An Experimental Investigation of Stress-Dependent Permeability and Permeability Hysteresis Behavior in Rock Fractures

Abstract

Stress-dependent permeability in rock fractures has been investigated since the 1960s [Snow, 1968]. The consequences of stress-dependent permeability in fractures have implications in oil and gas production, especially in tight oil/gas production [Abass et al., 2009] and shale gas production [Cho et al., 2012], coalbed methane production [McKee et al., 1988], and for CO2 storage projects. Changes in permeability are attributed to three factors: changes in mean aperture, changes in roughness, and changes in contact area. The relative contribution of each factor and how it depends on the stress history are investigated here using concurrent stress-dependent permeability and aperture measurements using X-ray CT scanning. Saw-cut fractures in two rocks are tested. We find that stress-dependent permeability and hysteretic behavior is influenced by both aperture change and roughness change. For a small aperture fracture tested here, changes in roughness dominate the permeability response to stress changes. The Modified Cubic Law [Witherspoon et al., 1980] and models from Walsh [1981], Zimmerman et al. [1992], and Sisavath et al. [2003] are compared with the experimental data, and results show that all four models do not result in sufficiently large permeability variation. Additionally, none of the previous models quantifies the relative contribution of aperture and roughness to permeability change. A new empirical model is proposed based on the Modified Cubic Law [Witherspoon, 1980] that provides a better match to the experimental data and accounts for both stress-dependent aperture and roughness.