Control of Fractured Reservoir Permeability by Spatial and Temporal Variations in Stress Magnitude and Orientation

Abstract

Abstract A case study of the Ekofisk field, a naturally fractured chalk reservoir in the North Sea. demonstrates the strong influence of horizontal stress anisotropy on fracture conductivity and reservoir permeability. Directions and magnitudes of horizontal in situ stresses, as well as the distribution and orientations of natural fractures, vary locally across the structural dome that forms the Ekofisk reservoir. Fracture permeability is stress-sensitive and decreases as effective stresses in the reservoir increase due to pore pressure reduction resulting from production of oil and gas. Changes in fracture permeability also depend on the orientation of fractures relative to the evolving anisotropic stress field in the reservoir. Steeply dipping fractures aligned parallel to the local maximum horizontal stress direction show the smallest decline in permeability as the reservoir is depleted and can control permeability anisotropy in a naturally fractured reservoir containing multiple fracture sets.