Effects of Mandrel, Borehole, and Invasion for Tilt-Coil Antennas

Summary

Several recently published studies discuss the concept of inductiveresistivity-logging devices with oblique transmitting and/or receiving coils.Both wireline induction and logging-while-drilling (LWD) propagationresistivity-tool concepts have been considered. Directional resistivitymeasurements and improved anisotropy measurements are among the benefitspromised by this type of device. Analyses based on point-magnetic dipoleantennas were used to illustrate these potential benefits.The effects ofa metallic mandrel, borehole, and invasion were not considered because of theabsence of a suitable forward model.

This paper characterizes mandrel, borehole, and invasion effects for avariety of candidate tilt-coil devices with antenna array parameters similar tothose of the previous studies. The characterization is based on calculationsfrom a new forward model that includes tilted transmitting and receiving coilsof finite diameter embedded in a concentric cylindrical structure.

Important details of the forward model used in the calculations are alsoprovided.

Introduction

Conventional propagation resistivity devices are routinely used forgeosteering applications. Because data from these devices have essentially noazimuthal sensitivity, the LWD engineer is greatly aided by a prioriinformation regarding the proximity of the target bed relative to othergeologic features such as shales and water-bearing zones. Suitable a prioriinformation is often available from offset logs. In cases in which offset logsare not fully useful because of changing depositional environments or differenttectonic settings, azimuthally sensitive resistivity data would improve thequality of the geosteering effort.

One way to achieve azimuthal sensitivity to benefit geo-steering (and to useit for imaging) is to construct a tool similar to a conventional propagationresistivity device, but with the transmitters and/or receivers tilted withrespect to the axis of the drill collar. In fact, directional resistivity tools(DRTs) have been proposed in the literature for this pur-pose.1-3 To theknowledge of the authors, DRTs have only been analyzed with point-dipolemodels, which ignore both the drill collar and the finite size of the antennas.For apparent lack of a suitable forward model, mandrel, borehole, and invasioneffects have not been considered in the literature. A model has been developedthat accounts for tilted transmitters and receivers embedded in arbitrarylayers of a concentric cylindrical structure. Many important details of thismodel are discussed in Appendix A.

The term mandrel effect is used here to denote the difference between valuescalculated with a point-dipole model and the model that accounts for themandrel encompassed by the antennas. Mandrel effects on DRT measurements willbe grouped into three categories:

1. Absolute effects where the mandrel primarily attenuates the signalsbecause of a reduction in the magnetic moment of the antennas.

2. Residual effects that remain after an air-hang calibration is applied tothe data.

3. Perturbations to the azimuthal sensitivity of the measurement caused bythe finite size of the antennas and the drill collar.

Algorithms that transform raw tool measurements to resistivity values can bebased on computationally simple point-dipole solutions without significantlydegrading the accuracy of the results if mandrel effects associated withcategories 1 and 2 can be suppressed. For conventional LWD propagationresistivity measurements, mandrel effects of type 1 are addressed by air-hangcalibration. Algorithms that suppress type 2 mandrel effects are discussed inthe literature.4 Type 3 mandrel effects are not discussed here.