JPH1062555A - Method and apparatus for bored hole sound wave reflection detection layer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reflected sound wave detection layer art which enables measurement in a wide range of stratum. SOLUTION: A method is provided to form an image of a stratum on the perimeter of a bored hole 14. The diameter of the bored hole is determined, a sound wave attenuation characteristic of the stratum is determined and the range of interest is determined in the prospecting depth into the stratus from the bored hole. A tool 10 is positioned in the bored hole, a sound wave signal is transmitted into the stratum from the tool and the sound wave signal reflected on a construction in the stratum is received by the tool. The received sound wave signal is analyzed to determine the distance of the reflecting construction from the bored hole while an image of the position of the reflecting construction is generated pertaining to the bored hole based on the determined distance. teps as mentioned above are arranged. This tool has at least one monopole transmitter 18 and at least one sound wave receiver 20 isolated from the transmitter by a distance selected according to the range of interest of the prospecting depth. The frequency of the sound wave signal is selected according to the diameter of the bored hole, the sound wave attenuation characteristic of the stratum and the range of interest of the prospecting depth.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to a method for use in borehole logging including detecting sound wave reflectors in a formation surrounding the borehole using sound wave signals generated and received in the borehole. It concerns the device.

[0002]

2. Description of the Prior Art Sonic techniques for characterizing formations are known. All of these techniques involve transmitting acoustic signals from a source through a formation of interest to a receiver. The frequency of the signal can change from a very low frequency in seismic applications to an ultrasonic frequency via an acoustic frequency, depending on the particular technology used.
Most borehole logging provides a time measurement for the acoustic signal passing from the source to the receiver substantially directly through the formation in the wall of the borehole. Often, this measurement is designed to record these direct arrivals and to eliminate signals based on reflection. on the other hand,
In some cases, a received signal is filtered to remove a signal caused by reflection.

[0003] Seismic surveys generally use reflected acoustic signals at very low frequencies to detect structures below the ground surface. Signal sources and / or detectors are typically located at the surface. Borehole seismic technology places one of these inside the borehole.

[0004] In the borehole acoustic reflection survey, measurement is performed by an acoustic wave transmitter and a receiver arranged in the same borehole. This configuration is best suited for recording the reflected wave field from a sound reflector having a small angle with respect to the borehole axis. For example, near vertical cuts, faults and salt dome sides are good borehole reflection targets for near vertical wells.
Nearly horizontal floor boundaries, fluid contact interfaces (gas / petroleum or petroleum / water) and ore strips inside the reservoir are highly remote and good targets for horizontal wells. The desired event (reflection) in the borehole reflection survey is a wave propagating from the borehole to the reflector in the formation and returning to the borehole. However, a significant portion of the sonic energy from the transmitter propagates directly to the receiver array. These direct waves include compressive and shearing headwaves (head waves), tube waves (Stoneley waves), fluid and borehole modes, and various casing modes if wells are provided with casing. I have. Direct waves may also include various tool modes that propagate along the tool body. In sonic logging applications, some of these direct waves are used to log formation properties, such as head and Stoneley waves. However, in reflection probing applications, they are undesirable. These direct waves are typically significantly larger than the reflected waves.

