Natural, accidental, and designed tracer tests are commonly employed in fractured rock systems to assess permeability and to infer fracture characteristics such as aperture, fracture density, and interconnectedness. Fracture skins, which are ubiquitous in nature, can affect test interpretation. Fracture skins include both coatings of the fracture surface by mineral precipitates and zones of the matrix along the fracture that are altered by water-matrix mineral interactions. We analyze fracture skins in volcanic and sedimentary rocks. SEM and optical photomicrography demonstrate that the altered zones have higher concentrations of iron and manganese oxyhydroxides, clays, and organic matter, including bacterial colonies, which should increase sorptivity. Skin permeabilities, mean pore diameters, and diffusion coefficients are typically lower than in the unaltered matrix. Consequently, solute and colloidal transport may be strongly affected by increased sorption along the fracture surface and decreased diffusive attenuation into the matrix. Tracer tests cannot be related directly to the physical properties of the fractured rock unless the effects of the fracture skin are evaluated.
We model the solute transport with a combination of analytical and numerical models. Key parameters are shown to be the skin thickness and the ratios of skin-to-matrix porosities, diffusion coefficients, and retardation coefficients. In dual porosity media, back diffusion times are shown to be significantly longer than the initial exposure times of the solutes in the fractures. These phenomena indicate that remediation of contaminants or enhanced oil recovery may be more problematical than is commonly postulated. Wall rock alterations along fracture skins composed of vein fillings indicate that the fractures may have had several episodes of healing (filling with precipitants) followed by reopening. Therefore, the timing of natural fracturing events is important in fracture-dominated flow and transport because younger fractures may not have had time to develop an effective skin. Finally, fracture skins may significantly affect rock mass or soil stability by altering frictional and roughness coefficients and changing system permeabilities.
Fractures and fracture skins are ubiquitous in near-surface geological materials. Open fractures control the flow of groundwater and other fluids because of their great permeability. Fractures also are discontinuities which influence the strength of the materials. Fracture skins are the coatings or zones of alteration along fracture surfaces caused by mineral-water interactions, organic activity, and adherence of colloidal materials (including clay minerals) to the fracture surfaces. We have observed fracture skins on near-surface fractures in a wide variety of environments. These include igneous rocks (tuffs, basalts, and granites), sedimentary rocks (limestones, shales, and sandstones) and unconsolidated alluvial and glacial sediments. The prominent vertical fractures which show erosional relief have no obvious fracture skins. The less visible but more abundant fractures dip about 30 degrees to the left. This fracture set possesses fracture skins of calcite and gypsum coatings which, in some cases, completely occlude the fracture. We predict that fracture skins have properties which have significant geotechnical and environmental implications. We make some implications about effects on the frictional resistance of fractures with fracture skins.