AU617441B2 - Method and apparatus for converting tube waves to body waves for seismic exploration - Google Patents

Description

COMMONWEALTH OF AUSTRAL 6 1 7 4 4 PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority ''Related Art: Se 0 Nanie of Applicant: SAddress of Applicant: Actual Inventor: Address for Service: EXXON PRODUCTION RESEARCH COMPANY 3120 Buffalo Speedway, Houston, Texas 77001, United States of America GRAHAM ARTHUR WINBOW, MARK STEVEN RAMSEY and JAMES DAVID

FOX

EDWD. WATERS SONS, 50 QUEEN STREET, MELBOURNE, AUSTRALIA, 3000.

Complete Specification for the invention entitled: METHOD AND APPARATUS FOR CONVERTING TUBE WAVES TO BODY WAVES FOR SEISMIC EXPLORATION The following statement is a full description of this invention, including the best method of performing it known to us METHOD AND APPARATUS FOR CONVERTING TUBE WAVES TO BODY WAVES FOR SEISMIC EXPLORATION Field of the Invention The invention relates to the downhole generation of compressional and shear waves for use in seismic exploration of the earth surrounding a wellbore. In particular, the invention relates to 0 *o a method and apparatus for generating and converting tube waves in a 10 wellbore to compressional and shear waves at a selected depth downhole for reverse vertical seismic profiling, or cross borehole profiling of f* the earth surrounding the wellbore.

Background of the Invention Seismic exploration is the use of seismic waves to map subsurface geologic structures and stratigraphic features. The .O ultimate goal of seismic exploration is the location of economically producible oil, gas or mineral deposits. Most seismic exploration is conducted by locating an array of sensors, called geophones, on the surface of the earth. Explosive charges, vibrators, or other seismic energy sources are operated on the surface to create seismic waves in the earth. The seismic waves travel through the earth as body waves, that is, as compressional waves (P-waves) and shear waves (S-waves).

The seismic waves strike layers in the earth and are reflected toward the surface. The geophones detect the reflected waves. The resulting signals are recorded and processed in various ways to yield information about the subsurface.

Reverse vertical seismic profiling (reverse VSP or RVSP) is an exploration technique useful in obtaining information about the subsurface features in the earth surrounding a wellbore. Reverse VSP is performed by positioning a seismic energy source at selected depths Sin a wellbore. Motion detectors, or geophones, are placed on the earth's surface in a selected pattern. Seismic energy from the source enters the formation around the wellbore and is transmitted through the earth in the form of body waves. The motion detectors on the

*S

surface of the earth respond to energy transmitted from the downhole 0 source and energy reflected from subsurface features. The information 10 obtained is used to make predictions about the geologic structure and stratigraphic features in the earth around the wellbore.

Use of reverse vertical seismic profiling has been limited 15 because of the need for a downhole source that will generate sufficient energy to conduct the operation and will not damage the wellbore. Explosive charges may be used downhole and will produce sufficient energy to perform reverse vertical seismic profiling.

S However, the risk of damaging the wellbore is substantial. Air guns may also be used as downhole sources. There are several practical Sproblems with air guns, including reflections from the bubbles released when the gun is fired and the need to provide a source of high pressure air downhole. In addition, air guns may also damage the wellbore.

A paper titled "Radiation from a Downhole Air Gun Source", by Lee, et al., Geophysics, Volume 49, Number 1 (January 1984) describes use of a downhole air gun as a seismic energy source in a field cross hole seismic experiment. The paper states an air gun is an attractive seismic energy source for cross hole VSP. However, the paper notes that retrieving, fixing and repositioning an air gun are time consuming operations. This is because of the cables, hoses and wires needed to be connected to an air gun for its operation. The air gun apparently caused minimal damage to the wellbore.

The Lee, et al. paper recognizes that in addition to the P- and S-waves radiated into the earth near the downhole source, other S" 10 body waves are generated when the tube wave generated by the air gun 010 reflects from the bottom of the borehole or from other nonuniformities in the borehole. Tube waves are pressure pulses or pressure waves that travel longitudinally in a fluid filled pipe. At set* pages 30 and 31 of the paper, Lee, et al. concluded tube waves travel downwardly from the air gun, reflect off the bottom of the borehole and are again reflected by the air gun or the air bubbles formed near the air gun. The paper states whenever there are obstacles which can generate a tube wave reflection, "such as termination of a source hole, irregularities of a source hole, air bubble, presence of the tool (air gun) itself or if there are inhomogeneities in the medium penetrated by a source hole, secondary radiations and associated multiples can be generated." In a paper by Lee and Balch, titled "Theoretical Seismic Wave Radiation from a Fluid-filled Borehole", Geophysics, Volume 47, Number 9, (September 1982), tube waves in a fluid-filled wellbore are discussed. The paper indicates that tube waves in the wellbore may generate a high amplitude body wave in the earth surrounding the wellbore when the tube wave is reflected at the bottom of the wellbore.

