Method and system for determining formation anisotropic resistivity with reduced borehole effects from tilted or transverse magnetic dipoles

Abstract

Systems and methods are provided for determining subsurface formation properties using an antenna system disposed within a borehole traversing the formation. A logging system includes a well tool implemented with an antenna system having transverse or tilted magnetic dipoles. One antenna implementation uses a set of three coils having non-parallel axes. Through mechanical or electromagnetic rotation of an antenna about its axis in order to maximize or minimize couplings, the systems and methods are used to estimate formation anisotropic resistivity, providing reduced borehole effects in vertical and small deviation wells. This invention may also be applied in deviated wells when the borehole is perpendicular to the strike direction, as is often the case in wireline logging.

Description

[0001] 1. Field of the Invention[0002] This invention relates generally to the field of well logging. More particularly, the invention concerns improved methods and systems in which an antenna system having transverse or tilted magnetic dipoles is used to estimate formation anisotropic resistivity, providing reduced borehole effects in vertical and small deviation wells. This invention is applicable to induction or propagation type measurements, i.e., at low and high frequencies.[0003] 2. Description of Related Art[0004] Induction and propagation logging techniques have been employed in exploration operations for many years to measure the electrical conductivity (or its inverse, resistivity) of subsurface formations. These techniques entail deployment of antenna systems into a borehole to emit electromagnetic (EM) energy through the borehole fluid (also referred to herein as mud) and into the subsurface formations. Conventional antennas are formed from coils of the cylindrical solen...

AI Technical Summary

Problems solved by technology

When a borehole is added, these current paths are distorted.

A particularly troublesome property of the TMD is the extremely large borehole effect that occurs in high contrast situations, i.e., when the mud in the borehole is much more conductive than the formation.

When these currents pass by TMD receivers, they can cause undesired signals that are many times larger than would appear in a homogeneous formation without a borehole, resulting in erroneous measurements.

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