Abstract
Abstract. Geothermal energy is an important and sustainable resource that has more potential than is currently utilized. Whether or not a deep geothermal resource can be exploited, mostly depends on, besides temperature, the utilizable reservoir volume over time, which in turn largely depends on petrophysical parameters. We show, using over 1000 (n=1027) 4-D finite-element models of a simple geothermal doublet, that the lifetime of a reservoir is a complex function of its geological parameters, their heterogeneity, and the background hydraulic gradient (BHG). In our models, we test the effects of porosity, permeability, and BHG in an isotropic medium. Furthermore, we simulate the effect of permeability contrast and anisotropy induced by layering, fractures, and a fault. We quantify the lifetime of the reservoir by measuring the time to thermal breakthrough, i.e. how many years pass before the temperature of the produced fluid falls below the 100 ∘C threshold. The results of our sensitivity study attest to the positive effect of high porosity; however, high permeability and BHG can combine to outperform the former. Particular configurations of all the parameters can cause either early thermal breakthrough or extreme longevity of the reservoir. For example, the presence of high-permeability fractures, e.g. in a fault damage zone, can provide initially high yields, but it channels fluid flow and therefore dramatically restricts the exploitable reservoir volume. We demonstrate that the magnitude and orientation of the BHG, provided permeability is sufficiently high, are the prime parameters that affect the lifetime of a reservoir. Our numerical experiments show also that BHGs (low and high) can be outperformed by comparatively small variations in permeability contrast (103) and fracture-induced permeability anisotropy (101) that thus strongly affect the performance of geothermal reservoirs.
Highlights
The amount of geothermal energy that can be extracted from a reservoir depends, to a 1st-order approximation, on reservoir temperature, permeability, and utilizable reservoir volume
In models in which high permeability is combined with a background hydraulic gradient (BHG), the HDI becomes ellipsoidal
The ellipsoidal long axis of the HDI is parallel to the BHG direction and its aspect ratio is controlled by permeability and the magnitude of the BHG (Fig. 2e, f)
Summary
The amount of geothermal energy that can be extracted from a reservoir depends, to a 1st-order approximation, on reservoir temperature, permeability, and utilizable reservoir volume. While temperature is often well constrained, permeability and the utilizable reservoir volume are more difficult to predict (e.g. Bauer, 2018; Bauer et al, 2017; Kushnir et al, 2018; Laubach et al, 2009; Seeburger and Zoback, 1982). Agemar et al, 2014; Moeck, 2014; Tiab and Donaldson, 2004). The most important parameters recognized in the literature are porosity and permeability They are often highly heterogeneous because of layering, localized fracturing, and diagenesis The vast majority of hydrothermal systems can be considered dual-porosity systems, where porosity is provided by both pore space and fractures (Gringarten, 1984; Warren and Root, 1963).
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