-L -I -1 e U.S. Patent Number 3,979,724 to Silverman illustrates an application of the principle mentioned in the above paper by Lee and Balch. Silverman teaches generating a shock wave, or tube wave, in the drill string in a wellbore. The shock wave exits the end of the drill string and enters the fluid in the wellbore, generating a seismic wave in the earth. The shock waves used by Silverman apparently would not damage the wellbore. However, merely permitting the tube wave to exit the pipe and enter the fluid in the borehole, or alternatively, to reflect back up the drill string, is inefficient.

Only a relatively small amount of the energy in the tube wave traveling down the pipe will be radiated into the formation. As a reee result, the Silverman method and apparatus is inefficient in conversion of tube waves into P- and S-waves.

e g. eSe Patent No. 4,671,379 to Kennedy, et al. illustrates a downhole seismic energy source. A column of fluid in the wellbore is oscillated to produce a resonant standing wave. This is accomplished by isolating a column of water between two inflatable bladders and eneo exciting the column with an oscillating driver communicating with the 20 column of fluid. The patent states it provides a relatively efficient source of energy by operating at or near the resonant frequency of the column of fluid. A principal disadvantage of the device in the patent is the relatively complicated downhole equipment, shown in Figures 3 through 9, needed to practice the idea.

U.S. Patent Number 2,281,751 to Cloud teaches producing seismic waves by periodically varying the pressure on a fluid filled borehole. To the extent Cloud generates any tube waves, he relies r.r primarily on the difference in the cross section between the pressurized tube 14 and the fluid-filled bottom section of the hole 13 to convert the tube waves to body waves. The method and apparatus taught in the Cloud patent will also be inefficient, for the same reasons stated above in the discussion of the Silverman patent.

As shown above, there is a need for an apparatus and method for producing tube waves that will be conducted downhole and efficiently converted into compressional and shear waves radiating 10 into the earth surrounding the borehole. As recognized in the Lee, et *ee. al. paper, any obstruction in the hole, such as an air gun, will convert tube waves into body waves. However, the efficiency of conversion is low and an extended amount of time would be required for seismic work because of the low power output. Preferably, the 15 apparatus and method will permit the tube wave converter to be moved to any selected location in the wellbore with relative ease. In addition, the method and apparatus should be simple and rugged so it will withstand typical downhole conditions.

Summary of the Invention The invention is a method and apparatus for creating tube waves at or near the surface and then converting the tube waves to body waves in the earth at a selected depth. Tube waves are created, injected into the fluid in the wellbore at or near the top of the wellbore, and guided downhole by the wellbore or casing or tubing.

When the tube waves strike a unique converter suspended downhole at a selected depth, the tube waves are converted to compressional and -oshear waves and are radiated from the converter into the earth surrounding the wellbore. The tube wave converter is a relatively efficient downhole source and may be used for conducting reverse VSP or cross wellbore seismic experiments.

The invention comprises a source for producing tube waves in the wellbore. A tube wave source is located at a shallow depth or at the surface and communicates with the fluid in the wellbore. The tube wave source injects tube waves, which travel down the wellbore and strike the tube wave converter. The tube wave source may create 10 S pressure impulses, such as an air gun. Preferably, the source will *see I create a controllable swept frequency pressure wave train.

The invention further comprises a tube wave converter that is g.

15 positioned at a selected depth downhole. The converter is preferably o. an elongated body suspended in, but not necessarily clamped to, the wellbore, and having a length approximately equal to one-half to one Swave length of the formation P-wave at the frequency of operation.

The tube wave converter should have a strong acoustic impedance contrast with the fluid in the wellbore and should fill the hole as completely as practical.

Preferably the swept frequency pulse generated by the tube wave source is detected by a suitable detector on the tube wave converter. The resulting signal is transmitted uphole through the wireline from which the tube wave converter is suspended. This downhole signal is cross-correlated with the signals received by the geophones at the surface to give an image of the subsurface comparable -7to that created by an impulsive downhole source. The resulting signal may also be recorded and stored downhole for retrieval and cross correlation at a later time.

The invention solves several of the problems with downhole seismic sources. The tube wave source may be located uphole. The downhole tube wave converter is a simple design, yet is efficient in converting tube waves to compressional and shear waves. The tube wave S. oS converter is not an air gun or an explosive source, thereby reducing I the possibility of damaging the wellbore. The tube wave converter is located at any selected depth in the wellbore by wireline. No high pressure air lines are required and no complicated wiring is necessary. Practically speaking, the tube wave converter portion of the present invention is an inherently rugged and reliable device, 15 which those skilled in the art of downhole tools design will realize is a great advantage.

Brief Description of the Drawings The embodiments of the invention are shown in the drawings, in which like reference numbers indicate like parts. Note that for clarity, portions of drawings may be shown in orientations not indicative of the final assembly perspective. A description of each drawing is below.

FIGURE 1 is a schematic cross-sectional view of a reverse VSP project utilizing the invention.

-8- FIGURE 2 is a side view of a tube wave converter.

FIGURE 3 is a cross-sectional view of a first embodiment of a rotary valve for a pressure pulse generator assembly.

