EFFECTS OF NATURAL FRACTURE SURFACES: IMPLICATIONS FOR FLOW AND TRANSPORT

When open and connected fractures are present they dominate both fluid flow and transport of solutes. The surface of the fractures is a critical factor. We examine the effects of fracture roughness and fracture skins (the altered zones along the fracture surface) in sedimentary and crystalline rocks with: 1) numerical models; 2) flow experiments in natural fractures; and 3) high-resolution X-ray CT (HRXCT). Model results demonstrate that the skins have a significant effect on solute transport. Field observations suggest that alterations are dependent upon the fluid flow history and fracture orientation. Experimental data show that the fracture roughness is an important controlling factor, and that the empirical models developed with artificial fractures may not be relevant analogs for natural systems in which apertures and asperities may be similar in magnitude. HRXCT was used to characterize roughness across the quasi-three-dimensional extent of each fracture plane; these data show that the apertures and asperities are not stationary with respect to the one-dimensional statistics typically used to describe them, even at small scales. Consequently, the upscaling of fracture-dominated system hydraulic and transport properties, if possible, is extremely difficult. Critical properties that control scaling include: asperity and roughness (as we demonstrate), channeling, and skin properties. Continuing work is evaluating geological controls on fracture roughness and skin properties and if they can be correlated.