Effect of reservoir depletion and pore pressure drawdown on in situ stress and deformation in the Ekofisk Field, North Sea

Knowledge of in situ stress and how stress changes with reservoir depletion and pore pressure drawdown is important in a multi-disciplinary approach to reservoir characterization, reservoir management, and enhanced oil recovery projects. Over 20 years of petroleum production from the Ekofisk field has resulted in a 21-24 MPa reduction in reservoir pore pressure. The effect of pore pressure drawdown on the minimum horizontal in situ stress in the Ekofisk field has been determined from shut-in pressure data of 32 hydraulic fractures. The effective stresses in the reservoir increase linearly with pore pressure drawdown, but at different rates. The ratio of the change in effective minimum horizontal stress to the change in effective vertical (overburden) stress is approximately 0.20. Laboratory experiments, which simulate the stress path followed by reservoir rock during the production history of the Ekofisk field, clearly indicate that shear failure has occurred during compaction of high porosity chalk as the shear stress increased with pore pressure drawdown. It is suggested that shear failure during primary production has increased fracture density and reduced matrix block dimensions, and has therefore maintained reservoir permeability, which may account for the continued good producibility of the Ekofisk field, in spite of compaction. 9more » refs., 5 figs.« less