Dielectric measurements and interpretation, introduced with great promise in the 1980s, have not found widespread use owing to measurement limitations, moderate accuracy, and insufficient quality control. This paper presents a new-generation dielectric tool that overcomes these limitations and brings extra information for more accurate petrophysical formation-evaluation. One of the revolutionary advances offered by this tool is the continuous measurement of dielectric dispersion (variation of formation dielectric properties as a function of the frequency) at 1-in vertical resolution. The tool uses multi-spacings antenna arrays operating at multiple frequencies in the MHz to GHz range. Moreover, transmitter and receiver antennas have colocated longitudinal and transverse polarizations. The wealth of recorded data will allow interpreters to both adapt the answer product to the reservoir fluids and geology, and also to provide acquisition quality-controls and error estimates on the results. Carbonate and heavy oil reservoirs are becoming more and more important. In these two environments, the new tool provides unique answers to better characterize these reservoirs. This tool also addresses traditional dielectric formation-evaluation in fresh water and thinly laminated sands. Answer products offered by this new tool can be classified in three categories.Pore-fluid analysis from the muti-spacings high frequency measurementsHydrocarbon residual saturation and invaded zone water salinityInvasion profile: hydrocarbon saturation profile in heavy oil reservoirMatrix analysis from dielectric dispersionCarbonates: textural information (Archie m cementation factor)Shaly sands: high resolution clay volume and anisotropyGeological structure analysis from the muti-polarizations and high resolutionThin beds analysisStructural anisotropy measurement in very thin bedsGeological features extractionCarbonate classification This paper reviews the dielectric dispersion physics, then describes the tool's architecture, measurements, and data processing chain. Field test examples illustrate the enhanced interpretation potential of dielectric dispersion measurements from this tool. Introduction to dielectric dispersion physics The permittivity quantifies the sensitivity of a medium to an electric field excitation. Three main physical phenomena contribute to the permittivity: the displacement of the electronic cloud of atoms, the coherent orientation on pre-existing microscopic electric dipoles and the polarization effect at the interfaces. These phenomena are illustrated in Figure 1.
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.
In recent years, high-frequency dielectric tools have become an increasingly accepted service for providing saturation, water salinity, and textural information. To obtain these petrophysical quantities from the measured dielectric data, a mixing law is used that relates the bulk formation complex permittivity (relative permittivity and conductivity) to the constituents of the system, including the water present in the pores. Several mixing laws have already been published in the literature. This paper is concerned with an integral part of all of them, which is the prediction of the water complex permittivity as a function of salinity, temperature, and pressure.The water complex permittivity is known to vary with salinity, temperature, pressure, and frequency of operation. We have established a dielectric water model as a function of these four quantities, and have recently validated this model with dielectric laboratory experiments on several water samples at high temperature and pressure. In this paper we fully disclose the model so that it can be used and adapted by the industry.We also demonstrate with a sensitivity analysis that using the correct water model is critical to the accuracy of the final petrophysical outputs.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe analysis of deep-reading electromagnetic measurements is critical to the evaluation of hydrocarbon reserves. However, in thin bed formations, poor tool vertical resolution and corresponding low sensitivity to hydrocarbon presence make interpretation in the virgin zone difficult. A priori knowledge such as the formation geometry or auxiliary petrophysical information is necessary to overcome these difficulties. This paper presents a prototype code developed by Schlumberger S-RPC in collaboration with AGIP. Using this code, wireline or LWD, laterolog and induction measurements can be more correctly analyzed in thinly bedded environments (2-D geometry, fluid invaded layers perpendicular to the borehole).This code has been implemented in a software framework that provides a common environment specifically designed for electrical tool interpretation. Processing modules have a common interface and share common functionalities. Their organization reflects an implicit processing methodology, with progressive refinements that provides the interpreter with a robust and simple to use product, to better quantify reserves.A preliminary step is to determine the formation geometry, which is carried out by detecting bed boundaries and representing the formation as a vertical sequence of layers. Petrophysical analysis can be invoked to characterize certain formation properties such as shale volume and porosity. These steps are performed prior to resistivity log measurement analysis and serve as a form of a priori knowledge.Once the formation is described as a sequence of layers, wireline l ogging or logging while drilling (LWD) tool response can be computed using fast 2D simulators. The estimation of resistivity and the subsequent estimation of saturation will correspond to the minimization of a cost function, defined as the weighted squared difference between the measurement and the simulated response. Confidence outputs can be related to the local shape of the cost function at the end of the processing.Two important advantages of the new code must be emphasized: (1) the possibility to choose among several petrophysical models to better describe the environment and determine directly parameters such as hydrocarbon saturation, and (2) the possibility to group together beds which are too thin or too close to each other to be analyzed independently, into a so called single optimization interval described by a reduced set of parameters.This paper presents results obtained on selected benchmarks extracted from real data and compares them with those obtained through more traditional approaches.
The ARI* Azimuthal Resistivity Imager is a standard laterolog equipped with an additional segmented electrode. This segmented electrode provides twelve deep calibrated azimuthal resistivity measurements. As any electrical device, the ARI tool is sensitive to fractures filled with conductive fluid. Finite-element modeling was performed to study the tool response to fractures. It shows that techniques similar to those used to characterize fractures with the FMI* Fullbore Formation Microlmager can be used with the ARI tool. But it also demonstrates that the extension of the fracture (or the length of mud invasion for an oil filled fracture) and its width (aperture) are equally important parameters when characterizing the fracture. From the resistivity image. the geometry (if the fracture (dip and azimuth) can be computed. A lower limit of the fracture aperture and a lower limit of the fracture extension can be estimated from the azimuthal resistivities. Log examples illustrate the proposed technique.
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