ABSTRACT: Distribution of In-Situ Stress and Natural Fractures in the Ekofisk Field, North Sea: Implications for Reservoir Permeability

Lawrence W. Teufel, Helen E. Farrell

In-situ stress and natural fractures have been mapped vertically through the Ekofisk and Tor reservoirs and laterally across the structural dome that forms the Ekofisk field in the Norwegian sector of the North Sea. The reservoir rock is chalk, and a natural fracture system forms the primary conductive path for produced hydrocarbons. In-situ stress measurements have been made using hydraulic fractures, well-bore breakouts, and anelastic strain recovery measurements of oriented core. In general, the azimuth of the maximum horizontal stress is not uniform across the field but tends to be oriented perpendicular to the structural contours around the dome. Differences in stress directions and magnitudes at different positions on the structure could significantly affect conduct vity of natural fractures and reservoir permeability across the field because the deformation and conductivity of fractures are strongly stress dependent. Oriented core analysis and formation microscanner logs show a well developed radial fracture pattern around the structure, in addition to a northeast-southwest regional fracture pattern. The radial fracture pattern is closely aligned with the local maximum horizontal stress direction on the flanks of the structure and probably was created by structurally induced stresses during vertical doming. Accordingly, it is suggested that (1) the radial fractures will have the highest permeability, (2) the reservoir permeability will be anisotropic, (3) the maximum flow direction will not be uniform across the field but will vary radially around he flanks of the structure, and (4) as the effective stress increases due to production and reservoir depletion, new fractures may be created which are aligned with the stresses. Limited interference tests and water injection response in a waterflood pilot study conducted in the northern area of the field, near the crest of the structure, indicate that a permeability anistropy exists.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990