FIGURE 4 is a cross-sectional view of a second embodiment of a rotary valve for a pressure pulse generator assembly.

FIGURE 5 is a cross-sectional view of a third embodiment of a rotary valve for a pressure pulse generator assembly.

FIGURE 6 is a cross-sectional side view of an embodiment of the tube wave converter for use in uncased boreholes.

1" 5FIGURE 7 is a cross-sectional side view of the embodiment of "15 SFIGURE 4 with additional sealing elements.

i S. Detailed Description FIGURE I is a partial schematic illustration of the invention. A conduit i, such a tubing or casing, penetrates wholly or partially into a wellbore 2 in the earth. A lubricator assembly 3, or other suitable means for allowing cable passage between the exterior and interior of the tubing or drill string is connected to the wellhead and an electric wireline or mechanical cable 4 extends through the lubricator and downhole. The cable 4 is connected to one end of the tube wave converter 5. The lubricator 3 seals around the -9cable and permits the depth of the tube wave converter 5 to be adjusted in the usual manner. Typically, the cable 4 will be spooled onto a motorized winch (not shown) so the depth or axial position of the tube wave converter in the wellbore can be readily adjusted.

A pressure pulse generator assembly 6 creates alternating pressure pulses that travel downhole as tube waves. The pressure pulse generator assembly communicates with the conduit 1 through a I O length of tubing 7. The pressure pulse generator assembly includes a rotary valve 8 powered by a motor 9. A pump 10 draws relatively low I o pressure liquid from a fluid supply indicated at 11. Liquid exits the pump at a pressure higher than the wellbore pressure, flows through a shut-off valve 12 and flows to the supply side of the rotary valve 8.

S* The rotary valve is configured so that the ports connecting the high *15 pressure side of the valve and the tubing 7 are rapidly opened and at 00I least partially closed or blocked, to create alternating pressure pulses in the tubing 7. By controlling the motor speed and acceleration, it is possible to create a swept frequency pressure wave train in the conduit 1. An accumulator 13 may be used to reduce the effects of fluid inertia on equipment upstream of the proposed pulse generator assembly. In the accumulator a body of liquid 14 communicating with the outlet of the pump 10 and the inlet of the rotary valve 8 is overlaid by a body of high pressure gas Typically, the gas 15 and the liquid 14 will be separated by a diaphragm or piston 21, In operation, the pressure pulses, or swept frequency pressure wave train, generated by the pressure pulse generator 6 are coupled to the wellbore 2 through the tubing 7 connected to the wellhead. The pressure pulses are conducted down the wellbore by the wellbore casing 1. The tube wave converter 5 converts the pressure pulses to body waves. The body waves, indicated by the lines originating at the tube wave converter 5, travel to the surface 18 and are detected at the surface by geophones 19. The body waves also are *0 reflected from features in the earth around the borehole and the reflected waves are subsequently also detected by the geophones 19.

I o: o The resulting signals from the geophones are recorded and may be processed as is well known in the art.

*gee To produce the swept frequency pressure wave train referred *15 to above, the rotary valve body of the pressure pulse generator is initially rotated at a speed that will produce a selected frequency, for example, 20 Hz. Over a period of a few seconds, the speed of the rotary valve body is increased in a controlled manner to a selected upper frequency, for example 100 Hz. This will result in a swept frequency tube wave train similar to the swept frequency pulses injected by surface seismic vibrators and methods that are well known in the art, such as VIBROSEIS*. The swept frequency pressure pulses travel down the wellbore as tube waves to strike the tube wave *registered mark of Conoco.

-11converter. Preferably, the rotation of the motor and the rotary valve will be controlled so that a swept frequency pulse having any desired characteristics may be produced.

The tube wave converter is preferably an elongate metal body having a strong acoustic impedance contrast with the fluid in the wellbore and which fills the conduit 1 as completely as is practical.

The preferred embodiment of the tube wave converter is shown in FIGURE 2. The converter preferably has a generally cylindrical 10 central portion 30 and tapering, or generally conical, ends 31 and 32. To radiate as effectively as possible the length L of the *I converter should be at least about 1/2 and up to about one wavelength of a formation compressional wave at the desired operating frequency "SOo or at the central frequency of the swept frequency pulse for a swept C 15 frequency tube wave. The wavelength of a formation P-wave (formation j. o *P-wave velocity/desired operating frequency) will be known or is readily obtained by well-known methods.

o As stated above, it is known that any obstruction in a liquid filled conduit will radiate some P- and S-waves when struck by a pressure pulse in the liquid. In the Lee, et al. paper discussed above, tube waves striking a downhole air gun produced P- and S-waves. It has been determined that the efficiency of conversion of tube waves to P- and S-waves increases as the length of the obstruction increases, up to the preferred length stated above.

However, an elongate converter will function adequately, although with less efficiency even if its length L is less than the above preferred length. Therefore, elongate, as used above, means generally having a -12length substantially longer than diameter and in particular having a ratio of length to diameter substantially greater than the length to diameter ratio of typical air guns or the like that may be used downhole.

