Oil well acoustic logging tool with baffles forming an acoustic waveguide

An acoustic logging tool includes external baffle assemblies forming a waveguide structure at the acoustic source. The logging tool is designed for acoustic logging of earth formation surrounding a borehole. The external baffle assemblies form a waveguide structure that is designed to increase signal to noise ratio in an acoustic logging tool using dipole or other acoustic waves. In a preferred embodiment, the acoustic logging tool includes an elongated transmitter module, and a receiver sonde having a linear array of acoustic receivers. The transmitter module includes first and second cylindrical masses spaced apart along the axis by first and second spacers. The first cylindrical mass defines a first circular facing surface and a first cylindrical outer surface. The second cylindrical mass defines a second circular facing surface and a second cylindrical outer surface. A multi-pole acoustic source is fixedly mounted between the first and second circular facing surfaces, and located on the transmitter module axis between the first and second spacers. A first annular baffle assembly surrounding the first cylindrical outer surface has a first annular facing surface co-planar with the first circular facing surface. A second annular baffle assembly surrounding the second cylindrical outer surface has a second annular facing surface co-planar with the second circular facing surface. The annular baffle assemblies form an acoustic waveguide.

FIELD OF THE INVENTION

The invention relates to acoustic logging in oilfield geological formations. More particularly, the invention relates to apparatus and methods for increasing the signal to noise ratio in logging tools that use dipole or other acoustic signals.

BACKGROUND OF THE INVENTION

The field of sonic logging of boreholes in the oil and gas industry involves making acoustic measurements in the borehole at frequencies typically in the range 500 Hz-20 kHz. Below this range is typically considered as the seismic domain, above it the ultrasonic domain. A summary of the general techniques involved in borehole acoustic logging can be found in GEOPHYSICAL PROSPECTING USING SONICS AND ULTRASONICS, Wiley Encyclopedia of Electrical and Electronic Engineering 1999, pp, 340-365.

In certain well-bores, measurement of acoustic dipole signal can be difficult. This problem is nontrivial because acoustic source design is constrained by the limited space within the tool body and by the limit in power supply.

Schlumberger Technology Corporation, the assignee of this application, has provided a commercially successful acoustic logging tool, the Dipole Sonic Imaging Tool (DSI), that delays and attenuates acoustic waves propagating along the tool from the dipole source to the receiver array. The Schlumberger DSI tool attenuates acoustic waves in a manner substantially as set forth in the above-mentioned co-owned U.S. Pat. No. 5,036,945 to Hoyle et al.

The Schlumberger DSI tool is shown in schematic form inFIG. 1(prior art).FIG. 1(prior art) shows the DSI tool comprising a transmitter section102having a pair of (upper and lower) dipole sources103arranged orthogonally in the radial plane and a monopole source104. A sonic isolation joint105connects the transmitter section102to a receiver section106which contains an array of eight spaced receiver stations, each containing two hydrophone pairs, one pair oriented in line with a lower dipole source, the other with an upper (orthogonal) dipole source. An electronics cartridge107is connected at the top of the receiver section106and allows communication between the tool and a control unit108located at the surface via an electric cable109. With such a tool it is possible to make both monopole and dipole measurements. The DSI tool has several data acquisition operating modes, any of which may be combined to acquire waveforms. The modes are: upper and lower dipole modes (UDP, LDP)-waveforms, recorded from receiver pairs aligned with the respective upper and lower dipole source used to generate the signal; crossed dipole mode waveforms recorded from each receiver pair for firings of the in-line and crossed dipole source; Stoneley mode—monopole waveforms from low frequency firing of the monopole source; P and S mode (P&S) monopole waveforms from high frequency firing of the monopole source; and first motion mode—monopole threshold crossing data from high frequency firing of the monopole source.

A first advance by Schlumberger on the DSI tool increases the signal to noise ratio in a logging tool using dipole signals by shaking part of the dipole tool body axially to produce a pure, broadband acoustic dipole signal while coupling as little energy as possible into the rest of the tool body. The use of dipole signals made by shaking (axially) all or part of the dipole tool is disclosed in the above-mentioned co-pending U.S. application Ser. No. 09/537,836, filed 29 Mar. 2000. As noted above, co-owned international patent application no. PCT/IB01/00447, filed 21 Mar. 2001, claims priority to co-owned, co-pending U.S. application Ser. No. 09/537,836, filed 29 Mar. 2000, and was published 4 Oct. 2001 as international publication no. WO 01/73478 A2.

