Influence of effective pressure on the hydraulic properties of intact, micro-fractured and macro-fractured Westerly Granite under hydrostatic conditions

The Earth's crust contains networks of fractures that facilitates fluid flow at all depths, influencing processes such as seismicity and subsurface storage. Fracture transmissivity is governed by stress conditions at depth and strongly influenced by fracture scale and surface roughness - factors that remain only partly constrained. This study investigates the hydraulic properties of Westerly granite under hydrostatic stress cycled to 150 MPa, using intact samples and those with controlled fracture characteristics: (1) micro-fractures created by thermal treatment, (2) macro-fractures with fabricated roughness from smooth (Ra ≈1.5 μm) to rough (Ra ≈10 μm), and (3) tensile fractures produced by Brazilian splitting, representing natural tensile fractures (Ra ≈164 μm). All samples exhibit transmissivity reduction with increasing effective pressure. Thermally generated micro-fracturing can increase permeability by two orders of magnitude over intact granite (matrix permeability ∼10 −21 m 2 ), depending on treatment temperature, whereas a single fabricated macro-fracture can increase transmissivity by up to six orders of magnitude. Although well-mated fractures close rapidly under load, they remain 2–3 orders of magnitude more transmissive than intact matrix. At 150 MPa, smooth fabricated macro-fractures achieve transmissivities comparable to rough analogue tensile fractures. Contrastingly, unmated rough fractures resist closure due to asperity propping, maintaining higher transmissivities. Upon unloading, transmissivity does not retrace the loading path, particularly in unmated rough fractures, indicating both elastic and permanent deformation that limits fracture reopening. These results demonstrate how fracture morphology controls transmissivity evolution under variable stress - key to understanding crustal fluid flow and related geological processes.