Pore pressure/stress coupling and its implications for seismicity

Periodic pressure measurements made during the depletion of oil fields and virgin pressure measurements through normallyand over-pressured sequences in sedimentary basins both demonstrate that changes in pore pressure and minimum horizontal stress (σh) are coupled to one another. Pore pressure/stress coupling is predicted by poroelastic theory. Data from the North Sea (Ekofisk Field) and Texas (Travis Peak Formation of east Texas and Vicksburg Formation of south Texas) suggest that σh decreases at approximately 80% and 50% of the rate of depletion of reservoir pore pressure respectively. Virgin pressures in overpressured sedimentary basins suggest that σh increases at approximately 60-80% of the rate of increase in pore pressure in the Canadian Scotian Shelf, the Australian North West Shelf and the Gannet/Guillemot Fields area of the North Sea. The total vertical stress (σv) is given by the weight of the overburden and is unaffected by changes in pore pressure. Hence, contra to simple, uncoupled models of the effect of pore pressure on rock failure, differential stress in normal fault regime basins (σv σh) increases as pore pressure decreases, and decreases as pore pressure increases. Increased differential stress with decreased pore pressure can account for depletion-induced seismicity, despite effective stresses increasing. Decreased differential stress with increased pore pressure implies that a greater increase in pore pressure can be withstood prior to failure than would otherwise be predicted, and increases the propensity of tensile, rather than shear failure, occurring with overpressure development.