Paper A

Response of 2-Mhz LWD Resistivity and Wireline Induction Tools in Dipping Beds and Laminated Formations

Barbara Anderson, Stephen Banner, Martin G. Lüling and Richard Rosthal

Schlumberger

Abstract

The responses of wireline induction and 2-MHz resistivity tools for logging while drilling (LWD) are compared in dipping formations. The differences in mea­surement physics, depths of investigation and vertical resolutions between induction and 2-MHz tools lead to different responses in steeply dipped beds. Some new and interesting effects are presented that are characteristic of all 2-MHz LWD tools. These effects have been observed on field logs and have also been reproduced with computer modeling. In particular:

-    In thin (2 ft -10 ft) beds, dip and shoulder effects can cause separations between phase shift and attenuation resistivity curves and affect the readings for all 2-MHz tools. The amount of separation is a function of dip angle, resistivity level and resistivity contrast.

-    At dipping bed boundaries, 2-MHz logs can show horns caused by electric polarization of the interface. The horns are more prominent on phase shift resistivity logs.

-    Resistivity anisotropy causes a separation of 2-MHz phase shift and attenuation resistivity curves in laminated or shaly formations logged at an angle.

A thorough understanding of the effect of dip improves interpretation of 2-MHz tools.