Patent Grant
8437220
1. Field of the Invention
The present invention relates generally to telemetry apparatus and methods, and more particularly to acoustic telemetry isolation apparatus and methods for the well drilling and production (e.g., oil and gas) industry.
2. Description of the Related Art
Acoustic telemetry is a method of communication used, for example, in the well drilling and production industry. In a typical drilling environment, acoustic extensional carrier waves from an acoustic telemetry device are modulated in order to carry information via the drillpipe as the transmission medium to the surface. Upon arrival at the surface, the waves are detected, decoded and displayed in order that drillers, geologists and others helping steer or control the well are provided with drilling and formation data. In production wells, downhole information can similarly be transmitted via the well production tubing.
The theory of acoustic telemetry as applied to communication along drillstrings has a long history, and a comprehensive theoretical understanding has generally been backed up by accurate measurements. It is now generally recognized that the nearly regular periodic structure of drillpipe imposes a passband/stopband structure on the frequency response, similar to that of a comb filter. Dispersion, phase non-linearity and frequency-dependent attenuation make drillpipe a challenging medium for telemetry, the situation being made even more challenging by the significant surface and downhole noise generally experienced.
The design of acoustic systems for static production wells has been reasonably successful as each system can be modified within economic constraints to suit these relatively long-lived applications. The application of acoustic telemetry in the plethora of individually differing real-time drilling situations, however, presents other challenges and this is primarily due to the increased noise due to drilling and the problem of unwanted acoustic wave reflections associated with downhole components, such as the bottom-hole assembly (or “BHA”), typically attached to the end of the drillstring, which reflections can interfere with the desired acoustic telemetry signal. The problem of communication through drillpipe is further complicated by the fact that drillpipe has heavier tool joints than production tubing, resulting in broader stopbands; this entails relatively less available acoustic passband spectrum, making the problems of noise and signal distortion relatively more severe.
To make the situation even more challenging, BHA components are normally designed without any regard to acoustic telemetry applications, enhancing the risk of unwanted and possibly deleterious reflections caused primarily by the BHA components.
When exploring for oil or gas, in coal mine drilling and in other drilling applications, an acoustic transmitter is preferentially placed near the BHA, typically near the drill bit where the transmitter can gather certain drilling and geological formation data, process this data, and then convert the data into a signal to be transmitted up-hole to an appropriate receiving and decoding station. In some systems the transmitter is designed to produce elastic extensional stress waves that propagate through the drillstring to the surface, where the waves are detected by sensors such as accelerometers, attached to the drill string or associated drilling rig equipment. These waves carry information of value to the drillers and others who are responsible for steering the well. Examples of such systems and their components are shown in: Drumheller U.S. Pat. No. 5,128,901 for Acoustic Data Transmission through a Drillstring; Drumheller U.S. Pat. No. 6,791,474 for Reducing Injection Loss in Drill Strings; Camwell et al. U.S. Patent Publication No. 2007/0258326 for Telemetry Wave Detection Apparatus and Method; and Camwell et al. U.S. Patent Publication No. 2008/0253228 for Drill String Telemetry Methods and Apparatus. These patents and publications include common inventors with the present application and are incorporated herein by reference.
Exploration drilling in particular has become a highly evolved art, wherein the specification and placement of the BHA components is almost entirely dictated by the driller's need to drill as quickly and accurately as possible while gathering information local to the drill bit. A large variety of specialized BHA modules or tools are available to suit local conditions, and their inclusion in a BHA usually takes priority over the requirements of telemetry methods, acoustic or otherwise. The diversity of these BHA tools and the decision regarding whether or not to even include them in a drillstring, pose major issues for consideration; these issues have a significant impact when dealing with acoustic energy questions. Cyclic acoustic waves suffer multiple reflections and amplitude changes even in a very simple BHA, and the net effect of these changes may destructively interfere with the required acoustic telemetry broadcast signal. The reflections are caused by impedance mismatches which are the result of mechanical discontinuities present in all BHAs presently in use.
An initial response to this problem would be to place the acoustic telemetry device above the BHA and simply direct the acoustic energy up the drillstring, away from the BHA components. Unfortunately, this does not fully address the problem because typical acoustic transmitters emit waves of equal magnitude both up-hole and down-hole, and the downward travelling waves in particular may be reflected, thereby potentially resulting in destructive interference with the upward travelling waves. In the worst cases, this can cause virtually complete cancellation of the upward travelling communication signal.
