Laboratory measurements of fracture-aperture distributions have been made by use of computed-tomography (CT) scanning under various conditions of effective stress. The stress-dependent aperture-distribution data demonstrate that increasing stress results in two effects: The mean aperture will decrease and the variance of aperture distribution will increase. By analyzing the stress-dependent mean and variance of aperture distributions, this paper provides a more straightforward method for estimating stress-dependent permeability and capillary pressure in rock fractures.
A Berea sandstone is cut to create a fracture in the middle of the core. The sample is moderately homogeneous, with little porosity variation. The fracture surface is very rough and has large aperture variations. The length of the sample is 8 cm, and the radius is 5 cm. The porosity of the sample is 0.22. The rock sample is wrapped in a sleeve and then placed in an aluminum core holder. Water is injected around the sleeve to create the confining stress. Two high-accuracy pressure transducers are used to measure pressure at the inlet and outlet of the core, and electric heaters maintain the temperature of the core. Pore pressure is kept constant at atmospheric pressure, and temperature stays constant at 50°C. For permeability measurements, a dual-pump system is used to inject water into the core. A pressure-regulating pump is connected to the outlet of the core to maintain 2.07 MPa at the downstream end of the core throughout the experiment. The experimental setup is shown in Fig. 1 (above).
Permeability is measured with the steady-state method. Fluid is injected at three different rates while the pressure drop across the core is measured. On the basis of core size, fluid viscosity, slope of pressure drop, and fluid-injection rate, Darcy’s law is used to calculate permeability. The intact-core permeability is measured at different stress levels and is subtracted from the fractured-core permeability. Permeability is calculated using a mean aperture of 0.1 mm.
The core holder is placed in a medical X-ray CT scanner. Five repeated scans are taken and averaged to reduce the uncertainty of the CT scanning.
Investigating Aperture-Based Permeability and Capillary Pressure in Rock Fractures
01 September 2015