CA2453891A1 - Acoustic isolator for downhole applications - Google Patents

Abstract

A plurality of heavy mass irregularities (54, 56, 58) attached to an inner wall (60) of the drill collar (52) attenuate waves traveling through the collar.The plurality of heavy mass irregularities (54, 56, 58) are spaced and sized for the maximum attenuation of acoustic pulses in a predetermined frequency range. The mass irregularities may be rings secured to the inner surface (60) of the collar by neck pieces (62), extending outwardly from the outer circumference of the ring, The mass irregularities may be made of steep or tungsten and are between six and ten in number. The spacing of the irregularities may lie between twelve and fourteen centimeters. A center pipe (64) may be included to isolate the irregularities from the fluid flow associated with the drilling operation. The pipe may be of a soft material such as rubber to reduce transfer of acoustic noise along the drill string.

Description

~'~ 00/75694 PCT/LJS00l1S230 ACOUSTIC ISOLATOR h'OR DO~VVNI~IOLE A:pI'~,ICATIONS
SPECIFICA "ION
CROSS-REFEItEI~tCES TO REI,A'TEI~ APPL,I~A°I'IONS
The present application claims priority from and is based upon United States Provisional Patent Application Serial No. 60/I37,388 filed on June 3, 1999.
>3ACKGROUNID OF TIIE IieIVENTION
Field of t'he Invention The present invention pertains to logging while drilling apparatus and more particularly to acoustic logging while drilling apparatus and atterguation of acoustic pulses that travel parallel to the direction of drilling.
Related Prior Art To obtain hydrocarbons such as oil and gas, wells or wellbores are drilled into the ground through hydrocarbon-bearing subsurface formations. Currently, much current drilling activity involves not only vertical uiells but also drilling horizontal wells. In drilling; information from the well itself must be obtained.. While seismic data has provided information as to the area to drill and app:roxirr3ate depth of a pay s~~s=fT~~sr~g~r ~~~~.~ ~s' ii'0 00/75694 ~CTILTS00/tS230 zone, the seismic information can be not totally reliable at great depths. To support the data, information is obtained while drilling through logging while drilling .or measuring while drilling (IrIIVJT3~ devices. bogging or measuring while drilling has been a procedure in use for many years. This procedure is preferred by drillers because it can be accomplished without having to stop drilling to log a hole.
This is primarily due to the fact that logging an unf°mished hole, prior to setting casing if necessary, can lead to washouts, damaging the drilling worlr, that. has already been done. This can stall the completion ~fthe weld and delay production. Further, this information can be useful while the well is being drilled to make direction changes immediately.
Advances in the IVI~UD rneasurernents and drill bit steering systems placed in the drill string enable drilling of the horizontal 'borehoIes with enhanced efficiency and greater success. Recently, horizontal boreholes, extending several 'thousand meters ("extended reach" boreholes~, have bee~a drilled to access hydrocarbon reserves at reservoir hanks and to develop satellite gelds from existing offsbone platforms. Even more recently, attempts have been made to drill boreholes corresponding to three-dimensional borehole prafiles. Such borehole profiles often include several builds and turns along the drill path. Such three dimensional borehole profiles allow hydrocarbon recovery from multiple formations and allow optimal placement of wellbores in geologically intricate formations.
1-lydrocarbon recovery can be maximized by drilling the horizontal and complex wells along optimal locations within the hydrocarbon-producing formations.
crucial t~ the success of tl?ese wells is establishing reliable stratigraphic position control while landing the well into the target formation and properly navigating the Wt) 00!75694 I~CT/&7SOOJ~S23~
drill bit through the formatio~z during drilling. In order to achieve such well profiles, it is important to determine the true Iocation ofthe drill bit relative to the formation bed boundaries and boundaries between the various fluids, such as the oil, gas and water. Lack of such information can lead to severe '°dogleg" paths .along the borehole resulting from hole or drill path corrections to find or to ree:c~ter the pay zones. Such well profiles usually limit the horizontal reach and the final well length exposed t~ the reservoir. optimization of the borehole location within the formation also can have a substantial impact on maximizing production rates and minimizing gas and water coning problems. Steering efficiency and geological positioning are considered in the industry among the greatest limitations of the current drilling systeans for drilling 1 ~ horizontal and complex wells. availability of relatively precise three-dimensional subsurface seismic maps, location ofthe drilling assembly relative t:o the bed boundaries of the formation around the drilling assembly can greatly enhance the chances of drilling boreholes for maximum recovery. prior art down hole devices lack in providing such information during drilling ofthe boreholes.
i~ IVfodern directional drilling systems usually employ a drill string having a drill bit at the bottom that is ratated by a drill motor {commonly referred to as the "mud motor"). A plurality of sensors and Ta~I devices are placed in close pr~ximity to the drill bit to measure certain drilling, borehole and formation evaluation parameters.
Such parameters are then utiIi:zed to navigate the drill bit along a desired drill path.
2f~ 'typically, sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a formatIOn reslstmty measuring; device are employed to determine the drill string and borehole-related :parameters. The resistivlty measurements are ?zsed to determine the presence of hydrocarbons against W~ 00/7569 PCTlUS00/~ 5230 water around and/or a short distance in front o:f the drill bit. i~esistivity measurements are most commonly utilized '.o navigate the dr~~ll bit. 1-lowetver, the depth of investigation of the resistivity devices usually extends only two to three meters and resistivity measurements do not provide bed boundary information relative to the downhole subassembly. p'urthermore, the location of the resistivity device is determined by some depth measuring apparatus deployed on the surface which has a margin of error frequently greater than the depth of investigation of the resistivity devices. Thus, it is desirable to have a downhole system which can. accurately map the bed boundaries around the downllole subassembly so that the drill string may be steered to obtain optimal borehole trajectories.
The relative position uncertainty of the wellbore being drilled and the critical near-wellbore bed boundary or contact is defined by the accuracy of the IVgV6~L?
directional survey tools and the formation dip uncertainty. :It~Wf3 tools may be deployed to measure the eart~.n°s gravity and magnetic field to determine the inclination and azimuth. Knowledge oft:~e course and pGSition of the wellbore depends entirely 1~ on these two angles. l;7nder normal conditions, the inclination measurement accuracy is approximately plus or minus two tenths of a degree. Such an error translates into a target location uncertainty of about three meters per one the~5~san.d meters along the borehole. ,~ldditionaily, dip rate variations of several degrees are common.
The optimal placement of the borehole is thus very difficult to obtain based on the currently available M~V'D measurements, particularly in thu:~ pay zones, dipping formations and complex wellbore designs.

