Stress-dependent permeability of a de-mineralised fracture in shale

Abstract

The paper presents the results of laboratory direct shear tests on the hydro-mechanical behaviour of an extensional fracture in shale. The tests were conducted on fracture specimens obtained by mechanically splitting blocks of a naturally fractured shale along the cemented fracture and dissolving the calcite fracture filling with a strong acid. The experiments consisted of radial fluid flow tests from a single fluid source along the fracture surface. Fracture samples were tested under different levels of effective normal stress and shear displacement. The experimental results show significant reduction of fracture permeability during increasing contact normal stress across the fracture and after shearing of the fracture under constant high normal stress. However, the tested fractures never completely closed even under normal stresses close to or higher than the unconfined compressive strength of the intact shale. The fracture permeability remained much higher than the intact shale (or matrix) permeability even when the fracture surface has undergone some gouge formation from local asperity failure during shearing. The results indicate that fractures once created are difficult to close by mechanical loading. In tight formations like shales, fractures will always be conduits for fluid flow unless closed by cementation. Currently, it is widely assumed that fractures in sedimentary basins stay open only when there is an overpressure which could keep the fractures hydraulically parted. However, the results presented here indicate that given a sufficient hydraulic gradient, fluid can still flow along a fracture even in the absence of large overpressures.