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

Abstract

Repeated pressure measurements undertaken throughout the depletion of oil fields demonstrate that reduction in pore pressure is accompanied by a reduction in total minimum horizontal stress (σh), a phenomenon described herein as oil field-scale pore pressure/stress (Pp/σh) coupling. Virgin pressure measurements (i.e. those unaffected by depletion) through normally and overpressured sequences in sedimentary basins demonstrate that overpressure development is accompanied by an increase in σh, described herein as sedimentary basin-scale Pp/σh coupling. With depletion of the Ekofisk Field, North Sea, minimum horizontal stress decreased at approximately 80% of the rate of reduction of reservoir pore pressure (i.e. △σh/△Pp ≈ 0.8). Virgin pressures measured in exploration wells surrounding the Ekofisk Field (Norwegian quadrants 1 and 2) indicate that with overpressure development △σh/△Pp ≈ 0.73 (assuming shallow, normally pressured sequences are representative of overpressured sequences prior to overpressure development). Hence, despite the different temporal and spatial scales, the rate of decrease of minimum horizontal stress with pore pressure due to depletion of the Ekofisk Field is similar to the rate of increase of minimum horizontal stress with pore pressure due to overpressure development in the surrounding region. Basin-scale exploration pressure data in the Ekofisk region may thus provide an indication of the reservoir stress changes associated with depletion. Knowledge of such stress changes is critical because they can lead to the collapse of uncased wellbores, sand production and to faulting/fracturing and seismicity with field development.