w~ OOI75694 t'C1'/US00l15g30 Until recently, logging while drilling has been limited to resistivity logs, gamma logs, neutron logs and other non-acoustic lags sine; acoustic noise caused by drilling and acoustic pulses traveling upstring from the transmitter has presented problems in accurate detection and delineatioru 'these problems cannot be easily isolated by arrival time since the acoustic pulses are generated and detected S continuously. Recently, the use of acoustic sensors having a relatively short spacing between the receivers and the transmitter to determine the formation bed boundaries around the downhole subassembly leas been used. .An essential element in determining the bed boundaries is the determination of the travel time of the rex'lection acoustic signals from the bed boundaries or other interface anomalies. ~ prior art proposal has been to utilize estimates of the acoustic velocities obtained from prior seismic data or offset wells. Such acoustic veiocities are not very precise because they are estimates of actual formation acoustic velocities. Also, since the deptb.
measurements can be off by several meters from the true d~rpth of'the downhole subassembly, it is highly desirable to utilize actual acoustic 1'ormation velocities determined downhole during the drilling operations to locate bed boundaries relative to the drill bit location in the wellbore>
l~,dditionally, for acoustic or sonic sensor measurements, the most significant noise source is acoustic signals traveling froirl the source to the receivers via the metallic tool housing arid those traveling through the mud column ;surrounding the downhole subassembly (tube waves and body waves). In some applications acoustic sensor designs are used to achieve a certain amount of directivity of signals.
A
transmitter coupling scheme with signal processing method may be used for reducing the effects of the tube v~ave and the body waves. Such methods, however, alone do WO 00175694 PCTlUS00115230 not provide su~cient reduction of the tube and body wave effects, especially due to strong direct coupling of the acoustic signals between the transmitters and their assooiated receivers.
Some United States patents representative of the current art in determining subsurface formations are as follows.
United States patent number 4,020,452, titled °°Apparatus For Use ire Investigating Earth Formations", issued to Sean-Claude 'frog filler, et al., relates to an apparatus for mechanically filtering acoustic pulses in a well logging tool.
This apparatus includes of a substantially rigid member having interruptions in the longitudinal continuity of tile member. These interruptions provide tortuous paths for the passage of acoustic energy along the member. .P. plurality of masses are periodically spaced along the interior of the member and are each mechanically integral with opposite sides of the member at :Qocations chosen to enable the member and masses to cooperate as a mechanical filter. Ey so doing; the structure made of the member and masses will hare good acoustic delay and attenuation characteristics as well as good mechanical ch~~racteristics.
United States patent number 5,043,952, titled °°lvlonopole °l"ransmitter For a Sonic Well 'Tool°°, issued to David C. l-Ioyle, et aL, relates to a rnonopole transmitter for a sonic tool which includes an axial tube, a piezoceram:ic cylinder surrounding the axial tube, an endcap disposed at each end of and firmly contacting the cylinder, and an apparatus for holding the endcaps firmly against the axiat tube. The endcaps firmly contact the axial tube without simultaneously contacting an upper bulkhead.
The apparatus may include spring washers disposed between flee bulkhead and at least one endcap, or it may include a spring disposed between a nodal mourat and each endcap.

