Violation of the Kaiser effect by hydraulic-fracturing-related microseismicity

Abstract

Application of a load to different materials and rocks induces acoustic and seismic emission. If the load has a cyclic character, then as a rule such an emission will vanish for loads less than the maximum previously reached load level. This is the so-called Kaiser effect, which is an expression of memorized stress history of materials and rocks. Usually, this effect is assumed to be valid also in the case of fluid-induced seismicity. Here, we show that microseismicity occurring during hydraulic fracturing violates the Kaiser effect. In material sciences, such a breakdown of the Kaiser effect is termed the felicity effect. We observe that the felicity effect can be directly related to reopening of the hydraulic fracture forced by fluid injection in loading cycles. In contrast, closing of the hydraulic fracture following injection cessations seems to be governed by the Kaiser effect. We explain such a contrasting behaviour of microseismicity by an aseismic character of a slow fracture closing and by rubbing and pore pressure diffusion during a quick fracture reopening. This violation of the Kaiser effect for tensile fracturing due to hydraulic forcing on rocks is similar to the absence of this effect in the case of shear fracturing by seismogenic processes. Possibly, the felicity effect is a common feature of ongoing active faulting.