Joint Impact of Fracture Topography and Aperture Distribution on Flow and Transport

Abstract:

The need to find quantitative relationships between fracture topography, fracture shear strength and fracture permeability give the motivation to compare the differences in surface characteristics between natural and artificial fractures as well as scale effects.

Rock fractures and their mechanical, hydraulic and transport properties are of utmost importance for systems such as oil-, gas- and geothermal reservoirs, groundwater remediation and nuclear waste storage. Many subsurface systems are naturally low permeable, and single fractures and fracture networks often experience flow rates far above those in porous media. The topography of fracture surfaces determines mechanical and flow properties. Some well known characterizations of fracture topography include tortuosity, correlation length, the roughness measures Z2 and JRC and fractal fields.

Field experiments investigating fracture properties and their response to changes in boundary conditions on the reservoir scale are difficult to perform, and understanding the response on the field scale requires intricate knowledge about small scale processes, which can be studied in laboratory experiments. Experiments attempt to replicate natural environments as closely as possible and there are many methods to sample fractured material. While some studies used natural fractures overcored from existing joints, others used artificial fractures obtained by splitting, or shear fracturing solid rock, or synthetic surfaces created by using characteristics of natural fractures.

This work studies the impact of the intial sample characteristics and fracture selection by outlining some relevant differences that can be attributed to fractures of different nature as well as size. This study compares pre-existing and artificial fractures from crystalline rock focusing on fracture topography and aperture. To bridge the gap between laboratory scale experiments and field scale applications, numerous studies have investigated the scale dependency of fracture and faults surface characteristics and the resulting aperture.

This contribution provides insight into the above issues by jointly investigating fractures of various sizes and nature and illustrates our attempt to progress research on upscaling fracture behavior from laboratory experiments.