W~ 00175694 P~'PIUSOOI1~230 A nodal mounting tube may be disposed around the axial tube, a ring being disposed at each end of the nodal mounting tube, each ring being disposed outside of the cylinder for biasing the endcaps in tension against a ring thereby holding each endcap f rmly in contact against the axial tube.
United States patent number 5,510,52, titled '°Acoustic Atte;nuator, Well S Logging Apparatus and lvIethod of Well Logging°', issued to Tames R.
Dirchak, et al., relates to a sonic well tool fox' performing acoustic investigations of subsurface geological formations penetrated by a borehole. The well to~oi gene,~~ally includes a longitudinally extending body for positioning ira the borehole. The tool also includes a transmitter supported by the hody for transmitting acoustic energy c~nd a receiver supported by the body for receiving acoustic energy. The tool includes an acoustic attenuation section positioned on the body between the transmitter and the receiver.
This section includes one or more cavities defined by the bcsdy, inertial mass members positioned inside the cavities in a suitable manner to form a gap between the wall of the cavit3~ and the inertial mass members, and an acoustical attenuation fluid in the gap. The method for attenuating sonic waves generally includes transmitting a sonic wave from the transmitter to the tool, passing the sonic wave through the acoustic attenuation section, and receiving attenuated wave at the receivers.
United States patent number 5,036,945, titled "Sonic WelfTool Transmitter ..
Receiver Array including an Attenuation and Delay Apparatus°', issued to David ~.
~0 Hoyle, et al., relates to a sonic well tool that includes a transmitter array having at least one monopole transmitter and at least one dipole transmitter and a receiver array far receiving sonic pressure wave signals from a surrounding borehole formation. A
first attenuation and delay apparatus is positioned above the receiver array and a w~ 00!?5694 P~ d'l~JS0011523t1 second attenuation and delay apparatus is positioned below the receiver array in the sonic well tool. The first ats:enuation and delay apparatus in dudes an attenuation member comprising a plurality of interleaved rubber and metal like washers for attenuating compressional and flexural waves propagating along a metal center support rod to the receiver array and an inner housing corraprising a bellows section having a corrugated shape and a thin transverse dimension for delaying the propagation of compressional and flexural waves along the inner housing to the receiver array. The second attenuation and delay appararixs includes a plurality of mass loading rings surrounding the outer housing of the sonic well tool for attenuating the flexural waves propagating up the outer housing from a sonic transmitter ad a further inner housing including a further bellows section laavirag a corrugated shape and a thin transverse dimension for delaying the propagation of compressional and flexural waves up the tool, along the inner housing, to the receiver array.
The sonic well tool also includes a differential volume compensator for changing the quantity of oil encapsulated in the sonic well tool in accordance with changes in oil volume and changes in borehole temperature and pressure. The receiver array ir~,cludes a plurality of hydrophone sets, each hydrophone set including at least one pair and preferably two pair of hydrophones disposed in a cross section of the tool, one hydrophone of a pair being disposed opposite tree other hydrophone of the pair in the cross section.
United States Patent l~pplication Ser. No. 09/201,qg~, now United States Patent ****** to I~Iolz ~ .Dubzns3ry, having the same assignee as the present invention discloses the use of a section of a drill collar that has a pl-~ral,ity of shaped cavities filled with oil. The passage of an acoustic wave sets up a resonance of the fluid in the shaped cavity. The frequency of resonance depends upon the shape and size of the w~ OOI75694 PCT/ITSOOI15230 caeity and the properties of the fluid in the cavity. In one ernbodir~~ent of the invention9 the cavities are sphericalo mother embodiment of the invention uses cylindrical cavities with a piston restrained by a spring within the cavity.
Changing the spring constant provides additional control over the frequencies that are attenuated. The '9~~ appl~catlon also discloses the use of segmented isolators which the drill collar section is filled with layers of a composite material in which the layers have a different density. T he thicknesses of the individual layers is selected to attenuate certain frequencies.
Surnrr~ary of the Invention 1Q The present Invention provides a system and method foi- attenuation of acoustic waves that travel tl-.~ough a drill collar in a logging while drilling operation.