A second advance by Schlumberger on the DSI tool increases the signal to noise ratio in a logging tool using dipole signals by attaching regularly spaced mass blocks to the central mandrel within the spacer section and the receiver section of the dipole tool body. This causes the spacer section and the receiver section to behave acoustically like a mass-spring structure which does not interfere with the acoustic signals used for evaluation of the formation surrounding the borehole, while still providing suitable physical structure and support for the other parts of the tool. The use of regularly spaced mass blocks is disclosed in the above-mentioned co-owned, co-pending international patent application no. PCT/IB00/01696, filed 16 Nov. 2000, published 23 May 2002, as international publication no. WO 02/41034 A1.

SUMMARY OF THE INVENTION

The invention provides an acoustic logging tool having external baffles forming a waveguide structure designed to increase signal to noise ratio. The tool is designed for acoustic logging of earth formation surrounding a borehole. The tool includes a transmitter module with first and second masses axially-aligned and bracketing a multi-pole acoustic source between circular facing surfaces. The acoustic waveguide includes first and second annular baffle assemblies encircling the first and second masses, respectively. First and second baffle assemblies define first and second annular facing surfaces co-planar with the first and second circular facing surfaces.

In a preferred embodiment, the acoustic logging tool includes an elongated transmitter module defining a transmitter module axis, and a receiver sonde having a linear array of acoustic receivers. The receiver sonde is coupled to, and spaced apart from, the transmitter module. The transmitter module includes first and second cylindrical masses spaced apart along the axis by first and second spacers. The first cylindrical mass defines a first circular facing surface and a first cylindrical outer surface. The second cylindrical mass defines a second circular facing surface and a second cylindrical outer surface. A multi-pole acoustic source is fixedly mounted between the first and second circular facing surfaces, and located on the axis between the first and second spacers. A first annular baffle assembly surrounding the outer surface has a first annular facing surface co-planar with the first circular facing surface. A second annular baffle assembly surrounding the outer surface has a second annular facing surface co-planar with the second circular facing surface. The annular baffle assemblies form an acoustic waveguide. Preferably, the annular baffle assemblies each include a baffle having a cylindrical outer surface and a protective ring, and the protective ring has a shaped surface with an approximately conical cross section. Preferably, the acoustic source is a dipole source. Preferably, the linear array of acoustic receivers is mounted to a linear array of mass blocks.

Alternatively, at least one of the annular baffle assemblies is just a baffle. Alternatively, at least one of said annular baffle assemblies is elongated in a direction transverse to the tool axis. Alternatively, the multi-pole acoustic source is a quadrupole

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a novel acoustic logging tool having external baffles forming a waveguide structure at the acoustic source. The waveguide structure increases the signal to noise ratio in an acoustic logging tool by increasing the received amplitude of dipole flexural mode acoustic waves. This can increase the signal to noise ratio by up to at least a factor of six. Increasing signal to noise ratio is achieved without modifying the acoustic source or increasing power to the acoustic source. This novel acoustic logging tool makes it possible to perform acoustic logging in certain well-bores in which acoustic logging is not currently possible.

FIG. 2is a schematic illustration of a first preferred embodiment of an acoustic logging tool20in accordance with the invention. Tool20includes an acoustic transmitter module110including a centralizer112, a standoff114, a first dipole source16with a lower baffle17and an upper baffle18, and a second dipole source116with a lower baffle117and an upper baffle118. Transmitter module110is shown in more detail in FIG.3and comprises an electronics section120with appropriate electronics and drive circuitry for the acoustic sources, an oil volume compensator section122, a first dipole source16(nominal “Y” direction), a second dipole source116(orthogonal to the first dipole source16, nominal “X” direction) and a monopole source128. The dipole sources16,116are substantially as described in the applicants' co-pending U.S. patent application Ser. No. 09/537,836 entitled “Dipole Logging Tool”, filed Mar. 29, 2000. U.S. patent application Ser. No. 09/537,836 is hereby incorporated herein by reference. U.S. patent application Ser. No. 09/537,836 is the priority application of the above-mentioned International Application no. PCT/IB01/00447, “Dipole Logging Tool” published as WO 01/73478 A2 on Oct. 4, 2001. The monopole source128is substantially as described in U.S. Pat. No. 5,036,945. U.S. Pat. No. 5,036,945 is hereby incorporated herein by reference.

