JPH05134050A - Vertical-well type apparatus for seismic prospecting - Google Patents

Abstract

PURPOSE: To generate an earthquake wave having less noise by fixing an earthquake generation source in a bore hole or casing for suppressing occurrence of tube wave. CONSTITUTION: A generator 34 comprising a housing 40 is, through a connector 42, suspended with a multi-core wire cable 36, and lowered to a desired depth in an earthquake generation source shaft, and then press-contacted to a side wall 48 with expansion leg members 44 and 46. Bender assemblies 54 and 56 are fixed to a pair of horizontal beam members 50 and 51 in the housing 40, and a mass body 58 is connected so as to clamp the center area. The bender bar has a laminated structure wherein a ceramics piezoelectric crystal body is stuck on both sides of a thin metal plate. Under the condition, when a supply source is excited to deflex a bender assembly, the inertia of the mass body 58 resists a movement of the bender bars 54 and 56, and an occurring force is transferred underground through the housing 40, so that a pressure wave P which moves horizontally and an orthogonal shear wave S are generated. The earthquake wave is detected with a receiver placed is a sensing bore hole.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for carrying out vertical well seismic survey, and more particularly to a downhole seismic source using piezoelectric means and a mass body for generating seismic signals. is there.

[0002]

2. Description of the Related Art Many methods and apparatuses for carrying out seismic surveys from the surface and also from the surface of the water are well known in the art. Basically, these known exploration techniques generate seismic signals by generating one signal each time an explosion, a sudden release of compressed gas, or a large mass object is dropped on the ground at high speed, or a constant seismic signal is generated. A first ground surface working unit that generates a seismic signal by a mechanical vibrator that sends a signal generated by continuously sweeping a range, and at least one listening sound that senses and records the seismic wave generated and reflected by the first working unit Alternatively, a second working unit including a vibration receiving device is provided.
The second working unit is provided separately from the first working unit. A geological profile of the area is formed by processing the recorded data sent from the at least one second work station. All of these known methods and devices are in widespread use and are to some extent effective. However, since the upper surface layer of the earth has been weathered for many years, it acts to considerably attenuate the high frequency signal generated from the ground surface. This attenuation makes it almost impossible to obtain resolution for anomalies of less than 30 meters when the signal reaches many oil and gas layer depths. Therefore, the conventional surface type seismic survey can search a very wide area, but its resolution is limited.

The technique known as Sakui logging enables very accurate measurements of underground structures, but this device is limited in the distance from the hole that can be explored. Given that the distance is typically only a few centimeters and wells are not tens of meters long, they are typically drilled at intervals of tens of meters, so this type of seismic exploration is the It is quite clear to overlook potential value.

The newly developed technique is known as vertical well seismic survey, in which the seismic source is installed in the main well and the geophone is placed in at least one auxiliary well near the first well. To do. By processing the data recorded in the auxiliary wells, high resolution images of the entire area between the wells could be formed. In vertical well seismic exploration, if both the seismic source and the geophone are lowered into the ground, the ability to pass high frequency seismic signals is significantly improved, especially when the seismic source is fixed to the well wall. A vertical well seismic survey can provide a resolution of about 3 meters.

Since both the borehole seismic source and borehole geophone assembly operate in the same environment as a well logging tool,
They must share the same physical characteristics. Therefore, each is sealed in a container and suspended by a cable or wire, generally into a fluid-filled borehole,
It is necessary to be able to lower to a considerable depth where temperature and pressure extremes appear. There is also a limit to the ability of the multicore cable to communicate between the tool in the hole and the ground surface. It is desirable to control the seismic source by a borehole computer with instructions stored in memory and to provide the geophone assembly with borehole recording capabilities.

At the present time, there are only a few drilling earthquake sources on the market. One of them is a perforation vibrator operated by hydraulic pressure supplied from the ground surface through a coil tube. The tool is approximately 37 meters long and produces a strong sinusoidal signal in the fluid in the borehole. The frequency of this signal varies from 20 to 120 Hertz. Another piercing seismic source is an air gun, much like the known underwater air gun. It is powered by compressed air supplied from the ground surface via an umbilical cable containing rubber hoses. This device provides a high pressure, localized air jet to the fluid in the borehole.

[0007]

SUMMARY OF THE INVENTION The present invention provides an excellent seismic signal in the ground to significantly suppress the generation of tube waves, and is suspended by an industrial standard multi-core wire cable for air or liquid. It is an object of the present invention to provide a vibrating seismic source that can be operated in a hole containing any of the above.

