Technical Discussions

Effect of Mandrel, Borehole and Invasion for Tilted-Coil Antenna Arrays

» SPE 84245 (Copyright 2003, Society of Petroleum Engineers Inc.)

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

Terry Hagiwara, SPE, Shell; Erik J. Banning, Shell; Richard M. Ostermeier, SPE, Shell; S. Mark Haugland, PathFinder Energy Services

Abstract

Several recently published studies discuss the concept of inductive resistivity logging devices with oblique transmitting and/or receiving coils. Both wireline induction and LWD propagation resistivity tool concepts have been considered. Directional resistivity measurements and improved anisotropy measurements are among the benefits promised by this type of device. Analyses based on point-magnetic dipole antennas were used to illustrate these potential benefits. The effects of a metallic mandrel, borehole, and invasion were not considered due to the absence of a suitable forward model. This paper characterizes mandrel, borehole, and invasion effects for a variety of candidate tilt-coil devices with antenna array parameters similar to those of the previous studies. The characterization is based on calculations from a new forward model that includes tilted transmitting and receiving coils of finite diameter embedded in a concentric cylindrical structure. Important details of the forward model used in the calculations are also provided.

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New Discovery with Important Implications for LWD Propagation Resistivity and Interpretation

» (SPWLA 2001)

New Discovery with Important Implications for LWD Propagation Resistivity Processing and Interpretation

S. Mark Haugland, PathFinder Energy Services

Abstract

When LWD resistivity devices are used, phase shift and/or attenuation measurements are individually converted to raw resistivity values using transformations that - at a minimum - involve assumptions regarding the dielectric constant of the earth formation. It is important that these data are interpreted correctly and are as accurate as possible. A fundamental assumption used to calculate the raw resistivities is that the phase shift senses both the resistivity and dielectric constant within one volume, and that the attenuation senses both variables within a different volume. The volume associated with the attenuation measurement is thought of as being radially deeper with less vertical resolution than the volume associated with the phase measurement. An examination of this assumption lead to the discovery that the phase measurement is sensitive to the dielectric constant within substantially the same volume as the attenuation measurement is sensitive to the resistivity, and that the attenuation measurement is sensitive to the dielectric constant in substantially the same volume as the phase measurement is sensitive to the resistivity. The physical arguments and mathematical analysis presented here consistently support this conclusion. Application of this discovery resulted in alternative algorithms which give significantly improved results over currently practiced methods to compute the raw resistivities. Examples of applying these algorithms to both field and theoretical data are shown. These processing methods are suitable for real-time use since they are robust and run quickly.