The system includes a plurality of heavy masses attached to an inner wall of the drill collar. 'fhe heavy masses constitute mass discontinuities t'l~at attenuate waves traveling through the drill collar. In one embodiment of th.e invention, the mass discontinuities are rings and attachment is dope by neck pieces. These neck pieces extend out from the outer circumference of the rings and may be an original outer circumference of the ring that has been milled down by cutting out portions ~f the ring. This allows signifbcantly less than the entire outer circumference of the hanging rings to be in contact with tile inner surface of the drill collar. Thus, the rings will more efficiently attenuate ø.13e vibrational force of the acoustic pulses coming in contest with the hanging ring. The plurality of heavy hanging rings are spaced and sued for the maximum attenuation of acoustic pulses in a ;predetemmned range, preferably in the range of 1 t9 khz to 20 khz. 'l: he system m.ay include steel rings as the W~ 00!75694 p~'~1t1S00I15230 plurality of heavy hanging rings. In an alternate embodiment; tlse plurality of heavy hanging rings may be a heavier, more dense material such as tungsten. The plurality array have as many as ten rungs or as few as sip, with eight I~eing another possibility.
The spacing of the rings may vary between twelve and fourteen centimeters, depending on the material used. In a still further embodiment, a pipe may be placed ~ within the inner circumference of the rings to isolate the attenuation rings from the flow of drilling mud. The isolation pipe may be of any material, however, a material that is non-rigid that is less likely to conduct vibrational forces is preferred. In another embodiment of the invention, the mass discontinuities are attached to the drill collar over a substantial portion of their individual axial lengths. Such an arrangement acts i~ 0 as a Iow pass falter. When this mechanical arrangement is 'used with an electrical bandpass filter in the tools high frequencies are e~ciently attenuated. In yet another embodiment of the invention, the attenuator sLction comprises a cylindrical body with sections of different inside andlor outside diameters to produce a ringed pipe: the sections of different diameter each have a characteristic passband and a reject band for 1 ~ attenuation of signals.
Brief Deseripfion of The Drawings Figure ~ is an illustration of a drill system having a measuring while drilling device mo=anted in the drilling apparatus.
20 Figure 2 illustrates raypaths of acoustic signals between the transmitter and the receiver.
Figure 3 is an illustration of an attenuation system for use on a well drilling collar.
~0 W~ OOI75694 PCTItJS00/I5230 Figure 4 is a graphic~~l representation ixlustrating the effects of an increased nll%nbeP of attclluatlOn elements t7f a System a5 that illustrated in Figure i Figure ~ is a graphical representation i'alustrati.ng thE; effects. of increasing the 'weight of attenuation elements of a system as that illustrated in Figrzre 1.
Figure 6 is a graphical representation illustrating the attenu3t3on effect of the system of Figure 1.
Figures 7a and '7b show a comparison of the invention of Fig. ~ v~°ith one iri which the mass discontinuities are attached to the drill collar over a substantial length.
Figures ~a - ~c show alternate embodiments of the :invention in which attenuation is accomplished by means of recesses that produce mass dyscontinuities ira a body of the attenuator.
Figure 9 shows a cor~~parison of frequency spectra o~~ attenuators having different types of recesses having a fixed length.
P'ig. 1i0 shows alternate embodiments of the inventior°g in wlgich the diameter of the attenuation sections is vaaied.
I)escrlptiean of the ~refer~°ed Fmbodirnen8 The present invention provides a system and method for attenuating acoustic waves in a down hole tool that is being used tc~ obtain infor..rnation ;about subsurface formations, some of which a~°e believed to be luolding hydrocarbon deposits. G. ~. is a schematic illustration of the; use of a l~easuremeni-~hile~-drilling apparatus while drilling a well. At the surface of the earth h a drilling rig a is used to Il w~ 00/756~d P~T'I~S00115230 drill a'6orehole 2~ through subterranean formations ZSa, 2!5b, 25c etc. Those versed in the art would know that a drillship or a platform could be used to drill a borehole into subtera-anean formations covered by a body of water. .~ drilling tubular 1~9 that could be made of drill pipes or coiled tubing is used to rotate a drillbit 17 at the bottom, the rotating action of the drillbit and axial pressure catvin~; out the borehoh.
V~hen coiled tubing is used for the drilling tubular, a drilling oto~°
{not shown} is used to impart the necessary rotary motion to the drillbit.
l~ variety of transducers are used downhole in a serAsor assembly 11. This sensor assembly makes measurements of properties of the :E~ormations through which the borehole is being drilled. These could include electromagnetic, gamma ray, 1.0 density, nuclear-magnetic resonance, and acoustic sensors. war illustrative purposes only, an acoustic transmitter array 31 and an acoustic receuver atra;y 33 are ia~dicated.
Those versed in the art would recognize that other Configurations of the aCOUStac transmitters and receivers could be used.
Turning now to F"l~,e Z, the transmitter 31 and the receiver 33 are shown '~ 5 inside the borehole 2~. The annulus between the drilling t;idular 1~ and the borehole 23 is filled with a drilling ih:id. 'The fluid is e:onveyed dovrn the borehole inside the drilling tubular to the drillbit: and returns up the hole via th~; annulus.
excitation of the transmitter produces acousti ~ signals. ~ portion of the signal, denoted by the raypath 43, is referred to as the direct arrival and travels through the tool tc9 the receiver. The 20 transmitter also produces an acoustic signal ire the borehoie b'luid tl-cat enters into the formation. ~ne portion of it, illustrated by the raypath 41 l:ravels a.s a body wave through the formation and carries information about the forca~ation that it traverses.

