Summary
A recently introduced rapid petrophysical wireline evaluation has been
tested and demonstrated to provide accurate petrophysical answers in a
deepwater Gulf of Mexico well. This new interpretation scheme uses the
combination of elemental concentrations from nuclear capture spectroscopy logs
and standard neutron, density, resistivity, or “triple-combo” log measurements
to get a more accurate description of the lithology and matrix parameters. Of
particular value in the interpretation are the straightforward computation of
matrix density from elemental concentrations and the resulting accurate
porosity. Accurate clay and porosity values combine to provide a good estimate
of permeability. These are all integrated with resistivity logs to estimate
water saturation and irreducible water saturation to complete the petrophysical
interpretation (Herron et al. 2002).
In this study, the rapid petrophysical evaluations of porosity,
permeability, and irreducible water saturation are demonstrated to agree well
with core data. Initial disagreement between the interpreted lithology and core
data jeopardized future use of the new interpretation, but on further
investigation, the discrepancies were proved to be caused by the core
measurements, not the wireline data. Therefore, part of the study focuses on
the reanalysis and evaluation of core samples.
Introduction
Complex lithologies in the deepwater Gulf of Mexico introduce new
petrophysical challenges. Conventional wisdom or approaches to interpreting
wireline logs do not always provide the desired accuracy in petrophysical
answers. The promise of improved accuracy in lithology characterization and
porosity evaluation led to the acquisition of nuclear spectroscopy wireline
data and the subsequent testing and evaluation of an integrated evaluation
using the spectroscopy logs with the conventional “triple combo” of neutron,
density, and resistivity (Herron et al. 2002).
The inclusion of the spectroscopy logs provides the opportunity to
accurately characterize the rock matrix in terms of its lithology and matrix
properties and thereby enable a more reliable and accurate petrophysical
evaluation. Selected core samples from the study well were analyzed for clay
content, porosity, permeability, and irreducible water saturation, and the data
are used to validate the interpretation. This paper outlines the steps in the
interpretation and demonstrates their validity by comparison with core
data.
For the petrophysical properties of porosity, permeability, and irreducible
water saturation, the petrophysical interpretation shows very good agreement
with the core data. However, in spite of this agreement, the interpretation was
initially questioned because of a significant discrepancy between the computed
clay content and the core values measured by X-ray diffraction (XRD). XRD clay
averages less than half the amount computed from spectroscopy logs. To
investigate this discrepancy, additional core samples were analyzed with a
different analytical technique, dual-range Fourier transform infrared
(DRFT-IR), and it was concluded that the XRD data are in error. The new clay
and chemistry data are used to optimize the estimate of total clay from
chemistry. The original and optimized interpretations are both presented. The
new methodology was used in new wells from the same field and continues to be
applied to other deepwater Paleogene formations.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
3 June 2004
- Revised manuscript received:
20 September 2005
- Manuscript approved:
1 February 2006
- Version of record:
20 April 2006
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