The hydraulic properties of fractured Westerly granite

Kajendran, Mahesh (2025) The hydraulic properties of fractured Westerly granite PhD thesis, University of Liverpool.

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Abstract

Understanding fluid flow within the Earth’s crust is crucial for advancing our knowledge of geohazards, such as earthquakes, and efficiently harnessing resources hosted in the subsurface, such as geothermal energy. The continental crust at depth is predominantly granitic in composition, therefore the study of crystalline rock provides a good basis for understanding fluid flow within the crust. Granites are characterised by a high strength and low porosity, therefore fluid flow through the rock matrix is slow. Once fractured, their ability to store and transport fluid is increased by several orders of magnitude. This is seen in nature in highly conductive brittle structures such as fault damage zones and fractured reservoirs. However, laboratory experiments have sometimes failed to produce the behaviour implied by observation from nature. For example, laboratory measurements of samples from deep boreholes greatly underestimate permeability inferred from downhole measurements, and shear laboratory fractures often do not show increases in fracture transmissivity implied by field measurements. To improve our understanding, this thesis aims to investigate and quantify the controls of fluid flow in fractured granite as a function of effective pressure, fracture damage scale, shear loading, shear displacement, and fracture surface roughness. In this experimental work, three different transmissivity evolutionary situations have been compared; closure of micro- and macro-fractures under hydrostatic stress changes, transmissivity evolution of fabricated fractures with controlled roughness under differential stress during slip and transmissivity evolution of rough tensile fractures under differential stress during slip. Brittle damage in Westerly granite, ranging from micro- to macro-fractures, is shown to significantly increase permeability compared to matrix permeability, with up to a 2-order and 9-order magnitude increase over an intact sample, respectively. Elevated effective pressure (hydrostatic), results in a predictable permeability decrease due to crack closure, with smoother fractures and mated fractures displaying lower transmissivity, though unmated macro-fractures resist closure at higher pressures due to asperity propping. In contrast, thermally induced micro-fractured samples at high pressures approach similar permeabilities to an intact sample. Identifying the scale, roughness and matedness of fractures is important for modelling natural fractured systems. A new direct-shear triaxial method developed for transmissivity measurements on shearing fabricated granite fractures demonstrates the intricate relationship between slip displacement, stress conditions, surface roughness, and transmissivity evolution. Progressive, permanent transmissivity reduction occurs over 5 mm of slip, resulting in 2–4 orders of magnitude loss due to asperity abrasion and wear-product formation blocking flow paths. Rougher, unmated fractures initially maintain transmissivity more than smoother ones, but transmissivity systematically converges with increased shear displacement for all tests. The production of gouge limits transmissivity recovery under reduced normal stress, while reducing shear stress results in further decreases in transmissivity. Interestingly, fractures that are unloaded and reloaded 4 halfway through the experiment (with ~2.5 mm of slip) show appreciable transmissivity increase when they are critically stressed, presenting parallels with observations made in nature. The same direct-shear methodology is applied to shearing of rough, mated, tensile fractures which are more analogous to natural fractures. Effective normal stress conditions of 20, 30, and 50 MPa are used. The added geometric complexity to the fracture surfaces is reflected in the data, where initial sliding (1–2 mm) causes transmissivity increases as critically stressed asperities result in fracture dilation, with low effective pressures yielding variable, up to 3 orders of magnitude, increases, while higher pressures suppress dilation and produce smaller, more predictable increases. Transmissivity decreases at higher displacements (>2 mm) across all pressures due to asperity shearing and gouge production. Surface roughness and microstructural analysis support this, which reveal fracture surface smoothing, asperity degradation and wear debris accumulation. These data highlight that effective pressure should be considered when evaluating fractures subject to a large shear component.

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	Geology, Rock deformation, Fracture flow, Fracture roughness
Divisions:	Faculty of Science & Engineering Faculty of Science & Engineering > School of Environmental Sciences
Depositing User:	Symplectic Admin
Date Deposited:	01 Sep 2025 15:37
Last Modified:	01 Sep 2025 15:37
DOI:	10.17638/03193782
Supervisors:	McNamara, David Faulkner, Daniel Lavallée, Yan
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3193782
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