Mud Acoustics in Modern Drilling - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 280

Other early investigators have considered “drillpipe acoustical telemetry,”
that is, the transmission of downhole-to-surface data using longitudinal and
torsional drillstring vibrations. Bradbury and Wilhoit (1962) importantly
considered the effect of tool joints on the passage of plane longitudinal and
torsional waves along a drillpipe. Approximate solutions to the governing
partial differential equations for an idealized tool joint showed that tool joints
had negligible effect for exciting frequencies of the same order as common
rotary speeds, and that the drillpipe can be taken as a uniform pipe with
negligible error. Barnes and Kirkwood (1972) also considered longitudinal and
torsional vibrations as a means for MWD communication. For their analysis,
only zeroth mode vibrations (i.e., plane wave) were considered, the assumption
being that higher modes would be damped out over short distances. The basic
ideas on modal eigenfunctions and evanescent waves were discussed in Chapters
1 and 3. In their idealized model, where the drillstring is taken as an infinitely
long cylindrical steel pipe loaded at equal intervals with identical couplings, the
authors show that most of the “passbands” for compressional waves are about
twice the width of those for torsional waves (passbands are frequencies for
which waves propagate without loss). Refer to the paper for further details on
the exact geometry used and the numerical results obtained. In Drumheller
(1988), the drillstring is a periodic structure of pipe and threaded tool joints; the
transmission characteristics are complex and they exhibit a banded and
dispersive structure. This report describes a theoretical model, together with
theory, laboratory experiments, and field test results, possibly suggesting the
existence of frequency bands which are virtually free of attenuation and hence
suitable for high data rate transmission. Torsional wave propagation along
drillstrings, as a means of MWD acoustic telemetry, was reconsidered by Squire
and Whitehouse (1979). They importantly studied attenuative effects posed by
the drilling mud environment, with and without gelling, and results of field
experiments were reported. The conclusions cited in the foregoing papers,
which have not been independently verified by this author, are given for
completeness only. “Electromagnetic wave telemetry” provides still another
alternative, although it appears to be attenuative under many practical situations.
All of these technologies are manifestations of wave propagation, but
unfortunately, even a cursory review of any particular one is beyond the scope
of this topic. So much progress has been made during the past two decades that
it is impossible to survey basic achievements, e.g., patents number in the
hundreds and grow rapidly by the day.
By far, the most successful commercial MWD systems are based on mud
pulse telemetry , despite the fact that typical data rates do not yet exceed an
extraordinarily low 10 bits/sec (bit rate improvements have been largely
accomplished through data compression coding techniques rather than hardware
advances). Mud pulse telemetry systems provide the basic focus of this section,