Effects of Grain Size on Geometrical Characteristics of Hydraulic Fractures

Abstract

Small-scale hydraulic fracturing was conducted in the laboratory to create a tensile fracture parallel to bedding plane in a block of three marbles with different mean grain size, ranging from 0.093mm to 2.55mm. To determine the initial aperture distributions of the fracture, height distributions of a pair of surfaces of the fracture were measured along matched paths from a common reference surface with a profllometer which had a stylus tip of 0.025mm in radius. Spectral and statistical analyses were carried out for surface profiles and the initial aperture distributions of the fracture to correlate geometrical characteristics of the fracture to grain size of calcite.
Main results obtained in this study are summarized as follows:
1) The height of the surface profiles was fractal in the range of this study. The fractal dimension determined from power spectral density ranged from 1.392 to 1.425. The fractal dimension slightly increased with the mean grain size of marble.
2) The initial aperture distributions created by the two fractal surfaces were not fractal because the power spectral density was flattened in wavelengths greater than the mismatch wavelength of each marble.
3) The roughness of surface profile, the mean value of the initial aperture, the standard deviation of the initial aperture and the mismatch wavelength increased with the mean grain size of marble.