The tube wave converter should have a strong acoustic impedance contrast with the liquid in the wellbore. However, it is not necessary that the converter be a solid metal body. Another ,reason air guns, such as used by Lee in the paper described above, are 10 not particularly efficient in converting tube waves to body waves is :0.o that air guns will not have a strong acoustic impedance contrast with the liquid in the wellbore. The efficiency of the converter increases as its radius increases and approaches the internal radius of the *I wellbore. As a practical matter, the radius of a converter for use in a cased wellbore may be up to 90% or more of the radius of the casing. This will create the necessary strong acoustic impedance contrast and still leave adequate clearance to move the converter through the casing. If the converter is a solid metal body with a substantially uniform cylindrical cross section (that is, if the ends of the converter are not tapered), it will radiate P- and S- waves adequately if the above conditions for length and radius are met.

However, it will only radiate efficiently for a narrow range of frequencies.

To improve the efficiency of the tube wave converter over a broader bandwidth, it should preferably be shaped as shown in FIGURE 2. The length L 1

L

2 of the tapering end portions 31 and 32 should be comparable to the length B of the central -13section 30. The performance of the converter is not very sensitive to the exact shape of the tapering. To optimize the bandwidth of radiation, the central section 30 should be much shorter than the tapering end sections 31, 32. However, this will reduce the performance of the converter in the low seismic frequency band, that is, with frequencies from about 20 to about 70 Hz.

Clamping the tube wave converter to the casing is not O. necessary. The converter will work efficiently irrespective of i0 whether the casing is well bonded or not. This is because at seismic frequencies the predominantly radial pulse emitted by the converter will pass through casing and surrounding mud or cement.

*e The body waves radiated by the converter can be detected 15 either by receivers placed in a nearby borehole or by an array of detectors on the surface. For example an array of geophones or hydrophones may be placed in shallow holes filled with water or mud, which should ensure a good signal to noise ratio. Preferably, the e 0 signal detectors should not be placed closer than about 100 feet from the well. This is because tube waves carry significant energy close to the borehole and the detectors should be located away from the well to avoid detecting this energy.

The swept frequency tube wave train is preferably detected by one or more suitable detectors mounted on the tube wave converter.

The detector may be a motion or pressure transducer, or any other suitable detector ktiown in the art. The measured signal is transmitted uphole through the cable suspending the converter.

-14- Alternatively, the measured signal may be recorded downhole and recovered at a later time. This later recovery may or may not include results of downhole signal processing. At the surface this signal is cross correlated with the signal received by the detectors on the surface, or elsewhere, to give a resulting image of the subsurface comparable to that created by an impulsive downhole source, such as an air gun. This is similar to the technique used to process data from VIBROSEIS type surface seismic sources, and is well known in the art.

eI Relative to surface vibrators, the present technique has the advantage o I10 that the signal entering the earth is ell-defined which is not the coo* case for surface vibrators.

The pressure pulse generator assembly may create single impulses rather than swept frequency tube waves. Such an injector 15 could be an air gun, for example. In addition, tube wave converters of different or variable designs could be utilized in the invention.

The optimal wellbore fluid is clean water, with entrained gas removed. The entrained gas may be removed by conventional means prior to operations. Drilling mud, saline water, and most commercially available completions fluids are also considered acceptable, if extra weight is needed in the fluid column in the wellbore.

Preferably, the operation will be carried out in a cased well. If the well is uncased, the preferred method of using the invention will be to inject the tube wave down an open ended drill string, tubing, or other work string with the converter mounted at the end of the drill string, tubing, or other work string. This embodiment is described in more detail below.

Embodiments of the rotary valve for the tube wave injector are illustrated in FIGURES 3, 4, and 5. The rotary valve hydraulically opens and closes several times per revolution of the valve spool. For example, one turn of the valve spool may open and Sclose each port twice. If there are N ports and the shaft turns at

F

0 Hz, then the frequency of the produced tube wave will be 2 NF 0 Thus, to produce 100 Hz tube waves, the shaft should spin at 60 x 100 rpm/2N. If N 4, as shown in FIGURE 3, then a shaft speed 10 of 750 rpm is needed to produce 100 Hz tube waves. Thus, increasing the number of ports will reduce the speed at which the valve spool must be rotated. This will reduce wear on the valve. At present, up to 10 ports through the valve spool are contemplated for a Working system.

4 The valve should be carefully balanced to minimize stress on bearings and seals during operation. It is anticipated 'the pressure of the mud or other liquid imposed on the inlet side of the rotary valve will be in the range of 100 to 5000 pounds per square inch, and typically would be about 1000 psi. Depending on fluid conditions and well geometry, higher pressures may be used.

Referring to FIGURE 3, the details of the design of one embodiment of the rotary valve 8 are shown in more detail. The valve comprises a generally tubular valve body 36. Inlets 37 supply high pressure liquid to the rotary valve. The high pressure liquid communicates with the interior of the valve body 36 through valve body ports 39. Outlets 40 are also connected to the valve body 36 and -16communicate with the interior of the valve body through valve body ports 39. End plates 41 are connected to the ends of the valve body 36 with bolts and nuts 42, 43 or other suitable means. Seals 44 between the end plates 41 and the valve body 36 prevent leakage of the high pressure fluid from the valve.

