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AAPG Bulletin

Abstract

AAPG Bulletin, V. 89, No. 1 (July 2005), P. 61-80.

Copyright copy2005. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/07300403002

The rate of pressure dissipation from abnormally pressured compartments

Ann Muggeridge,1 Yafes Abacioglu,2 William England,3 Craig Smalley4

1Department of Earth Science and Engineering, Imperial College, London, SW7 2AZ, United Kingdom; [email protected]
2BP America Inc., Gulf of Mexico Shelf BU, 200 Westlake Park Blvd., Houston, Texas 77079; [email protected]
3BP Exploration plc., Sunbury-on-Thames, Middlesex, TW16 7LN, United Kingdom; [email protected]
4BP Exploration plc., Sunbury-on-Thames, Middlesex, TW16 7LN, United Kingdom; [email protected]

AUTHORS

Ann Muggeridge has been a senior lecturer in petroleum engineering at Imperial College London for the last nine years. Her research interests include reservoir characterization, reservoir simulation, the physics of fluid flow in the subsurface and upscaling. She has also worked at Scientific Software-Intercomp (United Kingdom) as a simulator technical support and at BP as a research reservoir engineer.

Yafes Abacioglu has been working for BP as a reservoir engineer since he obtained his doctorate in petroleum engineering from the University of Tulsa in 2001.

William England has 19 years experience working in the upstream oil and gas industry. After an initial eight years specializing in reservoir and exploration geochemistry, he has worked in a number of reservoir engineering roles, and is currently working on Angolan projects.

Craig Smalley has a long-standing interest in fluid movement in the subsurface. In the 1980s, working at Norway's Institute for Energy Technology, he pioneered new methods for identifying fluid variations and interpreting them in terms of reservoir compartmentalization. He has worked for BP for 16 years in a variety of roles. He is currently a subsurface advisor.

ACKNOWLEDGMENTS

Part of this work was funded by BP Exploration. The manuscript has benefited from the thoughtful reviews by several individuals, some anonymous, and we thank them all for their time and effort.

ABSTRACT

An improved analytic solution is presented that describes the dissipation of abnormal pressures via Darcy flow out of a compartment formed of high-permeability rock capped top and bottom by lower permeability rock. The solution tends to the solutions presented by previous workers in the limit of (1) a thick compartment and thin barrier and (2) a thick barrier and a thin compartment. The importance of including fluid compressibility when analyzing hydrocarbon reservoirs is demonstrated. Previous workers in this field have only included the effect of bulk rock compressibility and have neglected fluid compressibility.

The solution is applied to pressure compartments on the scale of a typical hydrocarbon reservoir and overpressured regions on the basin scale. It is shown that the pressure gradient in a typical hydrocarbon reservoir compartment will return to the hydrostatic gradient over timescales of the order of hours or days, but that abnormal pressures are likely to dissipate over periods of tens of thousands to hundreds of thousands of years. Thus, any reservoir compartment with a different pressure from its neighbors at discovery can be considered as an independent unit during production. However, abnormal pressures on the basin scale may take tens or hundreds of millions of years to dissipate. There is thus no need to invoke zero-permeability seals or capillary-pressure seals in order to explain the existence of abnormal pressures over geological time.

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