Abstract

Traditionally, wellbore images logs have been used to perform structural and stratigraphic analyses. This study presents an approach of the resistivity image oriented to the identification of aquifers, further enlarging the application scope of this tool. From the analysis of borehole resistivity image logs, special image characteristics were observed, which were not related to their traditional application. Consequently, an analysis was carried out integrating well logs and interpreted core data for two different areas and formations located in Argentina in the Neuquén Basin: once at Lower Troncoso Member of the Huitrin Fm (Aptian), and the other at the Sierras Blancas Fm (Kimeridigian). This particular response from resistivity images was calibrated to reservoir zones with high water saturation, corresponding to aquifers.

Certain features were observed in the studied cases which constitute a condition for positively identifying aquifers by means of borehole electrical images. These are as follows:

  • Clastic reservoirs (sand/clay).

  • Reservoirs with good petrophysical conditions.

  • Drilling mud salinity values similar to formation water salinity values.

The high resolution rendered by this image logging tool makes it possible to identify high-water-saturation zones and accurately locate oil-water contacts. No special processing is required, and decisions can be made based on the direct observation of the recorded image.

Introduction

Traditionally, wellbore images logs have been used to perform two kinds of interpretations:

  • Structural, such as the determination of paleo-stress fields, faults and fractured reservoirs analysis.

  • Stratigraphic, such as paleo-currents, reservoir geometries, widths and sedimentary paleo-enviroments.

The identification of high-water-saturation zones in a reservoir is an important task but it has proven to be very difficult if it is based only on logging interpretations. The intention of this study is to include the response from electrical images for the identification of high-water-saturation zoneswhich may produce high volumes of wateror only water.

Some special responses from borehole electrical images were observed in some wells of Borde Montuoso and Puesto Hernández fields in the Neuquén basin. The integration of openhole logs data, core studies and production information proved to be a useful complementary tool for the accurate identification of high-water-saturation zones and oil-water contacts.

Hence, the objective of this work is to provide information about the use of a tool with high vertical resolution and accurate readings and which under certain conditions can be used for the identification of high-water-saturation zones.

STAR Imager[SM]: Measurement Principles.

The STAR Imagen[SM] provides high-resolution resistivity formation images in conductive mud systems. Six pads with 24-sensors are mounted on articulated arms, resulting in a total of 144 micro-resistivity measurements, with a vertical and azimuthal resolution of 0.2" (~5 mm). Fig. 1.

A known constant voltage difference between the return electrode and pads is used to create a current flow through the formation. The pads and the current electrode are separated by an electrical fiberglass isolator.

Control circuitry is used to maintain zero voltage potential difference between the upper part of the tool and the measure electrode. Thus, current is forced to flow into the formation, in a perpendicular way near the pad face.

The individual current measurements recorded from each button is a function of the formation conductivity and the voltage applied. These measurements are scaled to resistivity values in order to correlate with conventional resistivity logs.

High resolution images are achieved by sampling at a high rate (120 samples per foot) the readings from 144 "buttons" mounted on the 6 pads.

Due to the short depth of investigation on this tool (around 0.3 in to 0.5 in, depending on the conductivity of the formation), image log measures the flushed zone (assuming that invasion in the formation occurs).

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