Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS)
We experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a...
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MDPI AG
2022-05-01
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| Series: | Geosciences |
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| Online Access: | https://www.mdpi.com/2076-3263/12/5/195 |
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| author | Johannes Herrmann Valerian Schuster Chaojie Cheng Harald Milsch Erik Rybacki |
| author_facet | Johannes Herrmann Valerian Schuster Chaojie Cheng Harald Milsch Erik Rybacki |
| author_sort | Johannes Herrmann |
| collection | DOAJ |
| description | We experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a given roughness. For slate samples, the hydraulic transmissivity of the fractures was measured at confining pressures, p<sub>c</sub>, at up to 50 MPa, temperatures, T, between 25 and 100 °C, and differential stress, <i>σ</i>, acting perpendicular to the fracture surface of up to 45 MPa. Fracture transmissivity decreases non-linearly and irreversibly by about an order of magnitude with increasing confining pressure and differential stress, with a slightly stronger influence of p<sub>c</sub> than of <i>σ</i>. Increasing temperature reduces fracture transmissivity only at high confining pressures when the fracture aperture is already low. An increase in the fracture surface roughness by about three times yields an initial fracture transmissivity of almost one order of magnitude higher. Fractures with similar surface roughness display the highest initial transmissivity within slate, graywacke, quartzite and granite samples, whereas the transmissivity in claystone and granitic gouge material is up to several orders of magnitude lower. The reduction in transmissivity with increasing stress at room temperature varies with composition and uniaxial strength, where the deduction is lowest for rocks with a high fraction of strong minerals and associated high brittleness and strength. Microstructural investigations suggest that the reduction is induced by the compaction of the matrix and crushing of strong asperities. Our results suggest that for a given surface roughness, the fracture transmissivity of slate as an example of a target reservoir for unconventional EGS, is comparable to that of other hard rocks, e.g., granite, whereas highly altered and/or clay-bearing rocks display poor potential for extracting geothermal energy from discrete fractures. |
| first_indexed | 2024-03-10T03:49:17Z |
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| institution | Directory Open Access Journal |
| issn | 2076-3263 |
| language | English |
| last_indexed | 2024-03-10T03:49:17Z |
| publishDate | 2022-05-01 |
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| series | Geosciences |
| spelling | doaj.art-6a1c4d49aa7d40b4822b427c1290064f2023-11-23T11:12:05ZengMDPI AGGeosciences2076-32632022-05-0112519510.3390/geosciences12050195Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS)Johannes Herrmann0Valerian Schuster1Chaojie Cheng2Harald Milsch3Erik Rybacki4Institute of Drilling Engineering and Fluid Mining, Technische Universität Bergakademie Freiberg, 09599 Freiberg, GermanyGFZ German Research Centre for Geosciences, 14473 Potsdam, GermanyGFZ German Research Centre for Geosciences, 14473 Potsdam, GermanyGFZ German Research Centre for Geosciences, 14473 Potsdam, GermanyGFZ German Research Centre for Geosciences, 14473 Potsdam, GermanyWe experimentally determined the hydraulic properties of fractures within various rock types, focusing on a variety of Variscan rocks. Flow-through experiments were performed on slate, graywacke, quartzite, granite, natural fault gouge, and claystone samples containing an artificial fracture with a given roughness. For slate samples, the hydraulic transmissivity of the fractures was measured at confining pressures, p<sub>c</sub>, at up to 50 MPa, temperatures, T, between 25 and 100 °C, and differential stress, <i>σ</i>, acting perpendicular to the fracture surface of up to 45 MPa. Fracture transmissivity decreases non-linearly and irreversibly by about an order of magnitude with increasing confining pressure and differential stress, with a slightly stronger influence of p<sub>c</sub> than of <i>σ</i>. Increasing temperature reduces fracture transmissivity only at high confining pressures when the fracture aperture is already low. An increase in the fracture surface roughness by about three times yields an initial fracture transmissivity of almost one order of magnitude higher. Fractures with similar surface roughness display the highest initial transmissivity within slate, graywacke, quartzite and granite samples, whereas the transmissivity in claystone and granitic gouge material is up to several orders of magnitude lower. The reduction in transmissivity with increasing stress at room temperature varies with composition and uniaxial strength, where the deduction is lowest for rocks with a high fraction of strong minerals and associated high brittleness and strength. Microstructural investigations suggest that the reduction is induced by the compaction of the matrix and crushing of strong asperities. Our results suggest that for a given surface roughness, the fracture transmissivity of slate as an example of a target reservoir for unconventional EGS, is comparable to that of other hard rocks, e.g., granite, whereas highly altered and/or clay-bearing rocks display poor potential for extracting geothermal energy from discrete fractures.https://www.mdpi.com/2076-3263/12/5/195Enhanced Geothermal Systems (EGS)Variscan rocksslatequartzitegraniteclaystone |
| spellingShingle | Johannes Herrmann Valerian Schuster Chaojie Cheng Harald Milsch Erik Rybacki Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS) Geosciences Enhanced Geothermal Systems (EGS) Variscan rocks slate quartzite granite claystone |
| title | Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS) |
| title_full | Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS) |
| title_fullStr | Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS) |
| title_full_unstemmed | Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS) |
| title_short | Fracture Transmissivity in Prospective Host Rocks for Enhanced Geothermal Systems (EGS) |
| title_sort | fracture transmissivity in prospective host rocks for enhanced geothermal systems egs |
| topic | Enhanced Geothermal Systems (EGS) Variscan rocks slate quartzite granite claystone |
| url | https://www.mdpi.com/2076-3263/12/5/195 |
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