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Petroleum Geoscience

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Reservoir stress path during depletion of Norwegian chalk oilfields

Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK(e-mail: n.r.goulty@dur.ac. uk)

Pore pressure drawdown during reservoir depletion results in reduced horizontal principal stresses within a reservoir due to three distinct mechanisms: normal compaction, poroelastic behaviour and normal faulting. Established relationships, based on simplifying assumptions, give the ratio of the change in minimum horizontal stress, S_h, to the change in pore pressure, P, in terms of sediment properties for each mechanism. In spite of the approximations introduced by the assumptions, these relationships may be useful for discriminating between the mechanisms that control the reservoir stress path. For the Norwegian chalk oilfields, it is important to know whether normal faulting, in particular, is the governing mechanism because slip on active faults can shear well casings, and active faulting and fracturing can increase reservoir permeability. Previously reported field observations and laboratory measurements on chalk samples are compared to infer the mechanisms governing the reservoir stress path for the Ekofisk and Valhall fields. The amount of subsidence at the seabed observed at Ekofisk is evidence that the weaker horizons within the reservoirs are yielding plastically through pore collapse. Nevertheless, the reservoir stress path corresponds to that expected for poroelastic behaviour or normal faulting, and not that expected for plastic yielding.

KEYWORDS: horizontal stress, pore pressure, depletion, chalk, normal faulting