Quantitative EPT Interpretation in Australian Reservoirs
Authors: Penney R.K. , Calcraft A.P. , Pietsch A.P. 
Published: December 2007

Santos experience with Schlumberger's EPT (Electromagnetic Propagation Tool) in the late 1980's was disappointing due to lack of experience in using the EPT for quantitative interpretation and due to operational problems caused by poor tool handling and maintenance procedures.

Quantitative EPT interpretation allows the determination of a high resolution invaded zone saturation independent of cementation and saturation exponents, and clay conductivity effects. Combination of the EPT with the MSFL allows investigation of cementation exponent, m. Combination of the EPT with high sampling rate density logs allows identification of pay in thin beds.

Examples from reservoirs across Australia demonstrate that running an EPT is shown to be an advantage where:*uncertainty or variability in m is suspected*ultra-fresh formation water causes clay conductivity effects to outweigh hydrocarbon effects*thin bedded or laminated sands contain oil pay not identifiable on standard logs.

For zones of unrepresentative deep resistivities, such as thin bedded sands, calculation of water saturation from EPT-derived invaded zone saturation is also discussed.

Introduction

Santos introduced EPT logging in oil wells in 1981. Trials were carried out over Jurassic sands with fresh water, shaly sands and thin oil columns. The most successful use of the tool was in the Jackson oil field (Qld) discovery and appraisal wells. This led to a published example, although the EPT was still not incorporated in the Operators interpretations.

Use of the tool was discontinued in the late 1980's due to operational difficulties which appeared to be increased with borehole salinity exceeding 20 000 ppm NaCl. The first author's experience with EPT logging in high salinity environments in the Middle East caused this situation to be re-appraised. Investigation revealed a consistent lack of handling and maintenance care and allowed the tool to be promoted for successful reintroduction in 1995.

Dielectric Logging

The Electromagnetic Propagation Tool, EPT, of Schlumberger is a shallow investigation device that operates at a frequency of 1.1ghz. The EPT is configured with two microwave transmitters and receivers mounted in an antenna pad in an array to compensate for, and minimise, borehole effects.

The advantage provided by dielectric logging is based upon the dielectric permittivity of both the minerals and the hydrocarbons within sedimentary rocks being low when compared with that of water of any salinity. The dielectric constant of a formation can be derived from Maxwells equations from the attenuation, A and the propagation time, tpl, of an electromagnetic wave as it passes through the formation. As indicated by Poley et al., the propagation time is related to lithology, porosity and water saturation whereas the attenuation is related to salinity, porosity and water saturation. Figures 1 and 2, from Schlumberger Chart EPTcor-1 show the variation of brine and matrix propagation times, and hydrocarbons have typical propagation times of the order of 4 or 5 nsec/m.

Quicklook Interpretation

Two methods are routinely used by the authors for EPT interpretation, the tpo method and the Complex Time Average (CTA) method. The more complicated Complex Refractive Index Method (CRIM) using both attenuation and propagation time information is not routinely utilised.

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