Comparison of Surface Roughness and Transport Processes of Sawed, Split and Natural Sandstone Fractures

Sascha Frank,Stefan Wohnlich,Thomas Heinze

doi:10.3390/w12092530

Abstract

In single fractures, dispersion is often linked to the roughness of the fracture surfaces and the resulting local aperture distribution. To experimentally investigate the effects of diverse fracture types and surface morphologies in sandstones, three fractures were considered: those generated by sawing and splitting, and a natural sedimentary fracture. The fracture surface morphologies were digitally analyzed and the hydraulic and transport parameters of the fractures were determined from Darcy and the tracer tests using a fit of a continuous time random walk (CTRW) and a classical advection–dispersion equation (ADE). While the sawed specimen with the smoothest surface had the smallest dispersivity, the natural fracture has the largest dispersivity due to strong anisotropy and non-matching fracture surfaces, although its surface roughness is comparable to the split specimen. The parameterization of the CTRW and of the ADE agree well for β > 4 of the truncated power law. For smaller values of β, non-Fickian transport processes are dominant. Channeling effects are observable in the tracer breakthrough curves. The transport behavior in the fractures is controlled by multiple constraints such as several surface roughness parameters and the equivalent hydraulic aperture.

Highlights

To understand flow and solute transport in complex natural fractured systems, a profound investigation of flow and transport processes in a single fracture is often considered helpful to reveal the principal relationship between parameters and variables besides the difficulties of upscaling the findings to larger fracture networks [1,2,3,4,5]
We presented the results of the tracer tests as well as of the fracture surface analysis for the tracer tests as well as the quantities used to characterize the fracture surface morphology
The Darcy tests were used to determine the equivalent hydraulic aperture and the Reynolds number, and conclusions were drawn from the tracer tests regarding dispersivity, dispersion and flow velocity in the fractures using a continuous time random walk (CTRW)

Summary

Introduction

To understand flow and solute transport in complex natural fractured systems, a profound investigation of flow and transport processes in a single fracture is often considered helpful to reveal the principal relationship between parameters and variables besides the difficulties of upscaling the findings to larger fracture networks [1,2,3,4,5]. Hydraulic flow and solute transport in fractured systems are relevant processes in many applications such as the effective remediation of pollutants [6] or in geothermal reservoirs [7]. Dispersion is a crucial and characteristic process dominating the solute concentration distribution in a single fracture. Dispersion is caused by varying flow velocities across the flow cross section and along flow direction in connection with a complex geometry of the pore space as well as molecular diffusion [8]. Microscopic flow velocity and pore space geometry are determined by the fracture surface morphology and the resulting fracture aperture distribution, respectively [9]. The effect of surface roughness on dispersion is primarily through the control of the velocity field, based on the local aperture distribution for narrow fractures, especially if the fracture aperture is smaller

Results

Discussion

Conclusion