Crack Transmissivity and the strength of the lithosphere

Abstract

Understanding the flow of fluids (liquids and gases in the subsurface) is of vital importance to geoenergy exploitation and to underground storage of water, oil, economically important gases and for the disposal of waste liquids and gases. Individual cracks in rocks can be substantially more effective for fluid transport than flow through the porous matrix of the rock, by a factor of 1000 times, and a multicrack array even more so. However, for both porous matrix flow and crack flow the hydraulic transmissivity is greatly affected by the stress state in the Earth. The aim of this study has been to investigate the effect of in-situ stress on crack transmissivity for the case of rough fractures, including the influence of crack-parallel shear stress and frictional slip on cracks on the fluid flow. Controlled-geometry rough cracks were made by cleaving Penrhyn slate, and these were compared with smooth cracks made in Solnhofen limestone and in Carrara Marble. Samples were subjected to effective hydrostatic stresses up to 300 MPa, with cyclic pressurization and depressurization. Transmissivity measured using the flow of argon gas, was measured as a function of stress state using either the oscillating pore pressure method or the pulse transient decay method. The first pressurization would typically reduce the transmissivity more than one thousand-fold, but this was never wholly recoverable upon depressurization. After several pressurization and depressurization cycles behaviour would settle to a pattern of non-linear elastic variations of transmissivity by hundred-fold over the full pressure range. The first pressurization steps over the full pressure range would close down most transmissivity irrecoverably. This could be attributed to the formation of rock flour coating the crack surface through collapse of asperities and blocking transport paths. With the addition of shear stress leading to slip on the fracture surface the reduction of transmissivity was further exacerbated, until after several pressure cycles of accumulating damage on the crack surface the crack transmissivity could be reduced to a level approaching that of matrix permeation. These experimental results have significant implications for the engineering management of subterranean fluid flows. Exposing cracks to high effective normal stresses, for example by carelessly reducing existing fluid pressure, or permitting shear slip on faults by excessive fluid overpressuring can lead to damage and resulting irreversible reductions in crack transmissivity.

Date of Award	20 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Ernest Rutter (Supervisor) & Julian Mecklenburgh (Supervisor)