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AAPG Bulletin, V.
Sealing properties of faults and their influence on water-alternating-gas injection efficiency in the Snorre field, northern North Sea
1Roxar Software Solutions ASA, P.O. Box 165 Skyen, Oslo, Norway; email: [email protected]
2Compagnie Gnrale de Gophysique Norge, P.O. Box 243, Hvik, Norway
3Dong Norge AS, Kirkevegen 59d, Hvik, Norway
Einar Sverdrup received his M.Sc. degree in structural geology from the University of Oslo in 1985 and his Ph.D. on faults, diagenesis, and reservior performance in 2002. He was with Pelican (1985–1987) and the University of Oslo (1987–1990) before he joined Saga Petroleum in 1990. In Saga Petroleum, Einar worked in the Reservoir Department and paid particular attention to the integration of fault interpretations and seal analyses with other aspects of reservoir development. He left Saga in 2000 as a structural geology advisor and joined the Norwegian Geotechnical Institute, where he focused on fractured reservoirs. Since 2001, he has been the discipline leader of structural modeling in Roxar Software Solutions, with special interests in characterizing and implementing faults and fractures in reservoir models.
Jan Helgesen received his M.Sc. degree in physics from the Norwegian Institute of Technology in 1981 and his Ph.D. in acoustics in 1990. He joined Institutt for Kontinentalunderskelser Petroleum Research as research scientist in 1983 and worked on topics in seismic processing, amplitude vs. offset (AVO), and inversion. In the period 1986–1988, he was affiliated to the French Petroleum Institute and Elf Aquitaine in France. He later served as manager of geophysics and manager of the Reservoir Department at Institutt for Kontinentalunderskelser Petroleum Research. In 1995, he joined Saga Petroleum as a geophysical advisor to work on AVO, inversion, and seismic monitoring (4-D). Since 2000, he has worked as a technical advisor of research and development at CGG Norge, with special focus on reservoir-related processing and prestack depth imaging.
Jon Vold received his Master of Science degree from the University of Oslo in 1997. He then joined Saga Petroleum as a reservoir structural geologist, where he worked with fault modeling and fault seal evaluation. In 2000, he joined Norsk Hydro Research Center as a structural geologist, working with both exploration and reservoir geology. Since 2002, he has worked in Dong Norge as advisor of structural geology. Topics of interest include fault analysis, geometry, seal and fault modeling, seismic interpretation, and tectonic evolution.
We are thankful to Norsk Hydro (Saga Petroleum) for allowing us to publish the material contained in this paper. We also thank Dr. Graham Yielding (Badley Earth Sciences) and Prof. Roy H. Gabrielsen (University of Bergen) for valuable comments to an early version of the manuscript. The referees of this paper, Peter Gretener, Ben D. Hare, and an anonymous reviewer, are acknowledged for thorough and constructive criticisms.
Within the Snorre field, faults need careful consideration in reservoir modeling and simulation work. In this field, a static seal capacity for oil as much as about 5 bar (3.44 104 Pa) is observed across important faults. In a dynamic setting, the seal capacities for oil become about one order of magnitude higher than for the static situation. Water-alternating-gas (WAG) injection has been chosen as the main recovery mechanism. Because capillary seal capacities as much as 15 bar (1.03 105 Pa) are found for gas to flow across faults in the reservoir, the distribution of gas and efficiency of WAG to a large degree depend on the fault patterns. For the purpose of evaluating the flow paths of the injected fluids, tracer data and time-lapse seismic data were used.
Tracer and time-lapse data confirm that the WAG process in Snorre depends on the geometry and properties of medium- to small-scale faults. As a consequence of intrareservoir faults, injected gas is trapped in fault-bounded segments, whereas the water is allowed to spread out laterally. The observations suggest that flow of oil and gas across faults in the Snorre field must be treated individually in flow simulators.
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