The Ekofisk Field is a naturally-fractured chalk reservoir in the Norwegian Sector of the North Sea that has been producing hydrocarbons for over 20 years. In situ stresses in the field have been constantly changing because of the decline in pore pressure that has occurred in the reservoir as a result of production. The effect of production and pore pressure drawdown on the minimum horizontal in situ stress in the Ekofisk Field has been determined from shut-in pressure data from 37 hydraulic fractures conducted during the past 15 years. The effective stresses in the reservoir increase linearly with pore pressure drawdown, but at different rates. The reservoir stress path, defined as the ratio of the change in effective minimum horizontal stress to the change in effective overburden stress, is approximately 0.20. Laboratory experiments, which simulate the stress path followed by reservoir rock during the production history of the field, show that shear failure has occurred during compaction of high porosity chalk as the shear stress increased with pore pressure drawdown. Shear failure during depletion will increase fracture density and reduce matrix block dimensions in the fractured reservoir, thereby maintaining reservoir permeability as the apertures of older natural fractures are closed with increasing deformation. It is suggested that the shear failure process may account for the continued good producibility of the Ekofisk Field, in spite of compaction.

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