[0005] Reflection imaging around wells using downhole acoustic measurements has been previously proposed, especially for use in horizontal wells to determine the location of the top or bottom of the reservoir relative to the borehole. I was U.S. Pat. No. 4,833,658 discloses a method of processing a signal to determine the position of a reflector around a horizontal well. FIG. 1 shows the system described in the aforementioned U.S. Pat. The tool (device) 5 includes a drill string 30 connected to a drill derrick 31 on the ground surface.
Transmitters E ₁ -E ₄ and 12 receivers R _1- located in the horizontal borehole 1 at the ends of
It has a R _12. The transmitters are spaced apart from one another at regular intervals, for example, equal to 0.25 m, and the receivers are spaced apart from one another at regular intervals, for example, 1 m. The closest separation between transmitter and receiver is 1
m. The length of the tool is at most the borehole and the furthest reflective structure of interest (Interface 1).
3, 15). Tool 5 is logged along borehole 1 and transmitter E generates an acoustic signal at a frequency in the range of 5,000 to 10,000 Hz, and receiver R receives both direct and reflected waves. . The received signal is analyzed to determine the average velocity of the direct (diffraction) and reflected waves. A time section is determined from the signal corresponding to the reflected wave, and this time section is converted to a depth section by the average propagation velocity of the reflected wave. The position of the reflective interface is determined from this depth section. While the approaches and parameters proposed in the '658 patent work in some cases,
Many do not work. The '658 patent is not aware of this and does not suggest any appropriate modifications to the technology that would allow it to operate in all situations.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and makes it possible to perform a measurement over a wider geological area without the above-mentioned problems being serious. An object of the present invention is to provide a reflection acoustic logging technique.

[0007]

SUMMARY OF THE INVENTION A method of forming an image of a formation around a borehole according to the present invention comprises determining the diameter of the borehole, determining the acoustic attenuation characteristics of the formation, and determining the depth of the search from the borehole into the formation. Determine the area of interest for
Positioning a tool in the borehole comprising at least one monopolar transmitter and at least one acoustic receiver spaced from the transmitter by a distance selected according to the range of interest of the search depth; Transmitting at least one monopolar transmitter into the formation at a frequency selected according to the diameter of the borehole, the sound attenuation characteristics of the formation, and the range of interest of the exploration depth; A sound signal reflected from the structure is received by at least one receiver, the received sound signal is analyzed to determine a distance of the reflecting structure from the borehole, and a reflecting structure for the borehole based on the determined distance. Each of the above steps for generating an image at the position of.

An apparatus according to the present invention for forming an image of a formation around a borehole includes a tool body, a borehole diameter,
At least one monopolar transmitter for transmitting an acoustic signal into the formation at a frequency selected according to the predetermined sound attenuation characteristics of the formation and the range of interest for the exploration depth and located on the tool body; The sound signal reflected from structures in the formation, located on the tool body and spaced from the at least one transmitter by a distance selected according to the range of interest of the search depth. At least one receiving acoustic wave receiver, means for analyzing the received acoustic wave signal to determine a distance of the reflective structure from the borehole, and generating an image of the position of the reflective structure with respect to the borehole based on the determined distance. Means.

Configuring and operating the tool to optimize performance over a range of conditions not previously possible by taking into account the damping properties of the formation, the effect of borehole diameter and the desired probing depth. Is possible.

[0010] One particularly preferred mode of use of the present invention is in the case of forming an image of a formation around the borehole when the borehole is horizontal. The reflected signal can be analyzed to identify the location of the reflective structure, for example, a cutoff in the formation relative to the reservoir boundary, salt dome or borehole, and that location is used to characterize the formation. It can be represented as an image that can be performed.

It is preferred to use an array of one or more transmitters and a plurality of receivers. In some circumstances it is possible to use a single transmitter, but the search depth range is often limited. Therefore, it is preferable to use two or more transmitters to provide sufficient range in the spacing between the transmitter and receiver to provide a good search depth range. Two or three transmitters seem to provide the best compromise between range of borehole tool and physical and operational conditions. The axial receiver array provides increased reflected event amplitude with respect to the tube wave. A typical receiver array has eight axial receiver stations, each station separated by an equal distance, for example, 6 inches. The azimuthal receiver array allows for determining the azimuthal position of a reflective object in the formation with respect to the borehole. Preferably, four hydrophones are arranged around the tool axis at each receiver station. By recording the waveform at each hydrophone at each station, it is possible to compare the signals and to determine from which direction the reflection came.

The frequency of the transmitted sound signal is selected according to the borehole diameter, the properties of the formation and the desired search range. Frequencies can range from 100 Hz or less for long-range imaging in attenuated media to 20 kHz or more for short-range high-resolution imaging in unattenuated media. For a typical monopolar source suitable for the present invention, the frequency is typically between 1 kHz and 15 kHz.
Is within the range.

