Laboratory Investigation of Hydraulic Fracture Growth in Zimbabwe Gabbro

Direct measurement of the apparent toughness evolution during hydraulic fracture growth remains challenging in rocks due to the difficulty of measuring the time evolution of the fracture geometry. We report hydraulic fracturing tests performed in 250 x 250 x 250 mm Zimbabwe gabbro under different confining stress. Using time-lapse active acoustic surveys, we reconstruct the fracture front and fracture width using scattered and transmitted waves. After verifying that hydraulic fracture growth is dominated by the energy dissipated in fracture creation such that the fluid pressure can be considered spatially uniform in the fracture (negligible viscous flow energy dissipation), we estimate the apparent fracture toughness, using the theory of linear hydraulic fracture mechanics, by combining fracture geometry and pressure data. The apparent fracture energy is higher than that measured from semicircular bending tests. The created fractures evolve in a planar and radial geometry at the macroscopic scale, but we observe overlapping segments and bridges at the millimetric scale. This complex segmentation at a scale on par with the rock grain size is likely responsible for the observed higher fracture energy at centimetric scale. Such fracture segmentation and bridging will necessarily occur when encountering heterogeneities and result in a further increase of fracture energy at a larger scale even if the overall macroscopic geometry remains planar. The experimental data set reported here should guide modeling efforts necessary to better predict hydraulic fracture growth in rock masses.