'i~'~ 00175694 P~°~I11S0011523~
The receiver also detects other signals, such as tube waves that :involve a coupled wave between the fluid and the formation, Stoneley waves that are surface waves irI
the fluid, a.~Id signals reflected from acoustic reflectors within the formation.
In an tool, as in wireline tools, khe body wave 41 tlwough the formation usually arrives before the tube wave and the Stoneley wavE;., Hiow~:ver, in an tool, the direct arrival 43 through the tool commonly arrives bef~rc~ the desired signal component 41 that tames Iraf'ormation about the acoustic properties of the formation.
In addition, the drillbit 17 itself is continuously generating acoustic signals traveling through the drilling tubular ll. Consequently, it becomes ~dery difficult to determine a.
travel time for the formation body wave 41~
l.~ In order to attenuate the direct arrival 43, fhe to~1 a pulse at~enuator 40 is Located in tool 11 between transmitter 31 and an receiver :~~. (~nl.y ~ne transmitter and receiver are illustrated for demonstration. In practice, o:~ere may be several receivers and transmitters arid the present invention operates wukh any arrangement, the only requirement is that attenuator 40 be located betwes.rl the transmitter and the receiver.
In one embodiment of the invention, the acoustic ls~clator Is based upon ar!
array of mass rings attached to the inner wall of the drilling; collar. Such an array presents an interference filter providing a stop band at a pre:dete:rmined frequency for longitudinal sound waves propagating along tlm walls of a ~..ollar. 'The device exhibits suff'lcient damping wikhin the predetermined frequency ran~ee as wcrll as good mechanical strength. 'The efficiency of an isolator of this t3~pe increases proportionally to the number of the rings ~r as well as to the ratio ~/1 / p. , where I~~ is the mass of a single ring, ~t is a mass per unit length of the collar. I-Ience, the efficiency of the i3 ~~ 00175694 P~'~'/CJS00~13230 isolator is very sensitive to ~,ven minor changes in outer dimensions ofthe pipe as well as to the changes in demands to its wall thickness.
The attenuation provided by the isolator section is designed to be minus forty decibels within the frequenc;~ range of twelve through eighteen kilohertz. The isolator design satisfy the mechanical requirements specified concerning the limitations on inner diameter, outer diameter, minimal cross section area and others.
Figure 3 is a partial illustration of an attenuation system Sfi for a sound tool (not shown) in a drill collar vI2 using an array of hanging mass irregularities 54, 56, 5~
... may include up to ten elements) mounted on inner wall 6~ of drill collar 52. Mass irregularities 54, 56, 5~, .., a:e secured to inner wall 60 by neck pieces 62 which extend out from outer circumference 64, 66, 6g, ... of mass irregul~~rities 54, 56, 5~, respectively. Neck pieces 6s; are smaller both in depth and width t',Iaan outer circumferences 64, 66, 6~, .., ofmass irregularities 54, 56, ,:~, .., so that mass irregularities 54, 56, 5~, ... are held firmly against inner wall 6d~ but not so firmly that acoustic pulses tra~reling through drill collar 52 are transferred witY~out attenuation. In this manner, mass irregularities 54, 56, 58, ... are held firmly butt not tightly.
In an alternate embodiment, an inner pipe 64 may be provided to protect array of mass irregularities 54, 56, 58, ... from mud :flow. Inner pipe rnay be of any material to isolate mass irregularities 54, 56, 5r~ ... frorr~ the mud flow, however, a material that is non-rigid and has a degree of flexibility is preferred. ~ rraaterial that is less likely to transfer acoustic pulses toward the receivers is desired.
The operation of the attenuation filter rnay be understood by the following discussion. The attenuator sr:ction has N mass irregularities; or elements, each element having the shape of rings or donuts attached to the inner surface of a pipe at the points w~ 00175694 ~CTlZJS00115230 x = x~, (where j = 1,...r~}. Then origin of coordinates eoincide;s with the fast irregularity, i.e. xa = ~. The rrlass of a rang j is m~. The distance between two neighboring elements 3S<
~j ~ x~.l '~ xj.
At x > x", an incident longitudinal sound wave of a unit amplitude traveling towards the origin of coordinates may be denoted by ~ ~-~~~(s-%")-wrj where k = ~'c is a ~~avelength constant, 1 Q r.~ = ~ rcf is an angular frequency, c = the velocity of sound.
Due to the presence of an array there e~.ists (at x > xT~,~ a reflected wave ~~
"~~~~ik(x-x")-i~u9 where ~"(~,J is a reflection coefficient for an array of n irregularities.
i5 In the present invention, the dimensions of irregularities are small as compared with the wave length at a given frequency c~ = 2 arJdc. The densit;r ~ as well as linear mass of a pipe ,u are also of great importance in the attenuation. 1:n the present irwention, i~'~ OOI'75694 ~~~'IILTS00/~~23fl the mass m~ is much greater than ,u hJ, where h~ is the length of attachment zone for the mass j. Such an array.resents an interference Biter providing a stop band at a predetermined frequency range for longitudinal sound waves propagating in the walls of a pipe.
In the solution of a wave equation, the length of a <;ontact <bone, ~1, between a ring and an inner wall of a pipe is small as compared to thsr wavelE~ngth of interest ~.
