The dynamic excess pore pressure concept - A new possible fracture mechanism for fluid-induced seismic events

Based on a critical analysis of international literature, case studies of fluid-induced seismic events and using advanced ideas of kinematic as well as dynamic mechanical behaviour of joints and fracture zones in rocks under pore water pressure a new concept of induced failure of weakness zones in rock masses by variable pore fluid pressure is discussed. The concept includes different fluid conductivity of weakness zones, interference of time-dependent primary water pressure (e.g. pressure within fluid-injection wells or changing waterlevels in reservoirs) with time-dependent secondary pore pressure in weakness zones within the rock mass and the dilatancy and contractancy behaviour of deforming rough weakness zones. The time delay of pore pressure decrease in weakness zones with low permeability in comparison with highly permeable zones can result – together with recontractancy behaviour of the first weakness zones in the deformation processes – in remarkable excess pore pressures. The latter very strongly reduce the effective friction coefficients of the low permeable weakness zones and can trigger fluid-induced seismic events even for fast-decreasing primary fluid pressure. There are several case descriptions in the literature supporting this concept.