Permeability Evolution in Natural Fractures Subject to Cyclic Loading and Gouge Formation
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Increasing fracture aperture by lowering effective normal stress and by inducing dilatant shearing and thermo-elastic effects is essential for transmissivity increase in enhanced geothermal systems. This study investigates transmissivity evolution for fluid flow through natural fractures in granodiorite at the laboratory scale. Processes that influence transmissivity are changing normal loads, surface deformation, the formation of gouge and fracture offset. Normal loads were varied in cycles between 1 and 68 MPa and cause transmissivity changes of up to three orders of magnitude. Similarly, small offsets of fracture surfaces of the order of millimeters induced changes in transmissivity of up to three orders of magnitude. During normal load cycling, the fractures experienced significant surface deformation, which did not lead to increased matedness for most experiments, especially for offset fractures. The resulting gouge material production may have caused clogging of the main fluid flow channels with progressing loading cycles, resulting in reductions of transmissivity by up to one order of magnitude. During one load cycle, from low to high normal loads, the majority of tests show hysteretic behavior of the transmissivity. This effect is stronger for early load cycles, most likely when surface deformation occurs, and becomes less pronounced in later cycles when asperities with low asperity strength failed. The influence of repeated load cycling on surface deformation is investigated by scanning the specimen surfaces before and after testing. This allows one to study asperity height distribution and surface deformation by evaluating the changes of the standard deviation of the height, distribution of asperities and matedness of the fractures. Surface roughness, as expressed by the standard deviation of the asperity height distribution, increased during testing. Specimen surfaces that were tested in a mated configuration were better mated after testing, than specimens tested in shear offset configuration. The fracture surface deformation of specimen surfaces that were tested in an offset configuration was dominated by the breaking of individual asperities and grains, which did not result in better mated surfaces.
KeywordsFracture mechanics Fracture transmissivity EGS Fracture surfaces Aperture Gouge
The authors want to thank two anonymous reviewers for their constructive suggestions which helped to improve this work. The authors further want to thank the National Cooperative for the Disposal of Radioactive Waste (Nagra), Switzerland, and the CRIEPI fractured rock study Takana et al. (2014) for providing us with the specimen material for our study. The authors further want to thank the chair of geosensors and engineering geodesy at ETH Zurich for their support with the photogrammetry scanner. This work was partially supported by the GEOTHERM II project, which is funded by the Competence Center Environment and Sustainability of the ETH Domain. This project benefitted from partial funding from DOE DE-FE0023354.
- Esaki T, Hojo H, Kimura T, Kameda N, Deut Gesell E, Grundbau (1991) Shear-flow coupling test on rock joints. In: Proceedings—seventh international congress on rock mechanics, vol 1. Rock mechanics and environmental protectionGoogle Scholar
- Evans KF (2005) Permeability creation and damage due to massive fluid injections into granite at 3.5 km at soultz: 2. critical stress and fracture strength. J Geophys Res Solid Earth 110(B4):1–14Google Scholar
- Gentier S, Hopkins D, Riss J (2013) Role of fracture geometry in the evolution of flow paths under stress. In: Faybishenko B, Witherspoon PA, Benson SM (eds) Dynamics of fluids in fractured rock, American Geophysical Union, pp 169–184. doi: 10.1029/GM122p0169
- Hakami, E and Einstein, H H and Gentier, S and Iwano, M (1993) Characterisation of fracture apertures-Methods and parameters. 8th ISRM congress 1995Google Scholar
- Iwano M, Einstein H (1993) Stochastic analysis of surface roughness, aperture and flow in a single fracture. In: Proceedings of the ISRM International Symposium Eurock ’93. Lisbon. pp 135–1441Google Scholar
- Kim HM, Inoue J (2003) Analytical approach for anisotropic permeability through a single rough rock joint under shear deformation. J Geophys Res Solid Earth 108(8):5-1–5-10Google Scholar
- Louis C (1969) A study of groundwater flow in jointed rock and its influence on the stability of rock masses. Rock mechanics research, report 10Google Scholar
- Takana Y, Miyakawa K, Fukahori D, Kiho K, Goto K (2014) Survey of flow channels in rock mass fractures by resin injection. Asian rock mechanics symposium 8Google Scholar
- Tester JW, Anderson BJ, Batchelor AS, Blackwell DD, DiPippo R, Drake EM, Garnish J, Livesay B, Moore MC, Nichols K, Petty S, Toksöz MN, Veatch RW, Baria R, Augustine C, Murphy E, Negraru P, Richards M (2006) The Future of geothermal energy; impact of enhanced geothermal systems (EGS) on the United States in the 21st century. Technical report INL/EXT-06-11746, Idaho National LaboratoryGoogle Scholar
- Walsh JB (2003) A theoretical analysis of sliding of rough surfaces. J Geophys Res-Solid Earth 108(B8). doi: 10.1029/2002jb002127