Classes of sustained microfracturing produced after formation of hydraulic fractures

Aftershocks were observed to occur after hydraulic fracturing tests on mortar blocks at the laboratory scale. Five aftershocks sequences were recorded and fitted with the original and modified Omori laws. The modified Omori law was found to be a better model for the aftershock sequences studied. Both the estimated decay exponent p and the time shift c fall in the range found from observations of the geological aftershocks. The statistical similarity between the geological and the laboratory aftershocks suggests that there is a common underlying mechanism governing the generation of aftershocks across the scale. In this mechanism the aftershocks correspond to crack production by the mechanism of static fatigue. The power law decay of aftershock frequency (modified Omori law) is explained by the residual stress relaxation caused by the reduction of deformation moduli produced by crack accumulation. Two types of decay exponents p (exponents of the modified Omori's law) were observed: (1) p > 1 caused by static fatigue fracturing of initially non-fractured zones (bridges) within the fracture; (2) p < 1 caused by static fatigue production of parallel cracks outside the fracture surface.