Summary
Formation evaluation in unknown water salinity can be performed by solving
simultaneously for Sw and Rw from the logs
of resistivity and sigma. This has not been used extensively in the past
because the logs are often acquired on wireline at various times, leading to
unknown invasion effects that complicate the integration of the deep-reading
resistivity and the shallow-reading sigma logs. We will show that when the two
measurements are free from invasion effect and other interpretation parameters
(such as porosity and Archie m and n) are reasonably known, the
technique works well and gives reliable Sw and
Rw answers.
Both numerical and graphical solutions are presented. The numerical
technique uses a minimization routine to solve for a salinity value that is
consistent with Sw calculated from resistivity and from
sigma. The salient point of the paper is the graphical technique that provides
better insights into petrophysics than a set of equations. It uses an overlay
of equisaturation and equisalinity lines superimposed on the crossplot of sigma
vs. resistivity data. The overlay dynamically changes as porosity, lithology,
hydrocarbon type, temperature, and other parameters vary with depth. The
graphical technique has several applications. For the interpretation, it is
used to estimate Sw and salinity, and to identify zones of
changing properties. For quality control, it is used to validate the input
data. For processing, it is used to select parameters and analyze their
sensitivities to the results. For job planning, it is used to validate (or
invalidate) the application from input values of resistivity and sigma. The
graphical technique provides a valuable aid to the petrophysicist and gives an
independent verification of the numerical solution.
The proposed technique is illustrated with wireline induction and sigma logs
acquired in several wells in a mature Middle East carbonate field. The studied
wells are completed in open hole and are flowed during logging to ensure that
both resistivity and sigma logs are free from invasion effect. The results of
the resistivity/sigma technique compare very favorably with those of
carbon/oxygen (C/O) logging. Moreover, production-logging data confirmed the
results and showed that the resistivity/sigma technique provides more-robust
answers than C/O in low-porosity formations (below 15 p.u.).
© 2011. Society of Petroleum Engineers
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History
- Original manuscript received:
26 June 2010
- Meeting paper published:
21 September 2010
- Revised manuscript received:
15 January 2011
- Manuscript approved:
28 January 2011
- Published online:
12 August 2011
- Version of record:
15 August 2011
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