Role of Fracture Geometry in the Evolution of Flow Paths Under Stress

This paper summarizes more than a decade's research at BRGM on the hydromechanical behavior of natural fractures in granite under normal and shear stress. The paper's emphasis is on the importance of understanding the role of fracture geometry in fluid flow and, in particular, the evolution of fracture flow paths with changes in stress. Experimental results were obtained by modifying classical hydromechanical tests to allow detailed analysis of fracture geometry under zero load and of the spatial organization of flow. Fracture-wall geometry is analyzed using profilometry; a casting methodology is used to determine the geometry of the fracture's void space. Tracer tests show that the decrease in transmissivity that occurs with increasing normal stress is associated with increasingly distinct channeling. This channeling is strongly linked to correlation lengths identified from geostatistical analysis of surface profiles and data obtained from the casts of fracture void space. Modeling results show that deformation of fracture surfaces with increasing normal stress causes substantial, nonuniform changes in void-space geometry that can change the flow regime. To better understand the mechanical behavior of fractures under shear stress, image analysis techniques are used to identify geometrical parameters that affect the micromechanical behavior and the evolution of damage zones during shearing. Laboratory experiments indicate that a fracture's mechanical response to shear stress can be broken down into at least five phases, which are shown to be associated with changes in flow. In general, application of shear stress induces an opening of the fracture, sometimes preceded by a closure phase, that causes a very large increase in global transmissivity that is associated with a reorientation of flow subperpendicular to the shear direction. Reorientation culminates just after peak shear stress is reached. During the subsequent softening and residual phases, flow tends to return to a more isotropic pattern.