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Chapter 12 Reservoir Geophysics: Vp/Vs ratio versus differential stress and rock consolidation — A comparison between rock models and time-lapse AVO data

By
Kenneth Duffaut
Kenneth Duffaut
1Norwegian University of Science and Technology (NTNU), Faculty of Engineering Science and Technology, Department of Petroleum Engineering and Applied Geophysics, Trondheim, Norway, and Statoil Research Centre, Trondheim, Norway. E-mail: kdu@statoil.com.
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Martin Landrø
Martin Landrø
1Norwegian University of Science and Technology (NTNU), Faculty of Engineering Science and Technology, Department of Petroleum Engineering and Applied Geophysics, Trondheim, Norway, and Statoil Research Centre, Trondheim, Norway. E-mail: kdu@statoil.com.
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Published:
January 01, 2010

Abstract

The compressional to shear wave velocity ratio (Vp/Vs) is an important parameter in seismic amplitude versus offset (AVO) analysis, and this parameter plays a key role especially for lithology and fluid prediction methods. The P-wave velocity is a key parameter in traditional pressure prediction methods, because overpressure often results in a velocity reduction. However, for AVO-based pore pressure prediction methods, one expects that the Vp/Vs ratio also is a key parameter. The Hertz-Mindlin geomechanical model predicts a constant Vp/Vs ratio as the differential stress changes in a dry package of identical spheres. Ultrasonic core measurements show increased Vp/Vs ratios as the differential stress decreases, especially for unconsolidated wet sands. Thus, one is likely to assume that the Vp/Vs ratio is dependent on rock consolidation. By combining the Hertz-Mindlin model with the Gassmann model, we show how to obtain a simple rock-physics framework including both the differential stress and the degree of rock consolidation. We use the number of grain-to-grain contacts (coordination number) to represent the rock consolidation. For two field examples, we calibrate this consolidation parameter to in-situ stress conditions, then compare the predicted Vp/Vs ratios for the overpressured reservoir conditions with observed time-lapse AVO changes. The correspondence between modeled and AVO-estimated Vp/Vs ratios is good within the assumed accuracy of the real time-lapse AVO changes. In both cases, we observe an increase in the Vp/Vs ratio as the differential stress decreases. In the first case, a pore pressure increase of 5-7 MPa is measured, whereas the other case shows a pressure increase of approximately 15 MPa. The first reservoir represents a low-to-medium-consolidated sandstone reservoir of 33% porosity on average, whereas the second reservoir is amore consolidated sand with similar porosities (30%).

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Contents

Geophysical References Series

Geophysics Today: A Survey of the Field as the Journal Celebrates its 75th Anniversary

Sergey Fomel
Sergey Fomel
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Society of Exploration Geophysicists
Volume
16
ISBN electronic:
9781560802273
Publication date:
January 01, 2010

GeoRef

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