A portion of the direct tube wave signal propagates through the receiver (or transmitter) and is reflected from structures inside the borehole back to the receiver to prevent interference with the reflected signal from the formation. In order to achieve this, it is desirable to use one or more attenuators located within the tool string. These are preferably of the form described in co-pending US patent application Ser. No. 08 / 527,736, which is incorporated herein by reference.

[0014]

FIG. 2 is a schematic view showing a borehole acoustic reflection image forming system according to an embodiment of the present invention. The acoustic reflection image forming tool (apparatus) 10 is lowered into the borehole 14 by the outer multi-conductor cable 12, and the borehole 14 may or may not be provided with a casing. A sound wave measurement for forming an image of the formation 16 is performed. If the borehole is deviated from vertical, especially if it extends horizontally, if the gravity is insufficient or impossible to move the tool to the depth of interest The tool 10 can be pushed into the borehole by running the cable 12 through tubing such as a drill pipe or coil tubing. Tool 10 is provided with transmitters 18a, 18b, 18c and a receiver array 20 immediately adjacent thereto, which are disclosed in U.S. Patent Nos. 4,850,450, 5,036,945, and 5,043. , 952, which patents are incorporated herein by reference. The separation between the transmitter and the receiver array is described, for example, in US Pat.
This is accomplished by using a standoff 19 that incorporates an acoustic isolation joint as described in US Pat. No. 6,945. The length of the spacer 19 is selected according to the exploration depth range of interest. For example, a typical length of the standoff 19 is 32 feet (9.75 m) and the overall distance from the furthest transmitter 18a to the furthest position of the receiver array 20 is 43 feet (13.1 m). It is. One or more tube wave attenuators 22 can be provided at each end of the transmitter / receiver array section to reduce the interference effects of the reflective tube waves in the borehole. These attenuators and their functions are described in detail in co-pending US patent application Ser. No. 08 / 527,736, which is incorporated herein by reference. Such an attenuator can be used between the transmitter and the attenuator array, if desired. The downhole tool is completed by providing an orientation device (for example, a Schlumberger GPIT tool including a magnetometer and an accelerometer) 23 and a telemetry cartridge 24.

The tool 10 is adapted to move the borehole 14 up and down, and as the tool 10 moves,
The transmitter 18 periodically generates a sound wave signal. The generated acoustic signal propagates through the borehole and / or through the formation, where it is reflected by underground structures and the receiver in the receiver array 20 typically receives whatever is derived from the generated signal. The energy of is detected. The mechanism for moving the tool 10 within the borehole includes a cable 12 that extends to a pulley wheel 25 at the surface of the formation and then to a suitable drum and winch mechanism 26, which drum and winch mechanism 26 is desired. The tool 10 is moved up and down in the borehole in accordance with the above. Transmitter array 18 and receiver array 20
The electrical connection between the ground and the surface equipment is made through a multi-element slip ring and brush contact assembly 28 associated with the drum and winch mechanism 26. Unit 30 includes tool control and pre-processing circuitry that sends electrical signals to tool 10 and then receives other electrical signals (acoustic logs) via cable 12 and assembly 28. Unit 30 cooperates with a depth recorder 32 which derives a depth level signal from a depth measurement wheel 34 to correlate signals from receiver array 20 with respective depth levels in borehole 14.
The output of the receiver array 20, after optional pre-processing in the unit 30, is sent to a signal storage 36, which further associates the acoustic signal output with a respective depth level in the borehole 14. It is possible to receive signals from or through the depth recorder 32 for this purpose. The storage 36 can store the output of the receiver array 20 in the form of digital acoustic log measurements. The storage unit 36 may include a magnetic recording device such as a disk or a tape, and / or other storage media such as a semiconductor or an equivalent memory circuit.
Next, the stored digital data is processed to print the image of the underground stratum around the borehole as a print image or VDU3.
8 can be given as a display. An image of the reflective structure around the borehole is derived using Kirchhoff-type migration of the data as commonly used in seismic processing.

