ABSTRACT

Waterflooding often produces wide variations in formation water salinity, which preclude quantitative interpretation of resistivity logs. In principle, dielectric logs are well suited to estimation of remaining oil saturation in waterflooded reservoirs, because their measurements are less sensitive to salinity variations than are resistivity measurements. Salinity effects on dielectric logs are significant, however, and previously published interpretation techniques do not adequately account for these effects. Recently, the Electromagnetic Propagation (EPT) log was tested for residual oil determination in a West Texas field which has been waterflooded since 1961. The field produces from a highly stringerized carbonate formation with average porosity, 10%. Formation water salinity now ranges from 20,000 to 200,000 ppm NaC1. Mud filtrate salinity was 170,000 ppm NaC1. Two hundred feet of pressure core were taken for laboratory measurement of residual oil volume and saturation. The EPT log data were analyzed by three different techniques: the published "tpo method" and "complex refractive index (CRIM) method", and a variation of the CRIM method. The latter variation uses independent laboratory data on the complex dielectric permittivity of NaCl solutions to predict the EPT propagation time of the mud filtrate.

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