~CTnder these circumstances the propagation of the longitudinal wave can be described by the following differential equation:
z ~ a ~'.S ~~ -- ~tc ~ ~ - j --a ~ .~ - x~ ~: p ~l~
there:
1d his the Young's modulaas of the pipe rnater;~al, ~S is the cross sectioh. area of tire pipe walls a is the displacement, ~a is the lines:r mass of the pipe, and x is the lorgi~udinal coordinate.
~6 ~Y~ 00/75694 PC'a'/tIS001t5230 When considering propagation of a sinusoidal wave,, the displacement a nay 'be represented by a function of the form u(x)ex~( itAat), where, r~ is the angulax.
frequency, The differential ~t~ave equation then takes the forma ~a~ ~ -~- ~4l C!3 z fl -f- I ~7 ~ 2,l C~ ~JC -- x~ =
~2~
For an array of ~ mass irreg~arities, tF~e solution takes the fssrxn ~=i ~3) where, ~I is a~, initial wave amplitude, ~( x - x ~ ) = exp (u j x - x ~ f ) ~ (2 y .s k) is d:~reen function, and bJ = ~,.~.r~ is the magnitude of an irregularity.
'? 0 I-fence the transmission coefficient at a position x that is greater thaxa x" can be found ass w~ oo~'s69a ~~T~soon~23o T = a (x) / A, which may be expressed in decibels using the usual conversion factor.
The transmission coefficient ofthe array may also be obtained by other methods. One such method Fs an impedance approach, the relative input impedance is given by the formula:
Grr. - ~wP~~
where:
p --- pressure, c = velocity ~.~ sound in the medium, v = vibrational velocity, and p = density For an array of N elements, the impedance is calculated with the help of flee following recurrence procedure:
~~1 ' ~n 1 tarl~~ ~ 1 )_ ~ 7~~
p ' ~l 9J ~3~9~v.~
~ -- I,Z~ tcZB'1( I~I~
Figures 4 and 5 illustrate plots of transmission vs. i~equency. The influence of the number of elements is illustrated in Figa~re a. Transmission curves are shown W~ 00175594 PC'rlUS00/a5230 for six elements, eight elements and ten elements, The increase in. the number of elements only slightly changes the transmission cows at tlae borders of the predetermined frequency band. However, the attenuation values c~f the transmission curves in the middle of the frequency band are greatly affected. The period of an array 1 is important to place the transmission curves at the proper frequency. In the preferred embodiment an optimal value for the spacing betvaeen elements is 5.12 inches or approximately thirteen centimeters for the inner and csuter diameter used.
However, other spacings such as fourteen or 1-welve centimeters may also be used and provide acceptable results. The influence of the mass of a. single element is illustrated in Figure 5.
1 ~ Figure 4 illustrates attenuation curves for arrays of ten elements. Each curve is for elements of different weights. A first curve is for ter.~ elements;
each weighing eight kilograms, the second for elements weighing eleven kilograms and a third for elements weighing fourteen kilograms. An increase in the mass Dd results in changing the low frequency border. The high frequency border relnains essentially :15 unchanged. All the transmission curves show that transmission loss exceeds forty decibels within the predetLr~nined frequency band between tvb~el.ve and eighteen kilohertz.
The calculations were performed for a~x array of N identical equally spaced irregularities. Transmission coefficient was calculated vs. frequency within the ?.0 frequency range from five to twenty kilohertz.
Figure 6 is a graphical representation of the attenuation of a preferred embodiment of the present invention. in the preferred embodiment, ten elements were WO 00/75694 POTJtJS00115230 used with a spacing of thirteen centimeters between elements. pings of stainless steel were used as mass irregularities 54, 56, 58 .... It can be seen that the arrangement of the preferred embodiment provides attenuation of waves ir.~ the range ~f eight to eighteen kilohertz. By using his system, interference of waves traveling through the collar of a drilling tool can be greatly reduced and acoustic logging is possible during a drilling operation.
Figs 7a and 7b show a comparison between the erilbodi;~ne:nt discussed above with respect to Fig. 2 and an alternate embodiment of the invention using a different arrangement of attaching the mass discontinuities to the drill collar. Shown in the upper portion of Fig. 7a is a drill collar 152a to which a mass 154a is attached by means of a neck 158a. 'his corresponds to the arrangement discussed ab~ve with reference to Fig. 2. Shown in the upper portion of Fig. 7b is an alternate arrangement in which a mass 154b is attached to the drill collar 152b over substantially the full length of the mass. Shown in the lower portion of Fig. 7a is a schematic representation of the effecti~ a mass discontinuity l7Qa as seen by a propagating wave:
typically, such a mass discontinuity provides approximately b to 8 dB of attenuation of the wave. T'he lower portion of Fig. 7b shows the effective mass discontinuity 170b as seen be a propagating wave: effectively, an attenuation of 2 - 3 dB of attenuation is provided at each boundary. By an analysis such as discussed above with respect to equations 1 - 4, the arrangement of Fig. 7b is shown to act as a low pass 24 filter. By suitable choice of the spacing and size of the weights, the effective cutoff frequency can be made to be around l a kHz. When used in combination with an electrical bandpass filter knot: shown) on the tool, body waves through the drill collar may be effectively attenuated.