Each transmitter 18a, 18b, 18c has a monopolar source substantially as described in US Pat. No. 5,043,952 (incorporated herein by reference) and shown in FIG. doing. These are substantially the same as the monopolar sources used in conventional sonic logging applications. The source has a piezoceramic cylinder 40 held in an oil-filled cavity 42 defined by a corrugated container 44, the corrugation of the container 44 having a mud outside the corrugated container. And the oil inside the corrugated container allows a differential volume change. A power amplifier is attached to the piezoceramic cylinder 40 via electrodes 46 to poll the cylinder radially. Electrodes 46 are attached to the inner and outer surfaces of the piezoceramic cylinder 40 for applying a voltage to the cylinder,
The voltage expands the length and radius of the cylinder, whereby a volume expansion causes a compression wave to be generated and propagated into the borehole, and both the compression wave and the shear wave are propagated into the formation.

In order to radiate sound wave energy sufficiently,
The transmitter 18 is designed to operate at a near geometric resonance that inevitably rings for a long time. A damping mechanism has been introduced to stop this ringing, which has a rubber tungsten backing material 48 for the sonic signal source and provides good impedance matching and damping for the configuration to which it is attached. And have given. The rubber tungsten backing further prevents additional fluid mode excitation when the transducer is immersed in a liquid. The rubber tungsten composite has a butyl rubber skeleton filled with tungsten powder. The impedance and damping of the backing depends on the percentage of tungsten, the degree of compaction, the degree of vulcanization of the powder and the degree of adhesion of the rubber to the powder.

Although three transmitters are shown in FIG. 2, it should be understood that the present invention can be implemented with more or fewer transmitters. For example, two transmitters can provide a search depth range on the order of 30 feet (9.14 m), which may be appropriate in certain circumstances. The spacing between the transmitters has been chosen to achieve a wide range of search depths. Transmitter 18a
The distance between transmitters 18b and 18b is about 4 feet (1.22 m), and the distance between transmitters 18b and 18c is about 3 feet 6 inches (1.07 m). The considerations for selecting the best interval for a given environment are described in more detail below. A hydrophone is located at transmitter 18c for the purpose of monitoring the source output. The hydrophone can optionally be used as a short-spaced receiver. The spacing between the transmitter 18c and the receiver can be increased to increase the search depth. On the other hand, the spacing between the transmitters can be increased to provide multiple search depths.

The receiver array 20 has eight receiver stations that are vertically spaced 6 inches (15.24 cm) apart, with each station having 9 stations around the tool.
It has four hydrophones arranged in the circumferential direction at an interval of 0 degrees, and a total of 32 hydrophones are provided. This array is disclosed in U.S. Pat. No. 5,036,945.
This is described in more detail in The signals detected at each hydrophone are recorded separately and the signals received by the array are analyzed to provide the direction and distance of the reflective structure from the borehole.

The orientation device 23 is used to position and orient the tool in the borehole (orientation state).
It is possible to determine. It is therefore possible to determine from which direction the reflected wave arrives and whether the reflector is above or below a borehole in a horizontally extending well, or vertically. It is possible to determine the direction of the well relative to the borehole.

The imaging area (window) of the reflector away from the borehole is limited for several reasons. First, in most cases, this window (FIG. 4)
(Indicated by region X in FIG. 2) is terminated by a shearing head wave and a tube wave which are typically significantly larger than the reflected compression wave. In the case of a reflector parallel to the borehole axis, the arrival time of the compression wave reflection is approximately given by:

[0022]

(Equation 1)

Note that range is the distance of the reflector from the borehole, offset is the transmitter-receiver offset, and ν _p is the compression wave velocity. The direct shear wave arrival time that defines the end of the imaging window is given by:

[0024]