A feed-through section130is provided to allow power and signaling wiring to be connected to the portion of the tool above the transmitter module110. As shown inFIG. 2, connected immediately above the transmitter module110is a spacer section132. Two options are shown in theFIG. 2, a long section132aand a short section132b. The length of the spacer section can be selected according to the expected acoustic behavior of the formation to be logged. The spacer section132is described in more detail in relation toFIGS. 4A,4B and4C, and comprises an inner mandrel200formed from a titanium alloy pipe having a series of stainless steel mass structures210comprising blocks with a cylindrical outer surface212and a shaped inner surface214defining a cavity216mounted securely at regular intervals along the length of the mandrel200. The masses210are secured to the mandrel200by heating each mass210to cause it to expand and sliding it into place over the mandrel200using a bore220defined by the inner surface214of each mass210. The mass210is then allowed to cool and shrink around the mandrel200. By careful selection of the material and structure of the mandrel200and masses210, and appropriate positioning of the masses210along the mandrel200, the spacer can be configured to behave acoustically like a mass-spring structure which does not interfere with the acoustic signals used for evaluation of the formation surrounding the borehole, while still providing suitable physical structure and support for the other parts of the tool. Since there is no sleeve or housing around the spacer, and the mass blocks210are hollow and not sealed to each other, it is possible for borehole fluids to enter the cavity216in the mass blocks210and mud to build up inside the blocks and affect their acoustic behavior. In order to allow cleaning of the cavity216, bores218are provided through cylindrical outer surface212of the blocks210. The mandrel200is hollow and connected to feed-throughs230,240at either end of the spacer section132such that wiring (not shown) can pass through the spacer132between the transmitter module110and the receiver sonde134.

As noted above, the use of regularly spaced mass blocks in logging tools is disclosed in the above-mentioned international application number PCT/IB00/01696. International application number PCT/IB00/01696 is hereby incorporated herein by reference.

The top of the spacer section132is connected to a receiver sonde134comprising a receiver and near-monopole source region136, an oil volume compensator138and a sonde electronics section140, and which is provided with rubber standoffs142,144. A general view of the internal structure of the receiver sonde134is shown in FIG.5. The receiver and near-monopole source region136of receiver sonde134comprises an array145of receiver stations146(16 in this example although other numbers are possible) spaced along a central mandrel148, each station146comprising a receiver mounting block150connected to the mandrel148and having a number of sensing elements152(hydrophones) arranged equi-angularly around the circumference of the block150. In the present case, eight elements152are provided but other numbers, e.g. four, can also be used. Front end electronics boards (not shown) are associated with each receiver station146. Monopole sources154,156are mounted at either end of the receiver array145. The receiver and near-monopole source region136of receiver sonde134is encased in an armored sleeve158preferably made of a soft plastic material, and is filled with oil for pressure compensation. The oil volume compensator138is connected above the receiver and near-monopole source region136and connected to the interior thereof. The sonde electronics section140is connected above the oil volume compensator138and includes front end power supplies and step up transformers (not shown) for the monopole sources. Feed-throughs160are provided to allow wiring communication between the various sections of the sonde134. The upper part of the sonde134is also provided with feed-throughs162for connection to a master electronics cartridge164which also has a centralizer166. The cartridge164is provided with standard connectors168which allow connection to other tools in a logging tool string or to a telemetry cartridge which communicates with a surface system via a wireline logging cable (not shown).

The receiver sonde is shown in more detail inFIGS. 6,7A-7C and8. The basic structure of the receiver section136is a mandrel148and receiver-mounting mass block150, an arrangement similar to that used in the spacer section. Monopole sources154,156, essentially the same as that described in relation to the transmitter module above, are provided at either end of the receiver section136. The mandrel148extends between these sources154,156and the series of mass blocks150are mounted on the mandrel148in the same way as in the spacer section. Sixteen adjacent blocks150define receiver mountings170each of which carries a circumferential array of sensing elements (hydrophones)172spaced around the periphery thereof. One diametrically opposed pair of elements in each station are aligned with a respective one of the dipole sources. In this embodiment, eight sensing elements172are provided. It will be appreciated that the number of stations and the number of receiver elements at each station can be selected according to requirements, for example, twelve stations, each with four receiver elements could be chosen.

