Accurate Residual Oil Saturation from Dielectric Scanner Service's Measurements

Published: 07/06/2011

Blue hero texture

Inaccurate conventional saturation estimation

A UAE operator had been coring and logging wells in a carbonate reservoir to determine the residual oil saturation (ROS) for calculating reserves, defining the development strategy, and managing the reservoir. Conventionally, ROS in the invaded zone is calculated from the Archie equation as 1 – Sxo, where Sxo is the water saturation obtained from resistivity logging. However, uncertainties in the Archie parameters a, m, and n and water salinity variability adversely affect the accuracy of Sxo as a hydrocarbon mobility indicator.

ROS can also be obtained volumetrically from nuclear magnetic resonance (NMR) logging, which requires multiple trips and conditioning of the well, or from core analysis, which is expensive and forces the operator to wait for the laboratory results. The operator wanted an accurate, unbiased, and efficient method for determining ROS.

Water-filled porosity from Dielectric Scanner service

Dielectric Scanner multifrequency dielectric dispersion service delivers water-filled porosity measurements that are not influenced by salinity because the permittivity of water is easily distinguished by its significantly different value from that of the rock matrix or hydrocarbons. In addition to permittivity, conductivity and layer thickness are radially inverted from Dielectric Scanner service's high- resolution attenuation and phase-shift measurements at four frequencies. Dielectric Scanner service also provides textural information for determining the cementation exponents mn and cation exchange capacity (CEC) instead of relying on potentially incorrect estimations or waiting for core analysis.

Graphic: Accurate Residual Oil Saturation from Dielectric Scanner Service's Measurements
Rugose or washed-out holes do not degrade the measurements because the articulated pad design of Dielectric Scanner service greatly improves contact.

Accurate saturations in one trip

Combining Dielectric Scanner service with the logging platform delivered a continuous, high- resolution log of ROS in one trip. ROS in the near-wellbore region is computed from the difference between neutron-density total porosity and water-filled porosity from Dielectric Scanner service.

The high accuracy of Dielectric Scanner service's measurements was confirmed by the saturation results from core analysis.

Graphic: Accurate Residual Oil Saturation from Dielectric Scanner Service's Measurements
ROS determined from Dielectric Scanner service agrees well with core analysis measurements but required only one trip in the well, compared with NMR techniques, saving considerable rig time and operating costs.
Middle East, United Arab Emirates, Asia, Onshore

Challenge: Efficiently determine reliable values of residual oil saturation (ROS) in a Middle East carbonate reservoir.

Solution: Obtain Dielectric Scanner multifrequency dielectric dispersion measurements to accurately quantify water-filled porosity and ROS independent of water salinity and resistivity.

Results: Confirmed accuracy of one-trip Dielectric Scanner service’s porosity and saturation measurements through precise match to core analysis.

Products Used