(Equation 2)

The maximum distance from a borehole in which an image can be formed by a large offset approach is given by the above equation (1).
And (2) are equalized and solved for range, which is given by:

[0026]

(Equation 3)

For example, a typical value ν _p / ν _s =
If 1.73, the maximum range is range _max ≒
Given by 0.7 offset. In a more general form, ν _{s in} the above equation represents the velocity of the first dominant direct wave following the compression headwave. Furthermore,
There is also a minimum range limit. Reflections from very close reflectors may be masked by direct wave ringing. The duration of this ringing is inversely proportional to the bandwidth of the direct wave, which is highly dependent on acquisition parameters such as, for example, the spectral output of the transmitter. Resonant frequencies of the interfering direct waves should be avoided to reduce the ringing period. For example, if the compression speed is 10,000 f
For a formation of t / sec (100 μsec / ft), the minimum range is about 5 feet for a 1 kHz bandwidth and 10 feet for a 500 Hz bandwidth.

A second reason for range limitation is the rapid decrease in reflected wave amplitude due to geometric spread and attenuation. The decrease in amplitude due to geometric diffusion is approximately proportional to the total distance propagated by the reflection event or its arrival time.
The reduction in amplitude at large offsets due to attenuation may be more pronounced than geometric diffusion. The dependence of the reflected wave amplitude on attenuation, frequency and distance is given by:

[0029]

(Equation 4)

Note that f is a frequency, d is a reflected light beam path, and Q is a numerical value representing the attenuation characteristic of the medium. The Q value can vary significantly, from 5 for very damping media to 100 for ideally non-damping media. The above equation shows that the amplitude decrease due to attenuation increases exponentially with respect to the propagation path length of the reflection event, which increases with the offset. The frequencies used can range from 100 Hz (or less) for long-range imaging in attenuating media to 20 kHz (or more) for short-range high-resolution imaging in non-attenuating media.

The backbone of each pulse from the source and the number of measurements made at the receiver array for each pulse depend on the ability of the system to record the data. Using multiple pulses and recording data from different sets of hydrophones for each pulse, stacking the data to ensure that measurements are made for all transmitter and receiver combinations in the array It may be necessary or desirable to do so. This can be done while moving the tool through the borehole at normal logging speed.

The signals received by each of the hydrophones are processed in essentially the same manner as the signals received by the hydrophones in the seismic array, and an image is generated using the same techniques. Thus, images can be generated azimuthally along and around the borehole axis (depth axis). By recording the complete waveform at each azimuthal hydrophone location, it is possible to determine the time at which a given reflected wave is detected at each location, and thus from which direction it arrived . For example, in a horizontal borehole, a reflection is first detected by the uppermost hydrophone, then, if detected in a lower hydrophone, the reflective structure is above the borehole. And vice versa, the reflector can be identified as being below the borehole.

The above-described apparatus for performing reflection measurements in boreholes may be able to be used to obtain conventional diffraction data to evaluate formation properties in a known manner, in which case, for example, It may be possible to determine formation parameters using measurements deemed unnecessary for the present invention, such as headwave measurements. In addition, the general approach described above requires that the wells be well-closed and that the cross wells be well-defined.
Application to image formation is also possible.

The present invention is applicable to both wire line and drilling logging applications. In an LWD application, a portion of the bottom hole assembly above the drill bit is formed in the manner of the previously proposed LWD sonic logging tool.

Although the specific embodiments of the present invention have been described in detail above, the present invention should not be limited to only these specific examples, but may be variously modified without departing from the technical scope of the present invention. Of course is possible.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a conventional borehole reflection image forming system.

FIG. 2 is a schematic diagram showing a borehole reflection image forming system according to the present invention.

FIG. 3 is a schematic sectional view showing a monopolar supply source usable in the present invention.

