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Solid Earth An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/se-2019-112
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2019-112
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 17 Jun 2019

Submitted as: research article | 17 Jun 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Solid Earth (SE).

A numerical sensitivity study of how permeability, geological structure, and hydraulic gradient control the lifetime of a geothermal reservoir

Johanna F. Bauer1,2, Michael Krumbholz3, Elco Luijendijk2, and David C. Tanner4 Johanna F. Bauer et al.
  • 1Department of Rock Physics & Borehole Geophysics, Leibniz Institute for Applied Geophysics, 30655 Hanover, Germany
  • 2Department of Structural Geology and Geodynamics, Georg August University of Göttingen, 37077 Göttingen, Germany
  • 3independent researcher
  • 4Department of Seismic, Gravimetry, and Magnetics, Leibniz Institute for Applied Geophysics, 30655 Hanover, Germany

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, depends on, besides temperature, mostly the utilizable reservoir volume over time, which in turn largely depends on petrophysical parameters. We show, using a large series (n = 1027) of 4-dimensional 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. Further, 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 100 °C isotherm (HDI) reaches the production well. Our results attest to the positive effect of high porosity; however, high permeability and BHG can combine to outperform the former. Certain 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 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 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.

Johanna F. Bauer et al.
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Johanna F. Bauer et al.
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Short summary
We use a 4D numerical sensitivity study to investigate which geological parameters exert dominant control on the quality of a deep geothermal reservoir. We also constrain how the variability of these parameters affects a reservoir’s economic potential. We show that interplay of high permeability and hydraulic gradient are the dominant controls on reservoir lifetime. Fracture anisotropy, typical for faults, leads to fluid channelling, and thus restricts the exploitable volume significantly.
We use a 4D numerical sensitivity study to investigate which geological parameters exert...
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