Receiver-mounting blocks150comprise a relatively elongated, tubular body180having a bore182extending through the middle. An end section184of the bore182has a region186of reduced diameter which embraces the outer surface of the mandrel148. The outer part188of the block150is formed into a mounting cavity190for the sensing element172. These forms, or other similar structures can be used to define the acoustic behavior of the receiver section, particularly in the flexural mode. Each block150is connected so that it does not contact the adjacent blocks directly. The only continuous structure in the receiver is the mandrel148. Dummy blocks (such as shown inFIG. 8) can be provided at the ends of array145(shown inFIG. 5) to ensure consistent acoustic behavior of the structure near the ends of the array.

The sensing element172is preferably a piezoelectric pressure sensor. The preferred form of sensor comprises a piezoelectric cylinder with end caps connected by a screw extending through the cylinder. Another form of sensor is a polarized stack of piezoelectric plates. These can be in the form of a stack with a screw extending through the center of the stack to compress the plates. Alternatively, the plates can be located in a housing and separated from each other by electrodes to maximize the pressure effect on the plates. Whichever form of sensor is used, it is preferred that the axis of polarization is parallel to the longitudinal axis of the tool. The exact manner in which the sensing element172is mounted in the block150will depend upon the form of the sensing element used.

Front end electronics are mounted on circuit boards (not shown) located on mountings250(seeFIG. 9) positioned around the outer part of each block150, one set of boards on a mounting250being associated with each receiver station. The mountings250comprise four surfaces252located between circular end fittings254which fit over block150. The outer diameter of the end fittings is substantially the same as that of the mounting cavity190.

FIG. 10shows a schematic cut-away view of a portion of the first preferred embodiment ofFIG. 2, including the portion of the dipole source region119that includes lower dipole source16, lower baffle17and upper baffle18.FIG. 10shows tool20in borehole11. Tool20includes transmitter module21and receiver sonde50with receiver array145. Transmitter module21defines axis19and the cylindrical outer surface80that defines the diameter of the body of transmitter module21. Transmitter module21further includes a dipole source16(also shown inFIGS. 11A,11B and12), an array145of acoustic receivers, an annular lower baffle17and an annular upper baffle18. Lower baffle17and upper baffle18define annular facing surfaces85and87, respectively. The upper end of lower cylindrical mass40and the lower end of upper cylindrical mass41define facing surfaces86and88, respectively. Facing surfaces85and86below and87and88above define a waveguide structure including first-side waveguide71and second-side waveguide72. Lower baffle17and upper baffle18define outer cylindrical surfaces97and98, respectively.

Lower baffle17, lower cylindrical mass40, and lower protective ring43constitute baffle assembly49. This baffle assembly is preferably constructed by shrink-fitting lower baffle17and lower protective ring43onto lower cylindrical mass40. Alternatively, it may be cast and machined as one piece for unitary construction.

Dipole source16vibrates in the direction of arrows A—A, as illustrated inFIGS. 10,11A and11B.FIG. 10shows dipole source16transmitting dipole acoustic energy via first-side waveguide71and second-side waveguide72towards opposite sides of borehole wall12. The acoustic energy creates dipole flexural waves in opposite sides of earth formation14. Some of these waves, as illustrated by the dashed lines with arrowheads starting at second-side waveguide72, travel up formation14, substantially parallel to the borehole wall12, to cross again into the borehole where acoustic energy is detected at the receivers of receiver array145.

First spacer47and second spacer48define the distance between facing surfaces86and88. (Only the first spacer47is shown in FIG.10).FIG. 12shows both spacers in perspective view, each spacer preferably has the shape of a section of a cylinder. Spacers47and48fixedly attach upper surface86of lower cylindrical mass40to lower surface88of upper cylindrical mass41. Spacers47and48define the distance between lower baffle17and upper baffle18.