[0008]

The present invention differs from the above-described conventional mechanical devices in that it is an electronic mechanism. The present invention comprises at least one piezoelectric bender bar assembly that can be excited from the ground by a high power amplifier. This device is somewhat similar to a Navy sonar transducer and can operate with standard multi-wire cable. For example, a bender bar assembly can be constructed with a set of ceramic piezoelectric crystals attached to both sides of a thin metal plate, which is typically 7.6 cm wide and 61 cm long. The crystals of both sets are simultaneously excited by the application of high pressure of opposite polarity, whereby one crystal expands while the other crystal contracts. As a result, the bar is fixed at both ends so that the assembly bends in the central part. Although there are several resonance points, the movement of the bender bar almost follows the waveform of the applied electric signal.

When a piezoelectric crystal bender bar assembly is used as the source of a perforation earthquake, the assembly is preferably encapsulated within an oil filled flexible tube. The pressure wave generated by the movement of the bender bar is transmitted through the flexible tube and further through the drilling fluid. Part of the energy is transferred from the drilling fluid to the ground through the metal well casing. The seismic signal emitted from the borehole is
It is the pressure or P wave that moves in the horizontal direction. Although not fully understood, radiation patterns in the ground are believed to be nominally symmetrical about the axis of the well bore.

The seismic wave generated by the present invention has pressure (P) wave and shear (S) wave components. Since the traveling speed of the S wave is only about half that of the P wave, the P wave arrives at the geophone first. S waves are affected only by the rock structure and are not affected by liquid contact, whereas P waves are affected by liquid contact. The ratio of P waves to S waves can be used to predict the porosity of the rock through which the waves pass.
It has been reported that conventional devices can transmit seismic signals up to 3,000 Hertz through limestone between decorative wells 460 meters apart.

In the case of an earthquake source that transmits energy to the ground by pressurizing the drilling fluid, most of the energy (90-95%) remains trapped inside the drilling fluid in the form of a tube wave, This tube wave is a pressure wave that vertically moves vertically along the length of the fluid-filled casing.
Not only is this inefficient, but tube waves are a central cause of unwanted signals in the form of background noise. The tube wave is strongly reflected from the bottom of the hole and the top of the liquid surface, and to a lesser extent also due to anomalies in the casing. Each of these points transfers a portion of the tube wave energy into the ground, which is functionally another point of seismic origin. The seismic source of the present invention includes at least one bender bar assembly enclosed within a liquid tight steel housing similar to that used in well logging tools. Both ends of the bender bar assembly are secured within the housing and provided with means such as telescopic arms, pistons or electromagnets to secure the tool housing or sonde to the well casing or wall.
Preferably, the mass is coupled to the center of the bender bar assembly so that the inertia of the mass adds resistance to movement of the bender bar assembly. The force generated in this way is applied to the well wall through the tool housing,
Further transmitted to the ground. At least one geophone down in the second well receives and records the seismic signal for subsequent analysis.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A typical ground-based seismic survey system is shown in FIG. 1, in which a signal 12 generated by an earthquake source 10 is reflected by an anomalous portion 14 under the ground and connected to a recording means 20, respectively. The reflected wave 16 is detected by the listening device or sensor 18. Although this method has generally been used at an acceptable level, it has some major drawbacks. One problem is that it damps the high frequency seismic signals generated by the seismic source 10, since the topmost layer, here designated 22 is a formation that has been substantially destroyed by weathering. Also, by nature, this type of exploration generally cannot detect anomalies smaller than 30 meters.

FIG. 2 shows a drill well logging tool or sonde 2.
4 shows a typical well logging device being lowered into a well 26 by a multi-conductor wire 28. This type of tool uses one of a variety of methods to inspect underground structures through which a well passes. A major problem with this structure is that the typical well logging tool 24 can detect only a very limited distance from the well, and the dotted line 3
Generally in the centimeter range, as indicated by 0,
It is a decimeter level at most, and without the above-mentioned ground-based seismic survey, a large area between adjacent wells will remain unexplored. In this case, the logging 14 would not even detect the abnormal portion 14.

3 and 4 show the device of the present invention as used in a typical field. Multiple perforations 2 on site
6, 32 have been excavated beforehand. The holes 26 are called main or earthquake source holes and the other holes 32 are called auxiliary or sensing holes. Earthquake source 34 is wire and wire 36
Is lowered to the first depth in the earthquake source well 26 and fixed to the side of the drill hole. Next, at least one vibration sensor 38 is lowered into each vibration well 32 to a first predetermined depth, and seismic survey is started. After each probe, the depth of the sensor 38 is changed and the next probe is performed. This step is repeated while changing the depth of the earthquake source 34 and moving the geophone 38 to the same depth. For example, as shown in FIG. 4, it should be noted that the choice of seismic source wells and auxiliary wells is not critical to the operation of the invention, but is important in terms of the area to be explored. I want to.