A shaft 46 mounted in bearings 47 is connected to each end of a cylindrical rotary valve spool 45 to mechanically support the spool in the valve body. The valve spool 45 is a hollow cylinder, which may

S

-*log 10 be closed on one end and open at the other end. The ports 50 through •e the cylinder alternately align with and at least partially block off the valve body ports 39 to create a swept or fixed frequency pressure pulse train when the valve spool is rotated. Seals 44 between the shaft 46 and the end plates 41 prevent leakage of the fluid from the 04 15 valve body around the shaft 46. One end of the valve spool shaft 46 is connected to the shaft 49 of the drive motor (not shown), preferably through a clutch mechanism 48 to permit rapid engagement and disengagement of the valve spool from the motor. In addition, the clutch mechanism 48 may allow for slight misalignment between the motor and the shafts 46. Other drive components such as couplings, belts, transmissions, angle drives, and gear boxes may also be used as required. The drive motor may be of any type, such as electric, pneumatic, or hydraulic.

Typically, there will be some, usually small, clearance between the outer diameter of the valve spool 45 and the inner diameter of the valve body 36. The valve spool therefore typically will not completely seal off the outlets 40 when the ports 50 though 6 b -17the spool are not in alignment with the valve body ports 39. However, this will not affect the capability of the rotary valve to create the desired swept frequency pulse. Seals (not shown) may be located at other points, such as between the outer diameter of the rotary valve spool 45 and the inside diameter of the valve body 36. Such seals are discussed below.

It also should be noted that it is not necessary that there be net or continuous fluid flow from the rotary valve into the 10conduit 1 in the wellbore. It is sufficient that the pressure pulsed i 10 output of the valve is suitably coupled to the liquid in the well to transmit the pressure pulses downhole. This note applies to all embodiments of the invention.

15 FIGURE 4 shows another embodiment of the rotary valve 8 of 15 the invention. The parts of this embodiment that are common to the embodiment described above have the same part numbers as the e *e corresponding parts in FIGURE 3. The two principal differences between the two embodiments are the rotary valve spool 45 and the 20 valve body 36. In the embodiment in FIGURE 4, the valve spool is a disk, and the valve body 36 includes a septum or stationary disk with one or more ports 39. The ports 50 through the rotary disk alternately align with and at least partially block the matching ports 39 through the valve body 36 to create the swept or fixed frequency pressure pulse train in the manner similar to that described above. Seals (not shown) may be located adjacent to either the fixed ports 39 or the rotating ports 50, or both. Also, seals (not shown) may be located between the valve body inside diameter and the rotating -18spool 45 outer diameter, between the valve body 36 disk face and the face of the rotating spool 45, or between the shaft 46 and the valve body 36.

FIGURE 5 shows a third embodiment of the rotary valve having a cylindrical rotary valve spool. The parts of this embodiment that are common to the embodiments described above have the same part numbers as the corresponding parts in FIGURE 3 and FIGURE 4. In this embodiment, the inlet and outlet ports 39 are located on opposite sides of the valve body 36. One or more openings or ports 50 through 10 so:. the valve spool 45 allow fluid to enter through the spool and establish a direct fluid path through the valve when the openings in the valve spool are aligned with the inlets and outlets. As the spool is rotated, the ports 59 and the ports 39 alternately align and close to create the swept or fixed frequency pressure pulse train in the manner similar to that described above. A drain port (not shown) may be included through the end plates 41 to prevent pressure from building in the volume bounded by either end plate 41, the valve body 36, and the end of the valve spool 45. As in the other embodiments, Ce *0 "20 seals (not shown) may be located between the valve body 44 and the valve spool 45 in one or more locations.

FIGURE 7 illustrates an embodiment of the rotary valve in which seals 64 are included to improve the restriction of flow when the valve is in the closed position. An additional 0-ring 44 provides sealing between the shaft 46 and the septum. The seals are made of a suitable material such as polytetrafluoroethylene. They have a generally cylindrical body 65 and a mounting flange 66, which may be secured to the valve body by screws 67. The length of the body is selected so that the end 68 of the seal body 65 extends past the septum or valve body. The rotating valve disk thus will contact the end of the seal as the disk rotates. The seals shown, though functional as shown, may be altered or replaced by an alternate seal to improve sealing, or life, or both. It is also contemplated that one or more sealing elements may be used on any other embodiments of the valve.

