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Quantitative time lapse seismic analysis - rock physics, repeatability and inversion aspects

Bhakta, Tuhin
Doctoral thesis
Åpne
Fulltext not available (Låst)
Permanent lenke
http://hdl.handle.net/11250/293504
Utgivelsesdato
2015
Metadata
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Samlinger
  • Institutt for geovitenskap og petroleum [2168]
Sammendrag
Quantitative prediction of pressure as well as saturation effects from time-lapse seismic data

is one of the topics of interest for geophysical society since the last decade. Time-lapse

amplitude variation with offset (AVO) could be used for this purpose. The conventional

pressure-saturation discrimination method suffers due to leakage between estimated pressuresaturation

changes. However, the method works reasonably well for deeper reservoirs where

the P-to-S-wave velocity ratio (Vp/Vs) is around 2. Objective of this thesis is to investigate

the applicability of the conventional inversion method for shallow unconsolidated reservoirs

where Vp/Vs is often higher than 2. In addition, it is also important to investigate the inaccuracy

in the estimates (of pressure and saturation changes) due to improper implementation

of rock physics parameters as well as consideration of first order approximations in AVO

gradient and intercept changes. I propose a new method, based on a stepwise linear approximation

to the intercept and gradient reflectivity changes, to estimate pressure and saturation

changes. Similar to the conventional method the new method utilizes the near- and far offset

seismic surveys as two independent measurements, and then estimates the pore pressure and

saturation changes from amplitude versus offset (AVO).

As the conventional pressure-saturation discrimination method has inaccuracy mainly due

to gradient reflectivity attribute, it is crucial to explore the applicability of other seismic

attributes, such as PS-reflectivity changes and travel time shifts, to discriminate between

pressure-saturation changes. I investigate the applicability of various combinations of seismic

attributes for compacting reservoir scenario.

The usefulness of time-lapse seismic data greatly depends on the repeatability of the data

between different surveys. The more repeatable the data is, the more confidently and efficiently

it may be used in reservoir management. As the refraction method is emerging as

one of the promising and complementary 4D techniques, it is therefore necessary to perform

repeatability analysis on refraction data as well. In addition, repeatability analysis on

other seismic events (such as tank noise, normal modes, sea-bottom reflection) provide more

insight on the efficient implementation of 4D seismic method. I study repeatability issues using

two seismic data sets acquired at the permanent sea-bottom array that was installed at the

Ekofisk field (North Sea) in 2010. I compare pre-stack repeatability of various seismic wave

types, such as refraction, reflection, sea-bottom reflection, tank noise and water column noise.

In carbonate reservoirs, implementation of a proper rock physics model is extremely difficult

due the more complex pore-geometry in carbonate rocks. It is therefore important to

investigate various rock physics models and their applicability for carbonate reservoir. I investigate

the applicability of various rock physics models applied in three different wells at

the Ekofisk Field, North Sea.
Utgiver
NTNU
Serie
Doctoral thesis at NTNU;2015:127

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