Summary
Because of their sensitivity to ionic content and surface texture, wide-band
electromagnetic (WBEM) measurements of saturated rocks exhibit frequency
dispersions of electrical conductivity and dielectric constant that are
influenced by a variety of petrophysical properties. Factors as diverse as
fluid saturation, porosity, pore morphology, thin wetting films, and
electrically charged clays affect the WBEM response of rocks. Traditional
dielectric mixing laws fail to quantitatively and practically integrate
these factors to quantify petrophysical information from WBEM measurements.
This paper advances a numerical proof of concept for useful petrophysical WBEM
measurements. A comprehensive pore-scale numerical framework is introduced that
incorporates explicit geometrical distributions of grains, fluids and clays
constructed from core pictures, and that reproduces the WBEM saturated-rock
response on the entire kHz-GHz frequency range. WBEM measurements are verified
to be primarily sensitive (a) in the kHz range to clay amounts and wettability;
(b) in the MHz range to pore morphology (i.e., connectivity and eccentricity),
fluid distribution, salinity, and clay presence; and (c) in the GHz range to
porosity, pore morphology and fluid saturation. Our simulations emphasize the
need to measure dielectric dispersion in the entire frequency spectrum to
capture the complexity of the different polarization effects. In particular, it
is crucial to accurately quantify the phenomena occurring in the MHz range
where pore connectivity effects are confounded with clay polarization and
pore/grain shape effects usually considered in dielectric phenomena. These
different sensitivities suggest a strong complementarity between WBEM and NMR
measurements for improved assessments of pore-size distribution, hydraulic
permeability, wettability, and fluid saturation.
Introduction
A number of experimental and theoretical studies suggest the measurable
sensitivity of WBEM to various petrophysical factors, including porosity, brine
salinity, fluid saturation and wettability, clay content, surface roughness,
and even pore surface-to-volume ratio. Given the complexity of the different
phenomena under consideration, practical models are designed to fit measured
dielectric dispersions to ad-hoc models whose parameters are marginally
supported by quantitative petrophysical concepts.
Therefore, to assess whether accurate and reliable petrophysical
interpretations are possible with WBEM measurements requires an analysis that
(a) incorporates pore structure, pore connectivity, multiphase saturation and
electrochemical effects; and (b) quantifies the contributions of each factor in
the measured WBEM dispersions. However, extracting explicit petrophysical
information from WBEM responses is a difficult task. Myers (1991), for
instance, illustrated the non-uniqueness of WBEM measurements when a decrease
of water saturation, porosity, or brine salinity yielded similar responses.
Recent advances in NMR logging and interpretation (Freedman et al. 1990) can
eliminate some of these ambiguities with adequate experimental conditions, and
if rock wettability is known. Conversely, WBEM measurements could provide
independent wettability assessment in the cases where NMR measurements alone
reach their limits of sensitivity [for instance, the impact of fluid saturation
history on wettability determination was studied by Toumelin et al. (2006)].
Likewise, the interpretation of NMR measurements can be biased by unaccounted
rock morphology (Ramakrishnan et al. 1999) or by internal magnetic fields in
shaly or iron-rich sands (Zhang et al. 2003), whereas WBEM measurements provide
independent information on overall rock morphology. It is therefore timely to
consider integrating both technologies for improving petrophysical
analysis.
The objectives of this paper are twofold: (1) Review existing results on the
extraction of petrophysical information from rock WBEM measurements, and (2)
establish a proof of concept for the necessity to integrate electromagnetic
measurements on the wide-frequency band from the kHz range to the GHz range,
and study how WBEM techniques may yield petrophysical information unavailable
from other in-situ measurements. To reach the second objective, we introduce a
generalized pore-scale simulation framework that allows incorporating arbitrary
rock morphology and multiphase fluid distribution.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
13 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
12 September 2007
- Manuscript approved:
7 October 2007
- Version of record:
25 June 2008
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