0* FIGURE 6 illustrates an embodiment of the converter of the see. invention for use in an open or uncased hole. As stated above, preferably the hole will be cased. In an uncased hole, the tube wave :o oo. converter is connected in the end of a tubing or drill string 60 and tube waves are injected into the fluid filled tubing or drill s a* 15 string bO. The converter 5, which is positioned by moving the end of the tubing string to the desired depth, is connected to the tubing c string by bolts 61. The end 62 of the tubing string should be filled with a sound absorbent material 64, such as lead loaded rubber to reduce reflections. Openings 63 through the tubing string allow the o* S I 2 *converter to radiate P- and S-waves into the formation. The total 20 area of the holes should preferably equal at least about 30% of the area of the tubing over the center portion 30 of the converter Alternatively, the tube wave converter 5 may be made an integral part A of the tubing string or drill string.

The invention is used in a cased well by positioning the tube wave converter 5 at a selected depth downhole. As shown in FIGURE 1, the converteu: 5 is lowered on the cable 4 to the selected depth. In an uncased hole, the converter is positioned by locating the end of the tubing string, or drill string 1, as shown in FIGURE 6, at the selected depth by conventional means. As was described above, the pressure pulse generator assembly 6 is connected to the casing, tubing string or drill string as appropriate, to transmit a pressure pulse or pulses downhole. The pressure pulses strike the tube wave converter and P and S-waves are radiated into the earth by the converter.

Preferably, the swept frequency pressure pulse train is detected by a suitable detector (not shown) on the tube wave converter and the g resulting signals are transmitted uphole and recorded. Alternatively, the data from the detector may be stored downhole for retrieval 1 @6 later. The geophones 18 at the surface detect the body waves and the t o* resulting signals are recorded and preferably cross correlated with the signals from the detector at the converter.

an A specific embodiment of the invention has been illustrated Sand described above. Naturally, modifications of the above embodiment may be suggested to persons skilled in the art and it is intended that this patent application cover all such modifications that fall within the scope of the attached claims.

Claims (35)

1. A method for converting tube waves in a liquid filled wellbore into compressional and shear waves radiated into the earth surrounding the wellbore comprising: injecting at least one pressure pulse into the liquid in the wellbore to create a tube wave guided by the wellbore to a pre-selected depth; and positioning an elongated tube wave converter at the eo@ 10 pre-selected depth in the liquid in the wellbore, wherein the tube wave converter has a strong acoustic impedance contrast with the liquid in the wellbore.

2. The method of Claim 1 wherein the tube wave converter 15 has a length of at least about one-half of the wavelength of a 15 formation P-wave at the operating frequency of the tube wave converter, whereby the tube wave converter converts tube waves to compressional and shear waves and radiates these compressional and shear waves into the earth.

3. The method of Claim 2 wherein the tube wave converter has a length in the range of about one-half to one wavelength of a formation P-wave at the operating frequency of the tube wave converter.

4. The method of Claim 2 wherein the tube wave converter has tapered ends and a generally cylindrical center section. The method of Claim 1 wherein the tube wave converter has tapered ends and a generally cylindrical center section. -22-

6. The method of Claim 5 wherein the length of the center section of the tube wave converter is comparable to the length of each of the tapered ends.

7. The method of Claim 1 wherein the step of injecting at least one pressure pulse into the liquid in the wellbore comprises injecting a swept frequency pressure wave train into the liquid in the borehole to create a swept frequency tube wave train in the liquid in the wellbore.

8. The method of Claim 7 wherein the swept frequency range is about 20 to 200Hz. *0

9. The method of Claim 8 wherein the length of the tube wave 15 converter is in the range of about one-half of the wavelength of a formation P-wave at the center frequency of the swept frequency ,o pressure wave train. 23 Apparatus for producing compressional and shear waves at a selected depth in a liquid filled wellbore for use in seismic investigation of the earth surrounding a borehole, comprising: a pressure pulse generator assembly adapted to be suitably coupled with the liquid in the wellbore for creating a tube wave in the liquid; an elongate tube wave converter adapted to be positioned at a selected depth in the liquid in the wellbore, and having a strong acoustic impedance contrast with the liquid in the wellbore, wherein the tube wave converter will convert the tube to compressional and shear waves and radiate these compressional and shear waves into the earth at the selected depth.

11. The apparatus of Claim 10 wherein the length of the tube wave converter is at least about one-half of the wavelength of a formation compressional wave at the desired operating frequency. i

12. The apparatus of Claim 10 wherein the tube wave converter has a generally cylindrical center section and tapered ends.

13. The apparatus of Claim 10 wherein the pressure pulse generator assembly injects -a swept frequency pressure wave train into the wellbore.

14. The apparatus of Claim 11 wherein the length of the tube wave converter is at least about one-half of a I i wavelength of a formation compressional wave at the center frequency of the swept frequency pressure wave train. A method for converting tube waves in a liquid filled conduit in a wellbore into compressional and shear waves radiated into the earth surrounding the wellbore comprising: injecting at least one pressure pulse into the liquid in the conduit to create a tube wave guided by the conduit to a pre-selected depth; and positioning an elongated tube wave converter at the selected depth in the liquid in the conduit, wherein the tube wave converter has a strong acoustic impedance contrast with i10 the liquid in the conduit. *0S1 O* I 16. The method of Claim 15 wherein the tube wave converter *see fills the conduit as completely as practical and has a length of at least about one-half of a wavelength of a formation P-wave at the S: 15 desired operating frequency of the tube wave converter, whereby the tube wave converter converts tube waves to compressional and shear e wave and radiates such compressional and shear waves into the earth.

