Summary
Validated electrical type curves, which collectively describe a continuum of
conductivity behavior of fluid-saturated rocks, allow the petrophysical
evaluation of hydrocarbon saturation to be set within a generic reference
framework. As part of this process, the type curves permit pre-existing data
from other reservoirs to be examined as potential analogs. Through the type
curves, a reservoir rock can be classified in terms of its electrical
character, specifically adherence to, or departures from, classical clean-sand
(Archie) conditions and, by corollary, the degree of any shale (non-Archie)
effects. The classification guides the approach to future core-data acquisition
and to well-log analysis. In particular, in non-Archie reservoirs the type
curves indicate whether the formation-water salinity is sufficiently high for
the application of shaly-sand equations for the evaluation of hydrocarbon
saturation or whether recourse should be made to a (customized) pseudo-Archie
approach. Thus, the type curves are used to ensure that interpretative
algorithms are appropriate to the petrophysical task at hand.
The application of the type curves, using initializing log data from seven
shaly hydrocarbon reservoirs containing relatively fresh formation waters, has
illustrated how petrophysical interpretations away from the key intervals can
be screened with minimal supporting information through a pseudo-Archie
approach. Comparisons of best estimates of hydrocarbon-filled porosity with
previously reported values have indicated a potential volumetric upside in all
cases, with hydrocarbon saturations being up to 30 saturation units higher for
these complex reservoirs. This outcome is attributed to the generic nature of
the screening process, which takes account of the electrical character of a
reservoir without any of the procedural constraints that are associated with
conventional well-log analysis. To reduce further the risk of underestimating
hydrocarbon volumes, a set of equivalence charts has been constructed using
basic petrophysical properties. The equivalence charts allow a quick-look
recognition of any departures from Archie conditions and thence whether the
type curves are likely to be required.
The screening process has been synthesized into pragmatic workflows, whose
adoption should impart additional quality assurance to the petrophysical
evaluation of hydrocarbon volumes.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
12 July 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
6 August 2008
- Manuscript approved:
24 October 2008
- Published online:
2 March 2009
- Version of record:
26 February 2009
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