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AAPG Bulletin, V.
Uncertainty in well test and core permeability analysis: a case study in
fluvial channel reservoirs, northern North Sea, Norway
1Department of Petroleum Engineering, Heriot-Watt University,
Edinburgh, EH14 4AS, Scotland; email: [email protected]
2Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS, Scotland; email: [email protected]
3Research Centre, Norsk Hydro, Sandsliveien 90, N-5020, Bergen, Norway; email: [email protected]
4Edinburgh Petroleum Services Ltd., Riccarton, Edinburgh, EH14 4AP, Scotland; email: [email protected]
Shi-Yi Zheng specializes in pressure transient analysis and is involved in the teaching of well testing at Heriot-Watt University. As a research associate, his current research interests are the integration of well testing with other disciplines for fluvial and turbidite reservoir characterization. Shi-Yi obtained his B.Sc. degree from the Huadong Petroleum Institute in 1982 and Ph.D. from Heriot-Watt University in 1997. He completed his M.Sc. degree in petroleum engineering studies with distinction in 1991, while managing a project on two-phase flow in the casing screen annulus. Shi-Yi has 11 years oil production experience with the Chinese National Oil Company's Research Institute. He is a member of the Society of Petroleum Engineers and served as an SPE Forum discussion leader in 1998.
Patrick Corbett graduated in 1977 with a degree in geology (Exeter), followed by an M.Sc. degree in micropaleontology in 1978 (University College London), a postgraduate diploma in geological statistics in 1982 (Kingston), and a Ph.D. in petroleum engineering in 1993 (Heriot-Watt). From 1978, Patrick worked for 11 years in industry in various positions in international exploration and development for Unocal. He currently holds the position of Elf Professor in Petroleum Geoengineering in the Department of Petroleum Engineering, Heriot-Watt University. His research focus is on the integration of geoscience and engineering (geoengineering) through geological analysis, petrophysical measurement, and flow modeling. His current research areas include kv/kh modeling, well test interpretation, dynamic upscaling, secondary oil migration, overpressure containment, and petrophysics. He is a Chartered Geologist.
Alf Ryseth is a senior sedimentologist at the sedimentology and stratigraphy department of Norsk Hydro's research center in Bergen, Norway. He is currently working on regional geology and field development in the northern North Sea. He was educated in Bergen, Norway, and holds a Candidatus Scientarium degree in sedimentology (1987) and a Ph.D. in applied sedimentology and petroleum geology (1994) from Bergen University.
George Stewart, chairman of Edinburgh Petroleum Services and professor of petroleum engineering at Heriot-Watt University, has more than 21 years of experience of well test interpretation and reservoir engineering. He has been teaching at Heriot-Watt University since the inception of the Department of Petroleum Engineering in 1975 and was head of the department from 1981 to 1986. George has given numerous industry schools and seminars on well test analysis throughout the world and has authored many papers on the subject. He has also worked as a consultant for North Sea operators and has gained valuable insight into the most common well testing problems encountered by petroleum engineers working in the North Sea and elsewhere. His research interests include reservoir monitoring techniques (wire-line formation testing and production logging), reservoir description and phase behavior. George holds a Ph.D. in chemical engineering from the University of Newcastle upon Tyne and a B.Sc. in chemical engineering from the University of Edinburgh.
We acknowledge Arco British, Shell, Statoil, Mobil, Norsk Hydro, and Unocal for funding this work within the Integration of Geology and Well Testing for Fluvial Reservoir Description project at Heriot-Watt University. We thank Neil Hurley, Michael Heymans, and an anonymous referee; their challenging reviews have stimulated us to dig even deeper into this challenging subject. Kristian Soegaard kindly stepped in to help complete the revisions during one author's period of illness. We also thank Jos Okkerman for tracking down information on the stress sensitivity of the Oseberg rocks.
Reservoir permeability is one of the important parameters derived from well test analysis. Small-scale permeability measurements in wells are commonly made using core plugs or, more recently, probe permeameter measurements. Upscaling of these measurements for comparisons with the permeability derived from drill stem tests (DSTs) can be completed by statistical averaging methods. DST permeability is commonly compared with one of the core plug averages: arithmetic, geometric, or harmonic. Questions that commonly arise are which average does the DST-derived permeability represent and over what region is this average valid? Another important question is how should the data sets be reconciled where there are discrepancies?
In practice, the permeability derived from well tests is commonly assumed to be equivalent to the arithmetic (in a layered reservoir) or geometric (in a randomly distributed permeability field) average of the plug measures. These averages are known to be members of a more general power-average solution. This pragmatic approach (which may include an assumption on the near-well geology) is commonly flawed, owing to several reasons that are expanded in this article. The assessment of in situ reservoir permeability requires an understanding of both core (plug and probe) and well test measurements in terms of their volume scale of investigation, measurement mechanism, interpretation, and integration.
This article presents a comparison of core and well test measurements in a North Sea case study. We undertook evaluation of three DSTs and associated core plug and probe data sets from Jurassic fluvial channel sandstones in a single field. The well test permeabilities were generally found to differ from the core estimates, (Begin page 1930) and no consistent explanation could be found for the group of wells. However, the probe permeameter data were able to further constrain the core estimates. This study highlights the uncertainty in effective in situ reservoir permeability, resulting from the interpretation of small (core) and reservoir (well test) scale permeability data. The techniques used are traditional upscaling combined with the Lorenz plot to identify the dominant flowing interval. Fluvial sandstones are very heterogeneous, and this exercise is instructive in understanding the heterogeneity for the guidance of reservoir models in such a system.