Deviation from linear elastic fracture in near-surface hydraulic fracturing experiments with rock

This paper presents the results of near-surface hydraulic fracturing experiments in a medium-grained gabbro that make use of different specimen sizes in order to observe the influence of rock heterogeneity and/or non-LEFM behavior on fracture propagation. A comparison of the results with laboratory experiments in brittle elastic materials shows that fracture paths in rock usually do not monotonically approach the free surface. Moreover, they exhibit a high degree of non-symmetry, in contrast to experiments in glass or PMMA. An attempt to correlate crack path variation among different sized specimens to a size dependent fracture toughness has been initially inconclusive due to the noise caused by these crack path complexities. Nonetheless, these results provide an initial demonstration of non-LEFM behavior in hydraulic fracturing with implications for ongoing experiments aimed at quantifying the size effect on rock fracture toughness.