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ABSTRACT: Coupled changes in pore pressure and stress in oil fields and sedimentary basins

Hillis, Richard R. , University of Adelaide, Adelaide, Australia

Periodic pressure measurements made during the depletion of oil fields and virgin pressure measurements through normally- and over-pressured sequences in sedimentary basins demonstrate that changes in pore pressure (Pp) and minimum horizontal stress (Sh) are coupled to one another at both the field- and basin-scale. This phenomenon has been explained in terms of poroelasticity, or, alternatively, in terms of stresses being in frictional equilibrium with those required to induce faulting. Empirically, the predictions of both models are identical in normal fault regime sedimentary basins, differing only in the calibration constants used.

The spatial and temporal scales over which coupling occurs at the field- and basin-scales varies significantly. Nonetheless, coupling appears to operate similarly in both cases. With reservoir depletion in the Ekofisk Field of the North Sea, Sh decreases at approximately 80% of the rate of Pp. Virgin pressures in exploration wells in the Ekofisk region suggest that with overpressure development, Sh increases at approximately 73% of the rate of Pp. Hence, basin-scale virgin pressure data may be useful in predicting the field-scale stress changes associated with reservoir depletion that can lead to the collapse of open wellbores and to seismicity. Considering the basin-scale, the implications of Pp/Sh coupling are that a greater increase in Pp can be sustained within overpressured compartments than would be predicted by uncoupled models of Pp being limited by rock failure. Furthermore, tensile as opposed to shear failure is promoted by Pp/Sh coupling, because, as Pp increases in a normal fault regime basin, differential stress decreases.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia