Downhole pipe selection for acoustic telemetry

A system for transmitting signals along a downhole string including a plurality of serially connected tubular pipes such as drill or production pipes, a transmitter for transmitting a signal along the string and a receiver for receiving the signal placed along the string at a location spaced from said transmitting means, wherein the pipes between the transmitter and the receiver are ordered according to length of tube to minimize loss of signal from said transmitter to said receiver.

BACKGROUND OF THE INVENTION 
This invention relates generally to a method for transmitting data along a 
drill string, and more particularly to a method for increasing the data 
capacity for transmitted data through a length ordered drill string. 
Deep subterranean wells are typically drilled using drill strings assembled 
from 10 meter pipe sections connected end-to-end by heavy threaded tool 
joints. A drill bit is attached to a drill collar at the downhole end of 
the drill string, the weight of the collar causing the bit to bite into 
the earth as the drill string is rotated from the surface. Drilling mud or 
air is pumped from the surface to the drill bit through an axial hole in 
the drill string. This fluid removes the cuttings from the hole, provides 
a hydrostatic head which controls the formation of gases, provides a 
deposit on the wall to seal the formation, and sometimes provides cooling 
for the bit. 
Communication of information to the surface from downhole sensors of 
parameters such as pressure, temperature, drilling direction, or formation 
is desirable. Various methods of communicating that have been tried with 
varying degrees of success include electromagnetic radiation through the 
ground formation, electrical transmission through an insulated cable, 
laser communication through a fiber optic cable, pressure pulse 
propagation through the drilling mud, and wave propagation through the 
metal drill string. Each of these methods have advantages and 
disadvantages associated with signal attenuation, ambient noise, high 
temperatures and compatibility with standard drilling procedures. 
The most commercially viable of these methods has been the transmission of 
information by pressure pulses in the drilling mud. However, attenuation 
mechanisms in the mud limit the transmission rate to less then five bits 
per second. 
Acoustic telemetry through the drill string has been the goal of the 
industry for 50 years. The idea of acoustic telemetry is to produce a 
modulated elastic waves at the bottom of the well and let it propagate up 
the drill string to the surface and extract the data from the signal at 
the surface. At first glance, such a system should work, as the steel 
drill string is an excellent conductor of sound. However, in practice, 
these systems do not work. The received signal often does not correspond 
to the transmitted signal, and working systems would be limited to a low 
transmission rate on the order of less than 10 Hz. 
The theory of acoustic telemetry has been studied to provide an explanation 
for its unexpectedly poor performance. D. Drumheller, "Acoustical 
properties of drill string," J. Acoust. Soc. Am. 85, 1048-1064 (1989), 
analyzed a dispersion equation to determine that the group velocity of the 
distorted signal through a steel drill string has real roots only in 
spaced passbands. 
The assembled drill string has periodically spaced discontinuities in 
cross-sectional areas attributed to the tool joints, which form roughly 5% 
of the length of the drill string and have a cross sectional area 5 times 
greater than the remainder of the drill string. In U.S. Pat. No. 
5,128,901, Drumheller disclosed that the signal loss in a drill string is, 
in addition to attenuation, also a function of distortion caused by the 
drill string which was found to function as a comb filter having both 
stopbands with high attenuation and passbands with minimal attenuation. 
This behavior resulted from periodic reflections along the drill string at 
the joint collars. This patent disclosed that the frequency of the 
acoustic signal should be chosen to fall within the pass bands of the 
filter, and the signal also should be preconditioned to correct for the 
distortion. 
D. Drumheller, "Attenuation Of Sound Waves In Drill Strings", J. Acoust. 
Soc. Am. 94 (4), October 1993, pgs 2387-2396, discussed the three types of 
elastic waves that dominate in acoustic transmission in a drill string at 
the desired frequency range of 1-2 Khz: extensional, torsional, and 
bending waves. Communication with bending waves is not feasible because 
they tend to be the slowest of the three waves and are dispersive even in 
uniform piping. However, both torsional and extensional waves are long 
compared to the 125 mm diameter of the common drilling pipe, and these 
waves tend not to be dispersive in uniform diameter pipe. 
Extensional and torsional waves are partially reflected at each of the tool 
joints. The reflection coefficient of extensional waves depends upon the 
ratio of the cross-sectional areas while the reflection coefficient of the 
torsional waves depends upon the ratio of the polar moment of inertia of 
these cross-sectional areas. Consequently, torsional waves undergo 
stronger reflections at the tool joints. This is one of the primary 
reasons for preferring extensional waves as a means for communicating 
information from downhole to the surface. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a method to improve the 
transmission of extensional waves transmitted along a drill string. 
It is another object of this invention to provide a method to minimize the 
attenuation of a signal transmitted along a drill string from downhole to 
the surface where data is extracted from the signal. 
Another object of this invention to enhance data rate of acoustic telemetry 
used in commercial drilling from 50 to 100 times that of present data 
rates. 
Additional objects, advantages, and novel features of the invention will 
become apparent to those skilled in the art upon examination of the 
following description or may be learned by practice of the invention. The 
objects and advantages of the invention may be realized and attained by 
means of the instrumentalities and combinations particularly pointed out 
in the appended claims. 
To achieve the foregoing and other objects, and in accordance with the 
purpose of the present invention, as embodied and broadly described 
herein, the present, invention may comprise a borehole string having an 
acoustic receiver spaced from an acoustic transmitter, the tubes of the 
string between the receiver and transmitter being ordered by length to 
maximize signal transmission.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 1, a preferred embodiment of this invention includes a 
system 10 for transmitting data along a drill string 50 extending into a 
borehole into the ground 5. Drill string 50 is supported at the surface 
and rotated by a conventional drilling structure 20 that also pumps 
drilling mud (not shown) into the hole and adds additional sections of 
drill pipe to drill string 50 in a manner that is well known in the art. 
Such conventional structure is not part of this invention. Drill string 50 
also typically includes a tool 30, such as a weighted collar and a drill 
bit, at the downhole end. Conventional tool 30 is also not part of this 
invention. 
As shown in FIG. 1, drill string 50 is formed of n sections of drill pipe, 
understood that the value of n increases each time a new section of drill 
pipe is added to drill string 50 at structure 20. 
As each section of drill pipe is substantially similar to all other 
sections, a discussion of any section, such as P.sub.x, also describes 
each other section. Drill pipe P.sub.x consists of a tube T.sub.x of 
relatively constant cross sectional area along its length, and a threaded 
coupling defining a tool joint J.sub.x at each end of the individual drill 
pipe for fastening drill pipe P.sub.x to adjacent drill pipes. Drill pipe 
tool joints conventionally comprise either a threaded male or a threaded 
female section so that P.sub.x can be threaded onto adjacent section 
P.sub.x-1, and following section P.sub.x+1, can be threaded onto section 
P.sub.x. For the purpose of this invention it does not matter which 
coupler, male or female, is uphole. 
The formulation of this invention required an understanding of the effect 
of tube length on frequency transmission. FIG. 3 shows calculated 
passbands for extensional waves along a drill string where the length of 
each pipe is 10 meters, and another drill string where the length of each 
pipe is 9.5 meters. If a drill string is assembled from pipe of the 
original length and then connected to a string of shorter pipe, this 
comparison implies that only waves with frequencies located in passbands 
common to both pipe lengths will propagate the entire length of the drill 
string. In this example, all frequencies above 1.5 kHz would be blocked. 
Each actual drill pipe P.sub.x has a total length consisting of the length 
of tube T.sub.x and the lengths of joints J.sub.x. The length of all 
joints is constant for different sections of pipe; however, there is 
considerable variation in the length of tubes T among different sections, 
because tube length is not accurately controlled during the manufacture of 
drill pipe. Current practice in the drilling industry today is to use both 
new and reconditioned drill pipe in drilling operations. FIG. 2 shows the 
distribution of pipe lengths as a function of position in an actual drill 
string that was utilized during the testing of this invention in a 2-km 
hole. Pipe lengths for this test ranged from approximately 8.8 meters to 
9.9 meters. 
The actual transmission characteristics of the drill string of FIG. 2 have 
been measured. Accelerometers mounted on the drill string at the surface 
measured extensional wave motion that was generated by an explosive 
device, commonly known in the drilling industry as a back-off shot, 
lowered into the well. Such a shot generates a broadband signal over the 
frequency range at issue. The drill string was filled with drilling mud 
for the tests. 
FIG. 4 shows amplitude as a function of frequency as provided by a Fourier 
transform of the calculated and measured data from a pulse through the 
drill string of FIG. 2. The length of the drill string was approximately 2 
km; the back-off shot was detonated at a depth of 1.61 km; and the 
extensional wave was measured by an accelerometer at the surface. The 
calculated data was derived as discussed by Drumheller, J. Acoust. Soc. 
Am. 94 (4), referenced above and incorporated herein by reference, and 
accounted for the actual length of each section of the drill string. 
Neither the calculations or the measured data exhibit significant 
amplitudes above 1 kHz at this depth. The transform of the calculated data 
shown by the dotted line deviates from the measured data in the passbands, 
most significantly in the first pass band at 200 Hz, while comparisons at 
the second through fourth passbands are good. The interference patterns 
within each passband are also well represented by the calculation. This 
test proved that the calculations provide a realistic simulation of the 
transmission of signals as extensional waves through a drill string. 
As also set forth in the aforementioned Drumheller article, the calculated 
attenuation for this drill string peaks at about 520 Hz (at 15 db/km) and 
is much lower (below 5 db/km) above 1 kHz. Measured attenuation appeared 
to increase linearly with depth. The contradiction is explained by noting 
that the calculated levels were for extensional waves in a uniform pipe. 
The actual drill string has tool joints that produce reflections and 
thereby increase the effective propagation distance of the waves. The 
reflections typically manifest themselves as passbands and stop bands in 
the transmission spectrum. 
These interference patterns within each passband provide the inspiration 
for this invention. It is know that interference spikes can be created 
even in perfectly periodic drill strings. This occurs in short drill 
strings where waves reflect and interfere off the end boundaries of the 
drill string; the number of spikes within each passband being related to 
the number of pipes in the drill string. However, in the present case, the 
drill string is too long and the attenuation is too high to allow the 
constructive and destructive interference of end reflections. 
One possible source of this phenomena is the pipe length. If the drill 
string of FIG. 2 was length ordered, it would have the distribution shown 
in FIG. 5, with the shortest drill pipe is at the downhole end. FIG. 6 
shows the Fourier transform of a calculated shot similar to the shot in 
FIG. 4 except on the length ordered drill string. As can be seen from the 
calculated data, there is a significant improvement in transmission, 
especially for higher passbands. This suggests that the number of 
reflections as well as the effective length of wave propagation can be 
reduced by eliminating the variations on the tool joint spacing. The 
analysis further predicts that these reflections can be reduced by simply 
rearranging the drill pipe according to length. When done, the 
interference patterns disappear from the calculated spectrum. 
FIG. 7 shows the measured data from a shot similar to FIG. 4 except on a 
length-ordered drill string. Additional casing was also added to the hole 
prior to the measured shot of FIG. 7, so the setup was not identical to 
that of the measured shot with unordered pipe. However, the measurements 
did substantially agree with the predicted results. The calculated and 
measured attenuation for the second through fifth passbands of the random 
and ordered drill strings of FIGS. 2 and 4 are as follows: 
______________________________________ 
Drill String Attenuation (db/km) 
Pass Band 
Pipe 
Arrangement Second Third Fourth 
Fifth 
Random 12.3 18.8 27.8 .infin. 
Ordered 7.3 17.0 9.3 .infin. 
(calculated) 
Ordered 12.3 11.5 13.1 16.4 
(measured) 
______________________________________ 
These results show that a noticeable improvement in higher frequency 
performance will result from the use of a length-ordered drill string. 
In the tested embodiment, the pipes with the greatest variation in length 
from the majority of pipes were the shortest pipes. These pipes were 
placed in the hole first so their effect would be minimized except for 
shots at the maximum depth of the drill string. 
In actual operation, generated signals are most likely to originate at the 
downhole end of the drill string. It also would be desirable to place the 
drill pipes with the least variation in length, in the instant case these 
would be the longer pipes, in the hole first, and decrease the pipe length 
as they are placed in the hole. This arrangement would provide the most 
accurate high frequency transmission while the hole is being drilled, and 
suffer the degradation caused by significantly shorter pipes only if they 
are used when the hole reaches its maximum depth. 
Since the distribution in FIG. 5 shows only a slight variation in pipe 
length for about 1/2 the pipes in the string (between about numbers 60 and 
160), it is also contemplated by this invention that lengths of pipe that 
are relatively close to each other in length do not have to be ordered 
according to length; i.e., a few pipes being switched with other pipes of 
substantially the same length should not adversely effect the operation of 
the invention. This substantial ordering of pipes according to length will 
not work if pipes are not placed in the drill string adjacent to other 
pipes of substantially the same length. 
Another embodiment of this invention to improve the transmission 
characteristics of a drill string uses drill pipe with a tight 
distribution of lengths. For the tests, the length of pipes varied .+-.0.5 
m over the 10-meter nominal length of the pipe, a variation of about 10%. 
Calculations in the manner disclosed herein could provide a maximum range 
of lengths which would provide acceptable transmission performance for 
signals. Based on observations, the maximum allowable length variation for 
unordered pipes is about 0.5%. While this variation is much less than 
normal practice in the industry, it means that the 10-meter pipes should 
vary in length by no more than 50 mm, a precision that is easily 
attainable by the manufacturers of such pipes. 
An obvious embodiment of this invention is to concentrate the energy of the 
data transmission into one of the passbands of the drill string. It is 
obvious to one skilled in the art that earlier attempts to develop 
acoustic telemetry systems failed because of the spreading of energy over 
broad band of frequencies, echoes dispersed the wave and diluted the 
energy, and the physical characteristics of the drill string blocked large 
bands of energy. The invention permits energy to be transmitted and 
received at higher passbands. 
The particular sizes and equipment discussed above illustrate a particular 
embodiment of this invention. While the disclosure is directed towards a 
drill string, this invention may be used to transmit acoustical signals 
along any segmented tubular that is within a borehole and that is not 
acoustically damped by the sides of the borehole (such as casing). 
Production tubing, for example, is of thinner construction than drill 
string and has less massive couplings. Because of this construction, 
passbands to about 2.5 kHz are useable in production tubing, whereas 
passbands to only about 1 kHz are useable in drill strings. It is 
contemplated that the use of the invention may involve components having 
different sizes and shapes as long as the principle of ordering the 
segmented pipes in a drill string by length to maximize acoustic signal 
transmission is 
followed. It is intended that the scope of the invention be defined by the 
claims appended hereto.