It is known in other fields, for example in radio frequency (RF) transmitter design and electrical transmission lines, that wave reflections can be controlled by inserting simple specific impedance changes at certain distances from a transmitter, such that the combination of the original wave and the reflected wave combine constructively to produce a single wave travelling in one direction with increased amplitude. The standard approach is to insert a “quarter wave” (λ/4) impedance change (or odd multiples thereof) adjacent to the transmitter so that one wave (the “down” wave) is reflected in phase with the intended transmitted wave (the “up” wave) and constructively aids the intended transmitted wave by increasing its amplitude.
Downhole applications typically employ transmitters that emit stress waves of nearly equal, but not necessarily equal, magnitude in both directions. Moreover, each wave has the same sign in stress but opposite sign in material velocity. In such cases, the appropriate reflection device would be a λ/4 tuning bar (pipe section) placed below the transmitter. However, such a simple solution is often impractical because the equipment below the acoustic transmitter is designed to drill and steer the well rather than to aid telemetry. Equipment such as drill collars, crossover pipes, drilling motors and bits can easily nullify the benefit of simply introducing a λ/4 section of pipe below the acoustic transmitter because the equipment will generally be of differing lengths and impedances that can add to the λ/4 section and eliminate the intended benefit. This discussion assumes the reader is familiar with the phase change differences associated with waves passing from a given medium to that of greater or less acoustic impedance.
Other styles of transmitters which emit waves in both directions, but by design have different relationships between their stresses and material velocity would require tuning bars of different lengths, not necessarily λ/4 sections, further complicating the problem.
As mentioned above, downhole noise is also of concern in acoustic telemetry. The problem of downhole noise is addressed to some extent in U.S. Pat. No. 6,535,458 to Meehan, wherein is taught a baffle filter comprising a periodic structure of typically 20 m length interposed above or below the acoustic source; this is intended to cause stopbands over a certain range of frequencies, the position of the baffle being to protect the acoustic transmitter from the sources of the noise from the drill bit and motor. This teaching, however, does not address or anticipate the more serious problem of energy propagating in a “down” direction being reflected in a relatively unattenuated manner back to the transmitter where it may combine in a destructive manner with the energy propagating in an “up” direction, thereby causing possibly significant destruction of the signal intended to reach the surface.
As can now be seen, the required upward travelling acoustic telemetry waves are often interfered with by unwanted reflections from impedance mismatches below the transmitter. The known art of inserting a tuning bar of appropriate length is usually ineffective because the local conditions often necessitate the addition of further BHA components that cause further reflections that can often destructively interfere with the upward travelling wave.
It is an object of the present invention to control wave reflections, in particular, in such a manner as to mitigate the otherwise potentially destructive reflections. Specifically, the present invention comprises an apparatus for placement adjacent to the transmitter, and a method for using same, that will beneficially reflect waves, such that:
An isolator according to the present invention seeks to effectively isolate essentially all down waves from the subsequent (i.e. downhole) BHA components, thus curtailing the possibility of waves that would have entered the BHA and returned with potentially destructive phases. Positioning an isolator according to the present invention below the transmitter can, in effect, make the lower BHA components essentially “acoustically invisible” over a bandwidth useful for acoustic telemetry.
The present invention is also intended to be applicable in situations other than real-time drilling with drillpipe or production wells with production tubing. For example, many relatively shallow wells are drilled with coiled tubing. Although coiled tubing drilling systems do not have the passband/stopband features of drillpipe sections connected by tool joints, they do have BHA components similar to those in jointed pipe applications. Thus, the isolator and the isolation method taught herein are intended to apply equally to the situation of coiled tubing.
It is intended that the present invention be applicable in still further applications. For example, an isolation/reflection means as described herein can also be beneficial in production wells where there may not be a BHA as such, but there may instead be production components such as valves, manifolds, screens, gas lift equipment, etc., below the acoustic source. Thus, the apparatus and method taught herein are intended to apply equally to this situation.
