Case Study: Dielectric Scanner Service’s Measurements Correctly Identify Water-Wet Zones for Plugging

Completion of misidentified Wolfbone lower zones was costing USD 750,000 per well, West Texas

Challenge: Correctly identify oil-bearing intervals within the low-porosity, complex lithology for completion.

Solution: Obtain Dielectric Scanner multifrequency dielectric dispersion measurements to accurately calculate water-filled porosity insensitive to water salinity and conduct ELANPlus advanced multimineral log analysis to determine the lithology instead basing the saturation analysis and completion design on synthetic rock properties.

Result: Plugged previously completed lower stages now correctly identified by Dielectric Scanner service as exclusively water bearing to reduce water cut by 25%, as confirmed by production logging.

Uncertainty from synthetic rock properties in a complex lithology

The Wolfbone reservoir of West Texas consists of complex, low-porosity sequences of sand, shale, and carbonate layers. An operator had been using a computed log based on synthetic rock properties to determine perforation placement and stimulation design. However, lower zones that had been completed because they were identified on the computed logs as high permeability and oil bearing were producing only water in other wells in the area. The operator needed to know whether these zones were actually oil productive.

Saturation determination independent of salinity and resistivity

Dielectric Scanner service measures dielectric dispersion at a 1-in [2.54-cm] vertical resolution based on nine attenuation and phase-shift measurements at four frequencies. These multifrequency high-resolution measurements are radially inverted to obtain permittivity and conductivity at each frequency. The resulting determination of water-filled porosity is salinity insensitive because the permittivity of water is readily differentiated from that of rock matrix or hydrocarbons. The conductivity measurements provide reconstructed resistivity and water saturation for the invaded zone. Dielectric Scanner service measurements also yield useful textural information, including a continuous determination of the Archie mn exponents in carbonates for determining saturation beyond the invaded zone.

Dielectric Scanner service’s articulated pad greatly improves contact with the formation in rugose boreholes, a conditition that previous mandrel-type electromagnetic propagation tools were sensitive to.

Water-wet zones correctly identified and plugged

Instead of the high oil cut indicated on the synthetic logs, Dielectric Scanner service logging identified only water-filled porosity in the lower zones, and recomputation of the water saturation using the mn values from Dielectric Scanner service further validated that the zones were water-wet. ELANPlus analysis of the lithology also showed that the volume of clay and shale in the zones had been vastly overestimated, and this error had been propagated through the generation of synthetic rock properties, resulting in height growth into other stages during stimulation. The water production was confirmed by production logging, showing that the lower two stages were contributing 184 bbl/d of water, which was 24% of the well’s water cut.

Relying on the computed logs had cost the operator USD 750,000 to drill down to and complete the lower unproductive zones. That amount multiplied for 60 wells in the area totals to more than USD 45 million wasted on water production. With Dielectric Scanner service and production log confirmation that the lower zones are water-wet, the operator will no longer drill down to them, saving also 7 to 10 days of rig time for each well.

Download: Dielectric Scanner Measurements Correctly Identify Water-Wet Zones for Plugging (0.95 MB PDF)

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Multifrequency dielectric dispersion measurements speak volumes about carbonates, shaly sands, and heavy oil. Visit the Dielectric Scanner webpage