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Case study: Dielectric Scanner Service Unmasks Light Oil in Low-Resistivity Pay in South America

Salinity-insensitive measurements accurately calculate saturations to correct conventional resistivity analysis

Challenge: Locate light-oil-bearing sands in reservoirs with connate water that abruptly varies from fresh to saline.

Solution: Deploy Dielectric Scanner multifrequency dielectric dispersion service to obtain salinity-insensitive measurements differentiating water-bearing formations from oil reservoirs.

Result: Confirmed ambiguous conventional logging measurements and identified an additional 80 ft of pay.

Interpretation-misleading salinity variations

Petrophysical interpretation is demanding in a South American field where the connate water abruptly and repeatedly varies from very fresh to more saline. Traditional determination of the connate water salinity by using the apparent water resistivity (Rw) and deflection of the spontaneous potential curve often missed prolific oil-bearing sands because of the variation in Rw.

Salinity-insensitive Dielectric Scanner service

Dielectric Scanner multifrequency dielectric dispersion service measures dielectric permittivity and conductivity at four frequencies to provide dielectric dispersion at 1-in [2.54-cm] vertical resolution. Inversion of the measurements provides salinity-insensitive water-filled porosity and water salinity at two depths of investigation along with textural information for determining the Archie exponents mn in carbonates and cation exchange capacity (CEC) in siliciclastics. Previously, these parameters had to be estimated from log analysis or measured by laboratory core analysis. Pressure, temperature, and the permittivity and conductivity of the borehole mudcake are also measured to correct for environmental effects.

Accurate saturations where salinity changes

Dielectric Scanner multifrequency service was combined with a triple-combo logging platform to log a well with two intervals of interest.

In the lower interval, from X,500 to X,700 ft, it was a straightfrward process to confirm the conventionally measured invaded zone resistivity with Dielectric Scanner service’s resistivity (Track 5). Track 4 of the log shows extensive sands, consisting of an upper 100 ft of pay and a long residual section of an additional 100-ft accumulation. The hydrocarbon volume determined from comparison of Dielectric Scanner service’s water-filled porosity volume to the total porosity in Track 6 matches the resistivity indications. In Track 3 the dielectric-calculated water saturation for the invaded zone parallels the deeper resistivity-based saturation. Upon completion the interval is as prolific as indicated by the logs, delivering 15,000 bbl/d of oil.

In the second interval, from X,115 to X,215 ft, the conventional and Dielectric Scanner service’s resistivities significantly diverge. The dielectric measurements accurately show low-resistivity pay (Track 5). The conventional resistivity analysis and saturation went astray because a consistent value of Rw was assumed in an interval where the salinity varies. By not relying on estimated parameters, the invaded zone water saturation calculated form Dielectric Scanner service accurately corrects for the bias of the resistivity-based saturation in Track 3.


Download: Dielectric Scanner Measurements Unmask Light Oil in Low-Resistivity Pay in South America (2.05 MB PDF)

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