Influence of normal and shear stress on the hydraulic transmissivity of thin cracks in a tight quartz sandstone, a granite, and a shale.
Journal: Journal of Geophysical Research: Solid Earth
Publication Date: 20 January, 2018
Department of: Earth and Environmental Sciences
A novel approach to measuring the flow of fluids through cracks in rocks
Understanding underground fluid flow through cracks and the pores of the rock matrix is important for oil and gas recovery, underground gas (methane and CO2) storage, waste disposal down deep boreholes, geothermal energy extraction, and understanding earthquake origins. Crack flows have mostly been studied through complex numerical modelling and the results can seem opaque. This research shows how crack flow deep underground, and its dependence on normal and shear stress, can be measured in laboratory experiments by building on the analogy with physical laws of electric current flows.
Conventional wisdom says that crack transmissivity decreases with burial depth (closing cracks) but increases if dilatant frictional sliding on the crack initiates. However, this study shows that for widely disparate rock types, the small amount of frictional wear debris produced during initial sliding blocks flow paths much more effectively than crack dilatancy can enhance them. Whilst flow through cracks is much faster than matrix permeation for granite and a low-porosity sandstone, it is comparable in shales, giving a physical explanation for their effectiveness as underground permeability barriers (sealing layers). These results are expected markedly to facilitate modelling the deep underground behaviour of rock masses, based on real physical measurements.
- Permeability: Rock property that quantifies its ability to transmit fluid (liquid or gas).
- Transmissivity: The relative ease with which fluid flows through a feature such as a crack.
- Dilatancy: Volume expansion that occurs locally in a rock when microcracks are produced and fragments are displaced slightly, locally generating porosity.
- Frictional wear products: Small fragments produced by grinding of one rock surface against another.
- Normal and shear stress: Measures of the forces that respectively close cracks and drive sliding along them.