A Bayesian shifting method for uncertainty in the open-hole gamma-ray log around casing points

Oughton, Rachel H.; Wooff, David A.; O'Connor, Stephen A.

doi:10.1144/petgeo2014-006

A Bayesian shifting method for uncertainty in the open-hole gamma-ray log around casing points

Oughton, Rachel H.; Wooff, David A.; O'Connor, Stephen A.

Authors

Dr Rachel Oughton r.h.oughton@durham.ac.uk
Associate Professor Statistics

David A. Wooff

Stephen A. O'Connor

Abstract

The wireline gamma-ray log is sensitive to open-hole conditions and, in particular, the diameter. This means that the log can jump at casing points. Although environmental corrections exist, they can fail at these points. We present a Bayesian method for deriving a new quantity – the shifted gamma–ray index – that takes these shifts into account by fitting a piecewise linear function to open-hole data in a depth window around the casing point. Because it is Bayesian, the method enables us to assess our uncertainty about its performance. This method requires very little knowledge of the borehole or drilling conditions but relies on the assumption that the lithology is consistent. Investigating the other wireline logs enables us to assess whether this assumption is valid. We demonstrate our method using well data from offshore mid-Norway.

Citation

Oughton, R. H., Wooff, D. A., & O'Connor, S. A. (2014). A Bayesian shifting method for uncertainty in the open-hole gamma-ray log around casing points. Petroleum Geoscience, 20(4), 375-391. https://doi.org/10.1144/petgeo2014-006

Journal Article Type	Article
Acceptance Date	Sep 10, 2014
Online Publication Date	Oct 28, 2014
Publication Date	Nov 1, 2014
Deposit Date	Oct 3, 2014
Publicly Available Date	Oct 28, 2014
Journal	Petroleum Geoscience
Print ISSN	1354-0793
Publisher	European Association of Geoscientists and Engineers (EAGE)
Peer Reviewed	Peer Reviewed
Volume	20
Issue	4
Pages	375-391
DOI	https://doi.org/10.1144/petgeo2014-006