According the present invention, then, there is provided an acoustic isolator for use with tubular assemblies comprising:
a first tubular member of first physical length, first acoustic impedance, and first acoustic transit time;
a second tubular member of second physical length, second acoustic impedance, and of second acoustic transit time;
the first and second members not making contact or exchanging acoustic energy directly to each other;
a first upper coupling placed at the upper end of the first and second members, said coupling restricting the motions of said members and said coupling to be equal at their common points of contact thereby allowing exchange of acoustic energy between the drilling components above said coupling and said tubular members below said coupling;
a second lower coupling placed at the lower end of the first and second members said coupling restricting the motions of said members to be equal at their common points of contact thereby allowing exchange of acoustic energy between the drilling components below said coupling and said tubular members above said coupling;
the lengths, acoustic impedances, and transit times of said tubular members being adjusted so that by means of constructive and destructive wave interference the acoustic energy transmitted through the upper coupling results in reduced motion and force in the lower coupling and likewise acoustic energy transmitted through the lower coupling results in reduced motion and force in the upper coupling.
Thus it is to be understood that downward traveling acoustic energy may be reflected upward, and upward traveling acoustic energy may be reflected downward. Moreover, it is to be understood that acoustic energy could be arriving simultaneously from both directions and the acoustic isolator is simultaneously reflected back towards the drilling components that originally injected the energy.
A detailed description of an exemplary embodiment of the present invention is given in the following. It is to be understood, however, that the invention is not to be construed as limited to this embodiment.
In the accompanying drawings, which illustrate the principles of the present invention and an exemplary embodiment thereof:
In the following description, reference is made to “up” and “down” waves, but this is merely for convenience and clarity. It is to be understood that the present invention is not to be limited in this manner to conceptually simple applications in acoustic communication from the downhole end of the drillstring to the surface. It will be readily apparent to one skilled in the art that the present invention applies equally, for example, to subsurface stations in drilling applications, such as would be found in telemetry repeaters, or non-drilling applications as would be found in production wells.
Referring to the drawings more detail, the reference numeral 2 generally designates a parallel-path acoustic isolation system embodying an aspect of the present invention. Without limitation on the generality of useful applications of the system 2, an exemplary application is in a drilling rig 4 as shown in a very simplified form in
The focus of the present invention is to implement designs of isolators 26 comprising inner and outer tubular, coaxial isolation members 30, 32 (pipes of various types) such that judicious control of their impedances and transient times may result in a useful and necessary apparatus, i.e. the parallel-path acoustic isolator 26 which can be incorporated in the acoustic isolation system 2.
First, it should be understood that the wave speed c and characteristic acoustic impedance z of a pipe section i of uniform material properties and wall area are:
ci=√{square root over (Ei/ρi)} [1]
zi=√{square root over (ρiEi)}Ai=ρiciAi [2]
where ρi=material mass density
Also note that pipe section i with wave speed ci and length L has a transit time of
Δti=L/ci [3]
The basic principle of operation of this invention can be understood through an examination of an upwardly traveling incident simple wave W.1 (see
I=G(f)T [4]
where
I=amplitude of material velocity of the incident wave W.1
T=amplitude of material velocity of the transmitted wave W.6
G(f)=transfer function of parallel-path isolator, which is a function of f.
The object of designing an isolator is to make the transmitted amplitude T zero or nearly zero for arbitrary finite values of the amplitude I. This occurs in the neighbourhood of the poles of the transfer function G(f). The locations of the poles are given by:
z2(1−P12)P2+z1(1−P22)P1=0 [5]
where
Pi=exp(ikiL) [6]
Controlling the locations of the roots of [5] is key to designing an isolator, and this is best achieved by examining the function
S(f)=|z2(1−P12)P2+z1(1−P22)P1| [7]
which will be referred to as the pole equation. A plot of this equation reveals the frequencies fr where S(fr)=0. These frequencies are the solutions of [5]. Another simplified expression yields the solution for the reflected wave W.7 at the root frequencies fr:
where R=amplitude of wave W.7.
