Numerical, Experimental and Theoretical studies of the influence of the fracture roughness on its transport properties

H. AURADOU; L. TALON; A. HANSEN; J-P HULIN; I. IPPOLITO; R. CHERTCOFF; A. BOSCHAN

Workshop; Flows and mechanics in natural porous media from pore to field scale. IFP Energies nouvelles; 2011

In the past years fractures were often view as consisting of two flat parallel plates. However, as soon as a normal load is applied to the fracture, a strong localisation of the flow and consequently an increased of the dispersivity are observed. Pioneered laboratory experiments studied flow and transport in a fracture in situation of little contact. In such situations analytical model based on small perturbation analysis or stream lines simulations are able to reproduce the experimental observations. Yet those approaches fail as soon as the aperture becomes very heterogeneous; for fracture this happens roughly when more than 10% of the surfaces are in contacts. In this presentation, we will proposed new concepts and tools that may be use to predict the change of the transport property of tight fractures. Most of the current efforts are devoted to the study of the stream lines properties such as their localisation in the fracture plane or the velocity correlation along each of them. Instead of looking at properties along the flow direction, we propose here to focus on sections crossing the fracture normal to the flow direction. Among all possible sections, the one which has the lowest hydraulic aperture (noted bc) plays a crucial role. Under normal or transverse shear loadings, the fracture permeability diminishes. We will show that the permeability scales with the aperture bc. Remarkably, this scaling holds even in the presence of large contact area and for large change of the loading conditions or aperture field modifications; others lengths based on the statistical properties of the aperture field like the mean fracture aperture are found to fail to predict the permeability change over. The section with the minimal aperture is easily computed from the aperture field using minimal path algorithm, we will show that this length can be obtained from an hydraulic and a tracer test experiments. At the scale of a fracture, complete mixing is often not observed, yet the breakthrough curve may be used to gain information about the fracture. Previous studies dealt with the spatial distribution at the fracture outlet, here we study the distribution of the first arrival time when ordered in an ascending order. We show that the distribution scales with the order with an exponent independent of the fracture roughness. In field experiments only the breakthrough curve measured over the full fracture outlet is often available, we will show how to relate the shape of the breakthrough curve with the heterogeneity of the aperture field. Ordering statistic and minimal path algorithm are the basic tools used here to predict the fracture property. The theoretical predictions show good agreement with numerical and experimental data demonstrating that this approach may be used to predict the property of tight fractures. Future work will considered the application of such concepts to porous media.