Summary
Several recently published studies discuss the concept of inductive
resistivity-logging devices with oblique transmitting and/or receiving coils.
Both wireline induction and logging-while-drilling (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 because of 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.
Introduction
Conventional propagation resistivity devices are routinely used for
geosteering applications. Because data from these devices have essentially no
azimuthal sensitivity, the LWD engineer is greatly aided by a priori
information regarding the proximity of the target bed relative to other
geologic features such as shales and water-bearing zones. Suitable a priori
information is often available from offset logs. In cases in which offset logs
are not fully useful because of changing depositional environments or different
tectonic settings, azimuthally sensitive resistivity data would improve the
quality of the geosteering effort.
One way to achieve azimuthal sensitivity to benefit geo-steering (and to use
it for imaging) is to construct a tool similar to a conventional propagation
resistivity device, but with the transmitters and/or receivers tilted with
respect 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 the
knowledge of the authors, DRTs have only been analyzed with point-dipole
models, which ignore both the drill collar and the finite size of the antennas.
For apparent lack of a suitable forward model, mandrel, borehole, and invasion
effects have not been considered in the literature. A model has been developed
that accounts for tilted transmitters and receivers embedded in arbitrary
layers of a concentric cylindrical structure. Many important details of this
model are discussed in Appendix A.
The term mandrel effect is used here to denote the difference between values
calculated with a point-dipole model and the model that accounts for the
mandrel encompassed by the antennas. Mandrel effects on DRT measurements will
be grouped into three categories:
1. Absolute effects where the mandrel primarily attenuates the signals
because of a reduction in the magnetic moment of the antennas.
2. Residual effects that remain after an air-hang calibration is applied to
the data.
3. Perturbations to the azimuthal sensitivity of the measurement caused by
the finite size of the antennas and the drill collar.
Algorithms that transform raw tool measurements to resistivity values can be
based on computationally simple point-dipole solutions without significantly
degrading the accuracy of the results if mandrel effects associated with
categories 1 and 2 can be suppressed. For conventional LWD propagation
resistivity measurements, mandrel effects of type 1 are addressed by air-hang
calibration. Algorithms that suppress type 2 mandrel effects are discussed in
the literature.4 Type 3 mandrel effects are not discussed here.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
7 April 2004
- Revised manuscript received:
16 March 2005
- Manuscript approved:
22 March 2005
- Version of record:
15 June 2005
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