Pore pressure/stress coupling and its implications for rock failure

Abstract

Clastic dykes and sills witness that subsurface sediment mobilization is often controlled by the brittle failure of units 'sealing' overpressured and liquidized sediments. Brittle failure also imposes a limit on the buoyancy pressure that can be exerted by hydrocarbon columns. Conventional understanding of brittle failure induced by increasing pore pressure (P p) assumes that total minimum horizontal stress (σ h) is unaffected by changes in pore pressure. However, total minimum horizontal stress increases from shallow, normally pressured sequences to deeper, overpressured sequences. Data from the Canadian Scotian Shelf, the North Sea and the Australian North West Shelf demonstrate such P p/σ h coupling, with the minimum horizontal stress increasing at approximately 60-80% of the rate of pore pressure (i.e., Δσ h/ΔP p = 0.6-0.8). Hence, a greater increase in pore pressure can be sustained prior to brittle failure of units sealing overpressured compartments than would be predicted by conventional, uncoupled failure models. Furthermore, because total vertical stress is not similarly coupled to pore pressure, differential stress (σ 1-σ 3) reduces as pore pressure increases in normal fault regime basins. Thus, the mode of rock failure can not be inferred from differential stress in the stable state and P p/σ h coupling promotes tensile over shear failure.