'W~ OOI75694 PCTlUS00115Z30 Figs. 8a - 8c show alternate embodiments of the invention in which the isolator comprises a machin~:d cylindrical member. In Fig. 8a, 'the cylindrical member has an outer diameter of Oi) and an inner diameter of lfTp. 'I'he inner diameter allows passage of drilling mud. Thfv inside wall if the, cylindrical n~ernber has recess of length L therein. A body wave encounters regions of different cross sectional areas and mass densities, similar to the embodiments discussed above, resulting in attenuation of body waves.
Fig. 8b shows an arrangement in which the recess are on the outside ofthe isolator whole Fig. Se shows an arrangement in which there are recess on both the outside and the inside of the isolator.
Fig. 9 shows the results of a finite element ("FE") simulation of the various embodiments shown in Figs. 8a - 8c. "fhe abscissa is the frequency and the ordinate is the normalized amplitude of waves passed by the attenuator. Note that the amplitude scale is linear, rather than being in decibels. The curve 301 shows the spectrum for a cylindrical pipe. The curve m~03 shows the spectrum for cuts on thE: inside of the pipe, 305 is for recesses on the inside and outside of the pipe while 3Q7 is for recesses on the outside of the pipe. Similar FE simulations have been carried out for various lengths L of the recesses. Eased upon these simulations, for an Oft of 7.09", in a preferred embodiment of the invention, a value of L of 3.1 ~'~ (&.Scm) with recesses on both the inside and the outside of the isolator is used.
2Q The results in Fag. 9 ~:re for a plurality of equally spaced recesses having the same length and the same depth of the recesses. Other embodiments of the invention use a combinations of sections having different lengths and different depths of inner and outer recesses. Examples are shown in Fig. 30. Each section 400 may be wtD 00175694 P~:TT/IJS001I5230 considered to be a waveguide with an associated pass-band and a reject band determined by the inner diameter 403 and the outer diameter 40I. As may be seen in Fig.10, each section has an axis parallel to the longitudinal axis 4t95 of the body of the attenuator. By using such a combination of different inner and outer diameters , a broad range of frequencies may be attenuated. This attenuation is in addition to the attenuation produced by reflections between adjacent sections 400. In the presence of borehole fluid on the inside, and outside of the sections, the wavcguides are "leaky"
waveguides that allow energy to propagate inta the fluid. In a preferred embodiment of the inventions the inner diameters range fra:rn 2°' to 6" and the outer diameter ranges from 4°' to 10".
While there has been illustrated and described a particular Embodiment of the present invention, it will be appreciated that numerous charges anef modifacaticsns will occur to those skilled in the ~trt, and it is intended in the appended claims to cover all those changes and modifications which fall within the true spirit and scope of the presentinvention.