17. The method of Claim 15 wherein the tube wave converter is an elongate metal body having a generally cylindrical center section and tapered ends.

18. The method of Claim 17 wherein the length of each of the tapered ends of the tube wave converter is comparable to the length of the center section. i i I 25

19. The method of Claim 16 wherein the step of injecting the at least one pressure pulse comprises injecting a swept frequency pressure wave train and wherein the length of the tube wave converter is at least about one-half of a wavelength of a formation compressional wave at the center frequency of the swept frequency pressure wave train. The method of Claim 15, further comprising the step of detecting the compressional and shear waves radiated into the formation by the tube wave converter and the step of cross correlating the signal detected in the previous step with signals obtained from other compressional and shear wave detectors spatially removed from the tube wave converter.

21. Apparatus for producing compressional and shear waves at a selected depth in a liquid filled wellbore for use in seismic investigation of the earth surrounding a borehole, comprising: a liquid filled conduit, placed in the liquid filled wellbore; a pressure-pulse generator assembly adapted to be suitably coupled with a liquid in the liquid- o• filled conduit for creating a tube wave in the liquid-filled conduit; and an elongate tube wave converter, positioned at a selected depth in the liquid-filled conduit and having a strong acoustic impedance contrast with the liquid in the liquid-filled conduit, wherein the elongate tube wave converter will convert the tube wave into compressional and shear waves and radiate these compressional and shear waves into :9 the earth at the selected depth. o 'I 7 25a

22. The apparatus for producing compressional and shear waves at a selected depth, as defined in Claim 21 wherein the elongate tube wave converter is at least about one-half of the wavelength of a formation P-wave at the operating frequency of the tube wave converter.

23. The apparatus for producing compressional and shear waves at a selected depth, as defined in Claim 22 wherein the elongate tube wave converter is in the range of about one-half to one wavelength of a formation P-wave at the operating frequency of the tube wave converter. g <t '-N MLV 0>iiu, 26

24. The apparatus for producing compressional and shear waves at a selected depth as defined in Claim 23 for use with a source of a swept frequency pressure wave train wherein the elongate tube wave converter is at least about one half of the wavelength of a formation P-wave at the center frequency of the swept frequency. The apparatus for producing compressional and shear waves at a selected depth as defined in Claim 21 wherein the diameter of the elongate tube wave converter is sufficient to fill the conduit as completely as practicable.

26. The apparatus for producing compressional and shear waves at a selected depth as defined in Claim wherein the elongate tube wave converter has a cylindrical center section and tapered ends.

27. The apparatus for producing compressional and shear waves at a selected depth as defined in Claim 26 wherein the length of the center section is comparable to the length of each of the tapered ends. i: 28. The apparatus for producing compressional and shear waves at a selected depth, as recited in Claim 21 wherein the pressure-pulse generator assembly is comprised of: a valve body having an opening therein and at least two ports therethrough communicating with the S..opening in the valve body, wherein one of the ports is an inlet port adapted to be connected to a source of pressurized fluid and the other port is an outlet port adapted to be connected to the liquid-filled conduit in which the tube wave is to be created; N i^ j '.7 26a a valve spool rotatably mounted with respect to the valve body and having at least one opening therethrough positioned between the inlet port and the outlet port; and wherein the valve spool is rotatable from a first position wherein the opening in the valve spool permits fluid communication between the inlet and outlet ports and a second position wherein the fluid communication therebetween is at least partially blocked as compared to the first position, whereby rotating the rotor will create pressure pulses in the conduit. 'I IJ 27

29. The apparatus for producing compressional and shear waves at a selected depth, as defined in Claim 28 wherein the pressure pulse generator includes means for rotating the valve spool, whereby varying the speed and acceleration of the valve spool will produce a swept frequency pressure wave train in the conduit. The apparatus for producing compressional and shear waves at a selected depth as defined in Claim 28 wherein the valve spool comprises a shaft mounted for rotation in the valve body and a plate connected to the shaft perpendicular to the rotational axis of the shaft, wherein the at least one opening through the valve spool is formed through the plate.

31. The apparatus for producing compressional and shear waves at a selected depth as defined in Claim wherein the valve body further includes at least one opening therethrough at least partially aligned with the at least one opening through the valve spool at at least one S""rotational position of the shaft.

32. The apparatus for producing compressional and coo• shear waves at a selected depth as defined in Claim 28 oooo wherein the valve body comprises a cylindrical center section having two ends wherein at least one of the at least two ports is formed through the cylindrical center section and including two end plates connected to the end of the valve body; and the valve spool comprises a cylinder 14;' C) :0 oe 28 generally coaxial with the cylindrical portion of the valve body and having the at least one opening therethrough positioned to permit fluid communication between the opening in the valve body and the at least one port in the valve body at at least one rotational position of the shaft.

