Dielectric Logging: Principles, Applications, and Examples from the Brazilian Oilfields

Abstract

This work aims to present a review of dielectric logging, including physical principles, petrophysical evaluation, and applications. In addition, we will present a history of its use in Petrobras oilfields. The dielectric properties are generated by the charge alignment created by an excitation provided by an electric field. In this sense, polar characteristics of water molecules allow quantification of volume of water present in the reservoir, independently of salinity. Owing to the shallow depths of investigation, dielectric tools are useful in evaluating residual oil in high uncertainty salinity conditions, especially in mature fields, where uncertainties occur due to injection of water and/or steam. Although this logging tool has a long history of use in the petroleum industry, it was rarely used in Petrobras oilfields due to the high specificity, limitations of the technique, and/or high cost. Considering high frequencies used and proximity between electrodes, the tool provides very shallow measurements, which makes it impossible to evaluate the virgin zone in most reservoirs. Due to these characteristics, the tool was widely employed in low mobility hydrocarbon formations, where mud filtrate invasion tends to be smaller. The dielectric tool was used in Petrobras to evaluate reservoirs with very high viscosity and fresh water, in which it showed good results in the quantification of water saturation and hydrocarbon mobility. Additionally, the tool was used in Pre-salt in order to evaluate residual oil saturation to diminish the uncertain of microresistivity logs. Besides evaluating water saturation, many works have shown other applications for dielectric logging, such as to determine conductivity, salinity, wettability, Archie's "m" and "n" electric parameters, CEC, and evaluation of laminated reservoirs.