Long-Term Evolution of Fracture Permeability in Slate: An Experimental Study with Implications for Enhanced Geothermal Systems (EGS)

The long-term sustainability of fractures within rocks determines whether it is reasonable to utilize such formations as potential EGS reservoirs. Representative for reservoirs in Variscan metamorphic rocks, three long-term (one month each) fracture permeability experiments on saw-cut slate core sam...

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Published inGeosciences (Basel) Vol. 11; no. 11; p. 443
Main Authors Cheng, Chaojie, Herrmann, Johannes, Wagner, Bianca, Leiss, Bernd, Stammeier, Jessica A., Rybacki, Erik, Milsch, Harald
Format Journal Article
LanguageEnglish
Published Basel MDPI AG 01.11.2021
Subjects
Cores
Corrosion
Crack propagation
Deionization
Dissolution
Dissolving
Earth science
Enhanced geothermal systems
Equilibrium
Evolution
Experiments
Fluid flow
fluid–rock interactions
fracture
Fracture permeability
Fractures
Geothermal power
Mechanical properties
Membrane permeability
Metamorphic rocks
Minerals
Mountains
Permeability
Reservoirs
Salinity
slate
Sodium chloride
Stone
Sustainability
temperature
time-dependent
Germany
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Summary:The long-term sustainability of fractures within rocks determines whether it is reasonable to utilize such formations as potential EGS reservoirs. Representative for reservoirs in Variscan metamorphic rocks, three long-term (one month each) fracture permeability experiments on saw-cut slate core samples from the Hahnenklee well (Harz Mountains, Germany) were performed. The purpose was to investigate fracture permeability evolution at temperatures up to 90 °C using both deionized water (DI) and a 0.5 M NaCl solution as the pore fluid. Flow with DI resulted in a fracture permeability decline that is more pronounced at 90 °C, but permeability slightly increased with the NaCl fluid. Microstructural observations and analyses of the effluent composition suggest that fracture permeability evolution is governed by an interplay of free-face dissolution and pressure solution. It is concluded that newly introduced fractures may be subject to a certain permeability reduction due to pressure solution that is unlikely to be mitigated. However, long-term fracture permeability may be sustainable or even increase by free-face dissolution when the injection fluid possesses a certain (NaCl) salinity.
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ISSN:2076-3263
2076-3263
DOI:10.3390/geosciences11110443