A Novel Approach Based on Dielectric Dispersion Measurements to Evaluate the Quality of Complex Shaly-sand Reservoirs

Abstract

Abstract This paper presents a novel approach for the formation evaluation of shaly-sand thin layered reservoirs using dielectric dispersion wireline logging provided by a new generation tool. The study is focused on gas fields in the Mediterranean basin characterized by the presence of very thin inter-laminations (centimeter-scale or even less). In these complex reservoirs, the conventional petrophysical analysis is very challenging due to standard logging tools’ vertical resolution limits which prevent the quantitative estimation of the clay content, of a detailed water saturation profile and of the reservoir quality. The new-generation, cm-scale vertical resolution tool overcomes these limitations thanks to the continuous measurement of dielectric dispersion from the MHz to the GHz frequency range and to a fit-for-purpose inversion based interpretation model. The outcome is a high resolution estimate of the main petrophysical parameters including water volume fraction for determining hydrocarbon saturation, formation water salinity and an additional textural parameter of the rock matrix that is correlated to clay volume. The model has been developed and calibrated using a statistically representative database of core measurements (including porosity, mineralogy, granulometry, Cation Exchange Capacity as well as dielectric spectra measurements) from several wells drilled in shaly-sand reservoirs representing different geological environments. In all the analyzed wells a full wireline log data set was also available. The application of the new approach to the examined case histories provided satisfactory, high resolution results in terms of: lithological description of the formation with the added value of a quantitative estimation of the clay content; reservoir quality evaluation; gas saturation indicator thus reducing the uncertainty in the petrophysical characterization of complex shaly-sand reservoirs.