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

Oughton, Rachel H. and Wooff, David A. and O'Connor, Stephen A. (2014) 'A Bayesian shifting method for uncertainty in the open-hole gamma-ray log around casing points.', Petroleum geoscience., 20 (4). pp. 375-391.


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.

Item Type:Article
Full text:(AM) Accepted Manuscript
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Publisher statement:Petroleum Geoscience, 20(4), 2014, © 2014 EAGE/The Geological Society of London.
Date accepted:10 September 2014
Date deposited:28 October 2014
Date of first online publication:28 October 2014
Date first made open access:No date available

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