It is now instructive to examine a special case of [5] in which both the pipes 30 and 32 have the same impedance. Indeed for z1=z2 equation [5] yields:
(P1+P2)(1−P1P2)=0. [8]
The roots of [8] are obviously:
P1=−P2 [9]
P1P2=1. [10]
Substitution of [6] in these expressions yields the following frequency pairs
where n is an arbitrary integer including zero, and
Considering an incident wave W.1 whose frequency satisfies [9] will now provide an instructive discussion of the operation of the isolator. Upon initially encountering interface 28b a wave of this frequency produces transmitted waves W.2, W.3 in pipes 30 and 32 respectively. Waves W.2 and W.3 are in phase as they leave interface 28b, and because z1=z2 their forces and material velocities are equal. However, each wave travels at a different velocity upwardly towards interface 28a.
Because the frequency satisfies [9], waves W.2 and W.3 are caused to arrive at interface 28a with values of force and velocity that are opposite in sign to each other. Thus the total force and motion exerted by pipes 30 and 32 on interface 28a is ideally at or near zero, and little or no transmitted wave W.6 is produced in pipe segment 18a.
Parallel path isolators 26 can be designed from these expressions. The following examples illustrate how.
Table 1 contains material specifications and dimensions for pipes 30 and 32 of a parallel-path isolator. The sizes would be compatible with typical 6.5″ oilfield drilling tools. Notice that both pipes are chosen such that they have the same characteristic impedance z. The center frequency of the required isolation band is specified to be 660 Hz.
We are now able to employ solutions to [13] and [14]. They yield the following values for the length of pipes 30 and 32 respectively:
Setting the length of the isolator to the average of these two values (L=1.32 m) will center the pair of poles about 660 Hz.
The harmonic analysis using equation [4] is shown in
Note that at the frequencies corresponding to the zero points, 590 Hz and 730 Hz, there is no transmitted wave because |T|=|G(f)|−1=0 at these frequencies. However, if the frequency of the wave is unequal to either of the two pole frequencies it will not be completely reflected by the isolator, and some wave energy will enter pipe 18a.
In
Note the similarity in the plots of the pole equation [7] in
The column labelled Composite contains the averaged properties of the High Gravity/BeCu composite pipe 54/56, which also includes the averaged density and the parallel-coupled stiffness. The composite wave speed and impedance are computed from [1] and [2] using the listed composite values of stiffness, density and area. The isolator 52 is constructed of the mandrel 54 and the inner pipe 56 with the properties listed in the composite column and an outer pipe 58 (tubular member) with properties listed in the Stainless Steel column of Table 2. The length L of this isolator is 2.65 m.
This length as well as the outside diameter of the High Gravity Nylon inner pipe 56 is determined by iteration of parameters in the pole equation [7] until the plot in
A phase shift of π radians is achieved when the total delay equals half the period of a 660 Hz wave i.e. 758 μs.
The time for a wave to travel up transmitter 64 is a known property and for this particular example it is 20.5 μs.
The additional delay required for constructive combination is:
758−20.5−133.8=603.7 μs.
This delay must be achieved by a double transit of the steel tuning pipe 68, which has a known wave speed of 4961 m/s.
Thus the length of the tuning pipe 68 is
Using this length for the tuning pipe 68, harmonic analysis of the system yields the amplitude for waves W.8 and W.9.
The foregoing explains the innovative method by which an isolator can be built with bandstop properties determined by causing acoustic telemetry waves to travel along specific parallel tubular members such that the ensemble set of reflected and transmitted waves combine with phases that aid unidirectional requirements of an isolating filter.
It is shown how the components of the isolator may be tuned to respond to certain frequency bandpass structures inherent in drillpipe. This enables an acoustic transmitter incorporated in the BHA in a drilling environment to beneficially transmit in a net upward direction, thereby doubling its wave amplitude in that direction.
It is also shown how the components of the isolator may be tuned to respond to certain frequency bandpass structures inherent in downhole production strings, also aiding the transmission of acoustic telemetry signals in a specified direction of benefit to said telemetry.
A notable advance on the previous art is afforded by this invention is to be to provide impressive filter functionality in tubular mechanical materials appropriate to oil and gas drilling and production in a relatively small length considering that the wavelength in drill pipe at 660 Hz is approximately 8 m.
Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.
This application claims priority in U.S. Provisional Patent Application No. 61/148,995, filed Feb. 1, 2009, which is incorporated herein by reference.
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| Number | Date | Country | |
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| 61148995 | Feb 2009 | US |