The earthquake generator 34 according to the present invention is shown in FIGS.
In detail. The generator 34 is an elongated, generally cylindrical housing 4 that is typical of drilling tools and sondes.
0, one end of which is attached to the multi-core wire cable 36 by a connector 42. This cable, which has been used for a long time, supports a tool and transmits a signal to the ground. On the outside of this conventional housing 40, means 4 for fitting the tool housing 40 to the side wall 48 of a hole or casing (not shown).
4, 46 are provided. Securing the tool in this manner can be accomplished by one of many known means, such as the telescopic leg members 44, 46 shown. Electromechanical, electromagnetic, pneumatic or hydraulic devices may actuate the illustrated leg members or other fastening means not shown within the spirit or essential characteristics of the invention. Suitable fastening means are described in US Pat. No. 4,757,873, the disclosure of which is incorporated herein by reference.

Referring now to FIG. 6, the seismic generator 34 is shown in a vertical cross-section through the housing 40, with a pair of transverse beam members 50, 52 being spaced apart and parallel to each other. .. A pair of bender bar assemblies 54,
56 is fixed to the cross beam members 50 and 52 at each end, and the mass body 58 is connected so as to be sandwiched near the centers of the bender bar assemblies 54 and 56. A detailed cross section of an exemplary bender bar assembly is shown in FIG. Preferably, each bender bar assembly is a laminated structure having ceramic piezoelectric crystals 62, 64 attached to opposite sides of a thin metal plate 60 approximately 7.6 cm wide and 61 cm long. The mass 58 is shown as a block for convenience. The mass may be provided around the bender bar assembly or may be a heavy or dense liquid, such as mercury, in a sealed elastic container (not shown) between the bars. Note that although a pair of bender bar assemblies is shown, only one bender bar assembly can work.

The geophone assembly 38 (FIG. 3) will not be described in detail as suitable equipment is commercially available. For example, the Haliburton physical unit (Geo
The physical systems) SWC30 type geophone has three geophones in the housing, two geophones in the geophone are oriented in the horizontal direction, and one geophone is oriented in the vertical direction. As another geophone, an accelerometer that can be fixed to the wall of the hole in the same manner as the earthquake source of the present invention can be used. A hydrophone can also be used as a geophone. One or more hydrophones are suspended in the fluid of the auxiliary well to allow tube waves to be sensed. Although they are omnidirectional in nature, since the moving speed of the shear wave is about half that of the pressure wave, it is possible to distinguish between the pressure wave and the shear wave by their arrival time.

In operation, the seismic source 34 is lowered to a desired depth in the seismic source well 26 and fixed to the wall of the hole. Next, a source (not shown) is excited to flex the bender arm assembly. Mass 5
An inertia of 8 resists the movement of vane bar 54, 56. The force generated by this movement is the tool housing 40
(And a well casing (not shown)). The resulting seismic waves are pressure waves (P) that move horizontally (in both directions) along the bender bar axis. The horizontally polarized shear wave (S) moves horizontally in the direction orthogonal to the P wave,
As schematically shown in FIG. 8, the P wave is shown by the solid line and the S wave is shown by the dotted line in this figure. The vendor bar shall be moved to the left and right of the drawing sheet.

The present invention can operate without problems with either liquid-filled or dry holes as long as it can withstand the temperature and pressure conditions faced by the seismic source and geophone. In the present invention, even if the hole is filled with the liquid, since the earthquake source is fixed to the casing, many pipe waves are not generated. In some conventional devices, noise generated by tube waves was a problem.

Assigned to the same assignee as the present invention,
The earthquakes of the present invention taught by US patent application Ser. No. 07 / 409,907, filed Sep. 20, 1989, the disclosure of which is incorporated herein by reference and entitled "Coding of Earthquake Sources". The source can also be pulsed in code form. The seismic source pulsing of the present invention may require the use of a swept signal as used in vibroseis due to the low energy output of the bender bar assembly. There are many different types of these signals, but the central feature of all is that they are low power signals that are continuously applied over a relatively long period of time. The geophone signal has the effect of cross-correlating with the sweep, changing the energy into one pulse. Note that a sweep with good autocorrelation can be formed by a coded pulse train or continuous function.

One of the major problems in collecting drilling seismic data is the need to reposition geophones at various depths in the auxiliary well. If a number of geophones are simultaneously provided from the same wire and wire, productivity can be improved. One of the practical problems in doing this is sending large amounts of data to the ground surface over standard 7-core wire. Another issue is the amount of electronics that must be placed in a poor environment. However, none of these are insurmountable with commercially available well logging tools.

The "sign bit" method used for surface seismic data provides a method that can be used to record the drilling data generated by the present invention. Current processing techniques for well-well seismic data mainly use only the first-arriving waveforms of P and S waves. Unlike surface seismic data, the entire waveform is no longer used. This makes the sign bit technique more suitable for drilling data than surface data. Drilling seismic data is often contaminated with random noise from nearby generation work. This type of problem can be better handled with sign bit technology than with full precision equipment.

