High resolution, non-destructive measurement and characterization of fracture apertures

We measure at high resolution the fracture aperture of several consolidated materials and statistically characterize the aperture distribution for future studies on single and multiphase flow and transport through fractured porous media. The images of the real fractures in granite and sandstone rocks are obtained with a computer aided tomography (CAT) X-ray scanner. The minimum pixel size is 0.27 x 0.27 mm, but fractures as small as 35 mu m can be accurately measured, using a calibration standard for the variation in CT number due to the "missing rock mass" in a given pixel. The distribution of fracture apertures is best described by a lognormal function. This was also corroborated by comparing the theoretical to actual ratio of geometric mean to arithmetic mean apertures. The mechanical roughness of the fractures ranged from 0.17 to 1.5. The variability in fracture aperture displayed only short range structure, with correlation lengths of 0.8 to 7 mm at maximum, which is at most one-tenth of the minimum dimension (width) of the fracture plane. Fracture aperture distribution is rather heterogeneous. Potential flow channels can be deduced from the fracture aperture distribution. (C) 1998 Elsevier Science Ltd. All rights reserved.