Claims (17)

1. A system for attenuation of acoustic waves travelling through a longitudinal member capable of transmitting said acoustic waves therethrough comprising:
a plurality of spaced-apart masses firmly attached to an adjacent inner wall of said longitudinal member, each said plurality of masses having a predetermined spacing and a predetermined magnitude for attenuation of acoustic pulses in a predetermined frequency range, at least one of the masses having a length selected to set up a standing wave at a selected frequency therein.

2. The system for attenuation of acoustic waves according to claim 1 wherein said predetermined frequency range comprises 10 khz to 20 khz.

3. The system for attenuation of acoustic waves according to claim 2 wherein said plurality of masses comprises a material selected from (i) steel rings, and, (ii) tungsten rings.

4. The system for attenuation of acoustic waves according to any one of claims to 3 wherein said plurality of masses is between six and ten.

5. The system according to any one of claims 1 to 4 further comprising a center pipe fitting closely against an inner periphery of said masses for preventing contact between a borehole fluid and said plurality of masses.

6. The system according to claim 5 wherein said center pipe is constructed of rubber.

7. The system according to any one of claims 1 to 6 wherein said spacing of the masses is within the range of twelve to fourteen centimeters.

8. The system according to claim 1 or 2 wherein the masses comprise metal rings attached to the inner wall of the longitudinal member by neck pieces extending outward from an outer circumference of the rings.

9. The system according to claim 1 or 2 wherein each of said plurality masses is attached to the longitudinal member by at least one neck piece.

10. A system for attenuation of acoustic waves travelling through a longitudinal member capable of transmitting said acoustic waves therethrough comprising:
a plurality of spaced-apart masses firmly attached to an adjacent inner wall of said longitudinal member, each of said plurality of masses having a predetermined spacing and a predetermined magnitude for attenuation of acoustic pulses in a predetermined frequency range of 10 khz to 20 khz.

11. The system for attenuation of acoustic waves according to claim 10 wherein said plurality of masses comprises a materiel selected from (i) steel rings, and, (ii) tungsten rings.

12. The system for attenuation of acoustic waves according to claim 10 or 11 wherein said plurality of masses is between six and ten.

13. The system according to any one of claims 10 to 12 further comprising a center pipe fitting closely against an inner periphery of said masses for preventing contact between a borehole fluid and said plurality of masses.

14. The system according to claim 13 wherein said center pipe is constructed of rubber.

15. The system according to any one of claims 10 to 14 wherein said spacing of the masses is within the range of twelve to fourteen centimeters.

16. The system according to any one of claims, 10, 11, 12 or 15 wherein the masses comprise metal rings attached to the inner wall of the longitudinal member by neck pieces extending outward from an outer circumference of the rings.

17. The system according to any one of claims, 10, 11, 12 or 15 wherein each of said plurality masses is attached to the longitudinal member by at least one neck piece.