33. The apparatus for producing compressional and shear waves at a selected depth, as defined in Claim 32 wherein the valve spool is tubular and rotation of the valve spool alternately at least partially blocks and opens fluid communication between the at least one port in the cylindrical portion of the valve body and the opening in the valve body.

34. The apparatus for producing compressional and shear waves at a selected depth, as defined in Claim 32 wherein at least one of the end plates has at least one port formed therethrough. The apparatus for producing compressional and shear waves of a selected depth, as defined in Claim 32 wherein the cylindrical portion of the valve body has a plurality of outlet ports therethrough and the valve spool has a plurality of openings formed therethrough aligned with :i the ports in the valve body in positions such that rotation S" of the valve spool with respect to the valve body will create pressure pulses in the pressurized fluid. o* 29

36. The apparatus for producing compressional and shear waves of a selected depth, as defined in Claim 32 wherein the cylindrical portion of the valve body has at least one inlet port formed through a first side of the valve body and at least one outlet port through a second side of the valve body, and wherein the openings through the generally cylindrical valve spool are fluid passageways formed through the rotor adapted to permit fluid communication between the inlet port and the outlet port through the first and second sides of the valve body.

37. The apparatus for producing compressional and shear waves of a selected depth, as defined in Claim 36 including an inlet manifold connected to the inlet ports and an exhaust manifold connected to the outlet ports.

38. The apparatus for producing compressional and shear waves at a selected depth, as recited in claim wherein the pressure pulse generator is comprised of: a valve body having an opening therein and at least two ports therethrough communicating with the opening in the valve body, wherein one of the ports V 14 Sis an inlet port adapted to be connected to a source of pressurized fluid and the other port is an outlet port adapted to be connected to the S* borehole in which pressure pulses are to be created; a valve spool rotatably mounted with respect to the valve body and having at least one opening therethrough positioned between the inlet port and i the outlet port; and 30 wherein the valve spool is rotatable from a first position wherein the opening in the valve spool permits fluid communication between the inlet and outlet ports and a second position wherein the fluid communication therebetween is at least partially blocked as compared to the first position, whereby rotating the rotor will create pressure pulses in conduit.

39. The apparatus for producing compressional and shear waves at a selected depth, as defined in Claim 38 wherein the pressure pulse generator includes means for rotating the valve spool, whereby varying the speed and acceleration of the valve spool will produce a swept frequency pressure wave train in the conduit. The apparatus for producing compressional and shear waves at a selected depth as defined in Claim 38 wherein the valve spool comprises a shaft mounted for rotation in the valve body and a plate connected to the shaft perpendicular to the rotational axis of the shaft, wherein the at least one opening through the valve spool is formed through the plate. "41. The apparatus for producing compressional and shear waves at a selected depth as defined in Claim wherein the valve body further includes at least one opening therethrough at least partially aligned with the at least one opening through the valve spool at at least one rotational position of the shaft. g *o S S C kV- 9^\ S .A",A n a r iiii i- 31

42. The apparatus for producing compressional and shear waves at a selected depth as defined in Claim 38 wherein the valve body comprises a cylindrical center section having two ends wherein at least one of the at least two ports is formed through the cylindrical center section and including two end plates connected to the end of the valve body; and the valve spool comprises a cylinder generally coaxial with the cylindrical portion of the valve body and having the at least one opening therethrough positioned to permit fluid communication between the opening in the valve body and the at least one port in the valve body at at least one rotational position of the shaft.

43. The apparatus for producing compressional and shear waves at a selected depth, as defined in Claim 42 wherein the valve spool is tubular and rotation of the valve spool alternately at least partially blocks and opens fluid communication between the at least one port in the cylindrical portion of the valve body and the opening in the valve body.

44. The apparatus for producing compressional and shear waves at a selected depth, as defined in Claim 42 wherein at least one of the end plates has at least one port formed therethrough. •45. The apparatus for producing compressional and shear waves of a selected depth, as defined in Claim 42 wherein the cylindrical portion of the valve body has a plurality of outlet ports therethrough and the valve spool has a plurality of openings formed therethrough aligned with the ports in the valve body in positions such that rotation of the valve spool with respect to the valve body will create pressure pulses in the pressurized fluid. T C 32

46. The apparatus for producing compressional and shear waves of a selected depth, as defined in Claim 42 wherein the cylindrical portion of the valve body has at least one inlet port formed through a first side of the valve body and at least one outlet port through a second side of the valve body, and wherein the openings through the generally cylindrical valve spool are fluid passageways formed through the rotor adapted to permit fluid communication between the inlet port and the outlet port through the first and second sides of the valve body.

47. The apparatus for producing compressional and shear waves of a selected depth, as defined in Claim 46 including an inlet manifold connected to the inlet ports and an exhaust manifold connected to the outlet ports. DATED this 7th day of August, 1991. EXXON PRODUCTION RESEARCH COMPANY WATERMARK PATENT TRADEMARK ATTORNEYS, 290 Burwood Road, HAWTHORN. VIC. 3122 AUSTRALIA C C LPS:JZ (13.31) 9, *udx