM FE code called GEOCRACK to investigate the variations in the properties of rock joints during HDR geothermal exploitations. Kohl et al. (1995) studied the response of a fractured medium to hydraulic stimulation in an HDR system using a FRACTure coupled THM processes simulator. Hicks et al. (1996) employed HOTGRID code to model the potential coupled effects of THM processes in an HDR geothermal reservoir's heat extraction. Ghassemi et al. (2003) developed an integral equation to study the effect of induced thermal stress during an EGS reservoir extraction. Taron et al. (2009) proposed a coupled THMC framework by connecting TOUGHREACT and FLAC3D packages to analyse the physical interactions of deformable fractured porous media. The above literature presents a few THMC models that have been developed to investigate the various aspect of hydraulic fracturing effects on EGS reservoirs. However, despite these efforts, a need still exists for a standard code to model thermal aperture changes in EGS reservoirs, particularly in three- dimensions (3D). Thus, this study presents a new approach to establishing the thermal aperture variation in 3D EGS reservoirs using a coupled THM simulation. The new model incorporates a complete cycle of thermal, hydraulic and mechanical aperture change effects in an EGS reservoir. The goal of this research is to understand the extent to which thermal aperture changes might enhance the performance of the system. Three reservoir cases were developed: a model of hydro-mechanical aperture changes with a constant thermal aperture, a model of thermal aperture changes employing a