Size Effect on the Permeability and Shear Induced Flow Anisotropy of Fractal Rock Fractures

Abstract

The effect of model size on fluid flow through fractal rough fractures under shearing is investigated using a numerical simulation method. The shear behavior of rough fractures with selfaffine properties was described using the analytical model, and the aperture fields with sizes ¶ Corresponding author. This is an Open Access article published by World Scientific Publishing Company. It is distributed under the terms of the Creative Commons Attribution 4.0 (CC-BY) License. Further distribution of this work is permitted, provided the original work is properly cited. varying from 25 to 200 mm were extracted under shear displacements up to 20 mm. Fluid flow through fractures in the directions both parallel and perpendicular to the shear directions was simulated by solving the Reynolds equation using a finite element code. The results show that fluid flow tends to converge into a few main flow channels as shear displacement increases, while the shapes of flow channels change significantly as the fracture size increases. As the model size increases, the permeability in the directions both parallel and perpendicular to the shear direction changes significantly first and then tends to move to a stable state. The size effects on the permeability in the direction parallel to the shear direction are more obvious than that in the direction perpendicular to the shear direction, due to the formation of contact ridges and connected channels perpendicular to the shear direction. The variances of the ratio between permeability in both directions become smaller as the model size increases and then this ratio tends to maintain constant after a certain size, with the value mainly ranging from 1.0 to 3.0. 1840001-1

Size effect on the hydraulic behavior of fluid flow through rough-walled fractures: a case of radial flow

“…Moreover, the size-dependence of hydraulic properties is a critical problem in correlating the laboratory-scale investigations to in-situ applications. Before applying the results of laboratory-scale investigations to field-scale predictions, the scaling effect in hydraulic properties and their relationships must be addressed (Matsuki et al 2006;Singh et al 2016;Huang et al 2018;Sawayama et al 2021;Zhong et al 2021). Accordingly, this has led to efforts to address the size-dependency of fluid flow through fractured rock and to determine the corresponding optimum specimen size above which the apparent size effect is no longer important (Witherspoon et al 1979;Witherspoon 1981).…”

“…Although fractures in rock masses are typically present in interconnected networks, the behavior of the fluid in a single fracture must be understood first before more complicated field-scale fracture networks can be addressed (Merrill 1975;Brown 1987;Pyrak-Nolte et al 1988;Hakami and Larsson 1996;Brown et al 1998;Zimmerman and Yeo 2000;Konzuk and Kueper 2004;Xiong et al 2011;Zhang and Nemcik 2013;Javadi et al 2014;Huang et al 2018;Cao et al 2019;Zhang et al 2019). As one of the common approaches, the rock core contains an artificially created or a natural fracture that is used for laboratory investigations, where the fluid flow in the single fracture can be investigated under controlled conditions.…”

“…For this purpose, there have been a number of studies to characterize fluid flow through fractures ranging in size from several tens of centimeters to several meters (Yeo et al 1998;Lanaro 2000;Fardin et al 2001;Grasselli et al 2002;Fardin et al 2003Fardin et al , 2004Lanaro and Stephansson 2003;Meheust and Schmittbuhl 2003). Results obtained from numerical simulations have also shown that sample size plays a critical role for hydraulic properties of rock fractures, where the hydraulic properties determined by small laboratory samples only represent a fraction of characteristics of fieldscale fractures (Koyama et al 2006;Huang et al 2018;Sawayama et al 2021;Zhong et al 2021). Based on a new spectral method, Matsuki et al (2006) found that the effect of the fracture size on the hydraulic aperture disappears if the fracture is larger than about 0.2 m, when the fracture is closed without shearing and has the same mean aperture, since beyond this size the standard deviation of the initial aperture is almost independent of the fracture size.…”

“…Koyama et al (2006) suggested that one possible explanation is the higher possibility of larger samples having more number of severe asperities that dominate the shear dilation. Subsequently, the impact of shear direction on size effect was further examined by Huang et al (2018), who found that the size effects on the permeability in the direction parallel to the shear direction are more obvious than that in the direction perpendicular to the shear direction, indicating the formation of contact ridges and connected channels perpendicular to the shear direction. By employing the lattice Boltzmann method, Sawayama et al (2021) explored the scaling effect on fracture permeability of synthetic fractures with increasing scale, and they further proposed a scale-independent empirical formula based on simulated results.…”

Numerical Research of Fluid Flow and Solute Transport in Rough Fractures under Different Normal Stress

“…The larger the JRC value is, the rougher the surfaces are[9]. Many studies have shown that the aperture of fractures usually obeys a normal distribution[20,49]. According to the obtained apertures, Gaussian fitting was performed on the aperture of each fracture model.…”