Abstract

A new measurement-while-drilling (MWD) multi-resistivity tool has been introduced in the Middle East. It measures five resistivities, four of them focused, and a gamma ray.

The four focused resistivities all have the same very high vertical resolution, and different depths of investigation. The deepest measurement uses a ring electrode which measures resistivity all around the borehole, allowing a measurement even when the tool is sliding. The other three focused measurements are made with button electrodes which are azimuthally sensitive.

The true formation resistivity is recorded by the deepest measurement before invasion becomes severe. Button resistivities, thanks to their different depths of investigation, identify zones where invasion starts, and allow invasion diameter computations in favorable conditions.

Images of the formation resistivity at three different depths of investigation are produced. This enables geological features, including dips, fractures and their orientation to be studied. The combination of resistivities with three depths of investigations produces an image of the invasion profile around the borehole.

INTRODUCTION

A new measurement-while-drilling (MWD) multi-resistivity & imaging tool has been introduced in the Middle East. It measures five resistivity measurements, a gamma ray, and a single axis tool deviation.

This tool has been developed to satisfy conditions which were not covered by previous generation of tools, such as the 2M Hz electromagnetic tools.

The new features provided are:

  • Resistivity as close as possible and some times ahead of the bit, hence the name Resistivity-at-the-Bit (RAB*)

  • True resistivity in "laterolog" conditions (Rxo<Rt).

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