Aperture of Granite Fracture and Effects for Fluid Flow

Abstract

Fluid flow in rock mass is controlled by geometry of fractures which is mainly characterized by roughness, aperture and orientation. In this study we measure the aperture of rock fractures using a high resolution confocal laser scanning microscope (CLSM). Digital images of the aperture are acquired under applying five stages of uniaxial normal stresses. The error range is less than 0.125μm. It has been difficult to check the aperture change under stress application for the same specimen continuously. Our method can characterize the response of aperture. Results of measurements show that roughness geometry of fracture bears no uniform aperture. That is, it is changed in nonuniform manner under the different stress levels: Some parts bear a smaller aperture because of the applied stress, while some parts with very narrow aperture develop new cracks or shear displacement because of no space of aperture reduction. Laboratory permeability tests are also conducted to evaluate changes of permeability related to aperture variation due to different stress levels. The results do not imply a simple reduction of hydraulic conductivity under increase of the normal stress. This suggests that the mechanical aperture is different from the hydraulic aperture which is an effective conduit of fluid flow along a fracture. It is shown that the hydraulic aperture is slightly smaller than the mechanical aperture. Clearly the flow channels are changed due to the local change of geometry under the applied stress. The hydraulic conductivity does not follow the cubic law. This means that a parallel plate model is not suitable to express the hydraulic conductivity including local fracture geometry.