Lower baffle17is a rigid massive structure that is shown inFIG. 10as shrink-fit onto lower cylindrical mass40. Lower baffle17is protected and stiffened by lower protective ring43. Lower protective ring43is also shown shrink-fit onto lower cylindrical mass40. Upper baffle18is a rigid massive structure that is shown inFIG. 10as shrink-fit onto upper cylindrical mass41. Upper baffle18is protected and stiffened by upper protective ring44. Upper protective ring44is also shown shrink-fit onto upper cylindrical mass41.

In the preferred embodiment each baffle is preferably formed as one piece with its associated protective ring. Each protective ring is preferably shaped to present a smooth transition at the borehole wall. Lower protective ring43defines lower shaped surface45. Likewise, upper protective ring44defines upper shaped surface46.

FIG. 10shows baffles17and18, one spacer47, and dipole source16. This illustrates how baffles17and18and the two spacers47and48partially enclose dipole source16.FIG. 12is a perspective view of lower cylindrical mass40, spacers47and48, and dipole source16of the first preferred embodiment adapted for use in dipole mode. (Baffles17and18are not shown inFIG. 12for clarity of illustration).

The baffles and the spacers act as a waveguide structure defining a pair of waveguides that may be viewed as a first-side waveguide71and a second-side waveguide72. As illustrated inFIGS. 10 and 12, each of waveguides71and72has an inner region of substantially rectangular cross-section. The region is bounded at its sides by inner faces of spacers47and48. It is bounded below by the flat face of lower cylindrical mass40. It is bounded above by the flat lower end of upper cylindrical mass41(not shown in FIG.12). As illustrated inFIG. 10, each of waveguides71and72has an outer region that is bounded below by lower baffle17and above by upper baffle18, but is unbounded on its sides. Each waveguide effectively focuses acoustic energy onto an annular area on its side of the borehole wall with limited azimuthal dispersion by effect of the inner faces of spacers47and48, and with limited axial dispersion by effect of the lower and upper baffles17and18. Each waveguide channels acoustic energy that is radiated by dipole source16vibrating as illustrated inFIGS. 11A and 11B.

FIG. 10shows dipole source16coupled electrically to transmitter control electronics (not shown) via electrical wiring57and electrical wiring58(carried within flexible cable armor56). The electrical wiring passes through first electrical coupler61(shown in FIG.10), and the several connectors169, etc. (shown in FIG.2).

In the first preferred embodiment, each baffle has an annular shape and a cylindrical outer surface.FIG. 13Ais a plan view of lower baffle17of the preferred embodiment surrounding the structure ofFIG. 12, with dipole source16not shown.FIG. 13Ashows lower cylindrical mass40and first and second spacers47and48. First region73of lower baffle17corresponds to first-side waveguide71in FIG.10. Second region74of lower baffle17corresponds to second-side waveguide72in FIG.10. The diameter of lower baffle17is indicated by double arrow81.

A first alternative baffle shape is shown in FIG.13B.FIG. 13Bis a plan view of lower baffle65of the first alternative embodiment, including lower cylindrical mass40. First region75of lower baffle65corresponds to first-side waveguide71in FIG.10. Second region76of lower baffle65corresponds to second-side waveguide72in FIG.10. The effective diameter of lower baffle65is indicated by double arrow83.

A second alternative baffle shape is shown in FIG.13C.FIG. 13Cis a plan view of lower baffle66of the second alternative embodiment, including lower cylindrical mass40. First region77of lower baffle66corresponds to first-side waveguide71in FIG.10. Second region78of lower baffle66corresponds to second-side waveguide72in FIG.10. The effective diameter of baffle66is indicated by double arrow84.

Another alternative embodiment (not shown) of present invention includes a transmitter mount in accordance with U.S. Pat. No. 5,036,945 modified to add baffles as disclosed herein. U.S. Pat. No. 5,036,945, “Sonic Well Tool Transmitter Receiver Array including an Attenuation and Delay Apparatus”, issued Aug. 18, 1998, to Hoyle, et al., discloses a transmitter mount without baffles. In particular, see FIGS. 4, 4 C, and 4 D of U.S. Pat. No. 5,036,945.

FIG. 14shows a portion of an embodiment of quadrupole transmitter mount91partially enclosing quadrupole source90. When used with baffles in accordance with the invention, transmitter mount91may be seen as providing four waveguides, defined by the four spacers92-95, for use with a quadrupole source. Other multi-pole transmitter mounts may be constructed in similar manner.