FIG. 4 is a graph showing arrival at a head wave which limits detection of a reflected signal.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Sound reflection image forming tool 12 Cable 14 Borehole 16 Formation 18 Transmitter 19 Separator 20 Receiver array 22 Attenuator 23 Orientation device 24 Telemetry cartridge 30 Tool control and preprocessing circuit 32 Depth recorder 36 Signal storage unit 38 VDU ( Video display unit)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location G01V 1/28 G01V 1/28

Claims (20)

[Claims] 1. A method of forming an image of a formation around a borehole, comprising: (a) determining the diameter of the borehole; (b) determining the acoustic attenuation characteristics of the formation; and (c) determining the sound attenuation characteristics of the formation from the borehole. Determining an area of interest for the search depth; (d) positioning a tool in the borehole, wherein the tool comprises at least one monopolar transmitter and according to the area of interest for the search depth. At least one acoustic receiver spaced from the transmitter by a selected distance; and (e) the diameter of the borehole, the acoustic attenuation characteristics of the formation by the at least one monopolar transmitter. And transmitting an acoustic signal into the formation at a frequency selected according to the range of interest for the exploration depth; and (f) acoustic signals reflected from structures in the formation. (G) analyzing the received sound wave signal to determine a distance of the reflective structure from the borehole; and (h) a borehole based on the determined distance. Generating an image of the location of the reflective structure with respect to the method. 2. The method of claim 1, wherein the frequency is selected to avoid a resonance frequency of a head wave traveling from the transmitter to the receiver. 3. The method of claim 1, wherein the frequency is selected to optimize coupling of the reflected acoustic signal into the borehole. 4. The method of claim 1, wherein sound wave signals of more than one frequency are transmitted and detected. 5. The method of claim 1, wherein a plurality of transmitters are separated from the at least one receiver by different distances to transmit sound signals to increase a search depth range. Method. 6. The method of claim 1, wherein the acoustic signal reflected from an array of a plurality of receivers is received. 7. The method of claim 1, wherein reflected signals are received at a plurality of azimuthal locations around the tool. 8. The method of claim 7, further comprising analyzing the received signal to determine a direction of the reflective structure from the borehole. 9. The method of claim 7, wherein a complete waveform is recorded at each azimuthal position. 10. An apparatus for forming an image of a formation around a borehole, wherein: (a) a tool body; (b) a borehole diameter, predetermined sound attenuation characteristics of the formation and an exploration depth selected according to a range of interest. At least one monopolar transmitter located on the tool body for transmitting an acoustic signal into the formation at a frequency; (c) a distance selected according to a range of interest of the search depth. At least one sonic receiver spaced apart from the at least one transmitter and located on the tool body for receiving sonic signals reflected from structures in the formation; and (d) the borehole. Means for analyzing the received acoustic signal to determine a distance of the reflective structure from the e.g., (e) with respect to the borehole based on the determined distance. Means for generating an image of the position of the reflective structure, device characterized in that it comprises a. 11. Apparatus according to claim 10, wherein said acoustic receiver comprises an axial array of a plurality of hydrophones. 12. The apparatus of claim 10, wherein said acoustic receiver has a radial array of a plurality of hydrophones. 13. The apparatus of claim 10, wherein at least two axially spaced transmitters are provided on the tool body. 14. The apparatus according to claim 10, wherein the frequency of the sound wave signal is in a range of 1 kHz to 15 kHz. 15. The apparatus of claim 12, wherein the radial array comprises a plurality of hydrophones disposed at 90 degree intervals around the tool body. 16. The apparatus of claim 15, wherein each hydrophone records a substantially complete waveform. 17. The apparatus of claim 16, wherein the means for analyzing the received acoustic signal analyzes a waveform from each hydrophone to determine a direction of the reflective structure from the borehole. . 18. The apparatus of claim 10, further comprising one sonic receiver located at the at least one transmitter. 19. The apparatus of claim 11, wherein said axial array comprises eight receiver stations that are substantially equidistantly spaced. 20. The apparatus of claim 19, wherein each receiver station has four hydrophones disposed at 90 degree intervals around the tool body.