THE BOREHOLE ENVIRONMENT IN TRIAXIAL INDUCTION LOGGING

Abstract

The transmitter-receiver geometry used in traditional uniaxial induction logging was not accidental - a conductive smooth borehole gives a null response for axial magnetic field sources and sensors. However this nulling is not necessarily true for triaxial devices, which are designed to resolve features such as formation anisotropy or vertical fractures which are invisible to traditional tools.

Triaxial devices may be fabricated in the time or frequency domain. In the frequency domain, where the transmitter and receiver are separated, as in the EMI, Inc. device, borehole effects for conductive bore-fluids will be present and can overwhelm the response of formation features unless corrective means are taken, such as a focusing of the source fields. Borehole irregularities, such as breakouts, washouts, and key seats will also give responses which might mask formation properties. Time domain devices with separated transmitters and receivers, as discussed by Gianzero and Su, will have a time window over which the borehole response is minimized. Time domain devices with coincident sources and receivers, such as that discussed by Nekut, will have an appreciable borehole response. In the case of both frequency and time domain methods, numerical modeling of the borehole response is essential for full understanding and remediation of deleterious borehole effects.

Modeling borehole responses can be done using analytic modal responses, integral equations methods, or difference methods. Estimates of differential measurement sensitivities to borehole irregularities are approximated by a simple perturbation formula. Numerical modeling using integral equations for separated source-receiver geometries establishes bounds on the resolution of formation features in the presence of borehole fluid and borehole irregularities.