FIG. 15is a perspective, partial cross-section illustration of a wireline tool having a dipole source and baffles in accordance with the invention. The main performance characteristic (amplification ratio) of a given design was found to be determined largely by three design dimensions. The three design dimensions are baffle thickness “T”; baffle diameter “D”; and edge to edge distance between the two baffles “L”, as illustrated in FIG.15.

Amplification ratio as a function of each of these dimensions was calculated using a finite difference code. The schematic illustration ofFIG. 15shows a dipole source16with baffles in accordance with the invention. Lower baffle17and upper baffle18are located within borehole wall12and earth formation14.

In the computer simulation used to produce the data graphed inFIGS. 17A-19B, the mathematical model for dipole source16represented a piezoelectric ring. The mathematical model for lower and upper baffles represented steel blocks. The mathematical model for the 8.5 inch diameter borehole12and formation14represented a set of formation physical parameters. The physical parameters were shear slowness (dts=600 μ second/foot), compressional slowness (dtc=160 μ second/foot), and mass density (ρ=2 gram/cm3). The central frequency of the excitation function was 2.5 KHz.

Experimental Results

The inventors modeled the structure described above, simulating acoustic excitation, and calculating parameters using Finite Differences Code to show the benefits of the invention. Experimental data based on computer simulation shows that using a pair of rigid heavy blocks above and below the dipole source of a sonic logging tool will result in a significant increase in the amplitude of borehole flexural signals reaching the tool receivers. The blocks and baffles serve as waveguides that focus acoustic energy from the dipole source onto opposite annular areas of the borehole surface. Without this structure, the dipole source radiates energy more broadly, so a significant proportion of the available energy is absorbed by the borehole fluid and by the tool body. The denser, the stiffer, and the larger the blocks and baffles, the larger the increase in amplitude at the receivers. Based on the numerical results, a six times amplification can be achieved with reasonably sized baffles (3 inch thick and 8 inch diameter) in an 8.5 inch diameter borehole in a slow formation (600 μs/ft shear slowness).

Effect of Block Thickness

To determine the effect of the block thickness, block diameter D was set equal to 8 inch (20 cm) and the distance L between the blocks was set to 3 cm (1.18 inch). Block thickness T of the blocks was gradually changed from 0 inch to 6 inch (0 to 15 cm), and dipole flexural waveforms at the first tool receiver location were calculated.FIG. 16Ais a graph showing amplification ratio as a function of the block thickness. Amplification ratio is defined as (peak value of pressure waveform with the blocks) divided by (peak value of pressure waveform without the blocks).FIG. 16Ashows that as the block thickness increased from 0 inch to 6 inch (0 to 15 cm), the amplitude ratio increased from 1 to 8.FIG. 16Bshows that there were no significant changes in the shape of the waveform. This suggests that the blocks do not alter the waveform quality to any significant degree.

Effect of Block Diameter

To determine the effect of the block diameter, block thickness T was set to 3 inch and the distance L between the blocks was set to 3 cm (1.18 inch). Block diameter D was changed gradually from 0 inch to 8 inch (0 to 20 cm) and dipole flexural waveforms were calculated.FIG. 17Ais a graph showing amplification ratio as a function of the block diameter.FIG. 17Ashows that as the block diameter increased from 0 inch to 8 inch, the amplitude ratio increased from 1 to 6.FIG. 17Bshows that there were no significant changes in the shape of the waveform. FromFIGS. 18A and 18B, it can be seen that amplification ratio is more sensitive to changes in block diameter than to changes in block thickness.

Effect of Distance between Blocks

To determine the effect of distance between blocks, block thickness T was set to 3 inch (7.6 cm) and block diameter D was set to 8 inch (20 cm). Block distance L was changed gradually from 1.18 inch to 6.7 inch (3 cm to 17 cm) and dipole flexural waveforms were calculated.FIG. 18Ais a graph showing amplification ratio as a function of the distance between blocks.FIG. 18Ashows that as the distance between blocks increased from 1.18 inch to 6.7 inch, the amplitude ratio decreased from 6 to 1.7. The waveforms are shown in FIG.18B.