ABSTRACT:

Shallow and deep geothermal reservoirs are potential renewable energy sources. Heat extraction from geothermal reservoirs disrupts the natural temperature gradient and causes thermo-poro-elastic effects both in the reservoir and overlying formations. The drained thermo-elastic response of rocks at ambient pressure is well characterized. However, thermo-poro-elastic properties can change significantly at undrained conditions, and with increasing depth from the surface. This paper identifies the depth-dependent relationship between linear thermal expansion coefficient, bulk modulus, and Biot modulus to quantify the drained and undrained thermo-poro-elastic response of geothermal reservoirs and adjacent geological formations when subjected to temperature changes. The work is organized in three sections: (1) compilation of thermo-por-elastic rock properties from the literature, (2) analytical solution of thermal stress-relaxation for simplified boundary conditions, and (3) numerical simulation of the subsurface (from the reservoir to the surface) of heat extraction for a 100 MW geothermal closed-loop system. We also quantify the maximum temperature drawdown, for a given a thermal stress relaxation threshold.

1. Introduction

The subsurface temperature is hot enough to harvest geothermal energy throughout a significant portion of Earth's crust (McClure and Horne, 2014). The amount of fluid-saturated rocks that constitute geothermal reservoirs is substantially larger and more widely distributed than hydrocarbons trapped in sedimentary rocks (MIT, 2006). The main categories of geothermal energy are conventional hydrothermal, enhanced geothermal systems (EGS), and advanced geothermal systems (AGS). Hydrothermal reservoirs are generally characterized by naturally high well productivity and high temperatures (MIT, 2006). However, reservoirs targeted by EGS and AGS typically have very low porosity and permeability, are located at depths of 3-10 km, and are more abundant than hydrothermal reservoirs (McClure and Horne, 2014; Song, 2018). EGS consist of connecting injection and production wells with a stimulated fracture network (Wang et al., 2021). AGS relies on a continuous closed loop wellbore to extract heat from the subsurface (Fig. 1).

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