Most seismic devices digitize analog seismic signals with high precision by means of instantaneous floating point amplifiers and 16-bit analog-to-digital converters, but the sign bit method only signals signals with 1 bit (sign bit) accuracy. To measure. This can significantly reduce the amount of electronics required and the number of bits sent.

The sign bit method is not effective when a strong shock source having a high signal-to-noise ratio such as dynamite is used. The sign bit technique is only used for vibrosis sources where the relatively low source energy is spread over time and the signal noise ratio (before correlation) is low. The signal to noise ratio and the number of bits of resolution are significantly improved after correlation.

If the signal-to-noise ratio is less than 1 (before correlation), the sign bit technique can prove to be comparable to a perfect precision device. When the signal-noise ratio is improved to 1 or more, it is advantageous to increase the number of precision bits. The sign bit method is very effective for processing random noise. It is often superior to full precision instruments in the presence of high amplitude transient bursts that are typical of cultural noise. It does not work well with high coherent noise.

As a modification of the invention, it would be advantageous if either a pressure wave or a shear wave could be generated in a given direction. This is achieved by mounting two independent seismic source assemblies according to the invention at right angles to each other and in the same housing as close as possible.
An example of this is shown in FIG. 9 where the source assembly 66,
68 are stacked in close proximity within the housing 40. Each source assembly receives power from a dedicated power supply (not shown) and has mounting means 70, 72, 74, 76.
And at least one bender arm assembly 78,
80, 82 and mass bodies 84, 86 are provided. By appropriately shifting the relative phase of the signals applied to the two assemblies, the direction of the radiated wave can be effectively rotated about the well bore axis. The direction of the waves generated by the seismic source of the present invention is determined by incorporating any of the known sonde directing devices within the housing of the present invention.

As a further modification, multiple perforation sources can be used non-interferentially simultaneously by using coded sweep signals. This can significantly increase the data collection rate and reduce costs. There is no theoretical limit on the number of coded sources that can be operated independently at the same time. As a further modification of the present invention, the number of signals that must be generated from the sources can be reduced by lowering a series of acoustic or geophones spaced apart from each other into each receiving well.

The device described above is small and lightweight,
The very small number of moving parts makes it very practical and able to withstand the rough handling that one might expect to encounter in the field. Various changes can be made to the present invention without departing from the spirit or essential characteristics thereof. The above description is illustrative and does not limit the scope of the invention.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a typical conventional surface-type seismic survey device.

FIG. 2 is a schematic diagram of a typical conventional logging device.

FIG. 3 is a schematic diagram of the present invention.

FIG. 4 is a plan view of a typical field drilled hole.

FIG. 5 is a side view of the present invention placed in a well.

FIG. 6 is a vertical sectional view of an embodiment of the tool of the present invention.

FIG. 7 is an enlarged detailed longitudinal cross-sectional view of an embodiment of a bender bar according to the present invention.

FIG. 8 is a schematic plan view of a seismic wave pattern generated by the present invention.

FIG. 9 is a vertical cross-sectional view of a modified embodiment of the present invention.

[Explanation of symbols]

 40 Tool Housing 44, 46 Telescopic Leg Member 54, 56 Bender Arm Assembly 58 Mass 62, 64 Piezoelectric Ceramic Crystal

Claims (2)

[Claims] 1. A device for generating seismic waves, comprising a closed housing (40) for lowering into a hole, and fixing means (4) for fixing the housing to the wall of the hole.
4, 46), supporting means (50, 52) provided in the housing and attached thereto, and first and second piezoelectric ceramic crystal bodies (62, 64) to form an arm. At least a first bender arm assembly (54, 56) configured to attach a mass body (58) in an intermediate position and attached to the supporting means, and a housing by exciting the crystal body to bend the arm Drive means connected so as to generate seismic waves that propagate into the formation around the perforation when secured to the wall. 2. A device for generating seismic waves, comprising a closed housing (40) for lowering into a hole, and fixing means (4) for fixing the housing to the wall of the hole.
4, 46), supporting means (70, 72, 74, 76) provided in and attached to the housing, and first and second piezoelectric ceramic crystal bodies (62, 64), It is composed of a pair of arms that are spaced apart in parallel and a mass body (84; 86) attached to an intermediate position of the arms, and each mass body (84; 86)
First and second bender arm assemblies (78, 80; 82) attached to the support means, the linear movement paths of which are substantially orthogonal to each other, and the first and second bender arm assemblies (7, 7), respectively.
8, 80; 82), and by bending the arms by exciting the crystal in a predetermined phase relationship and sequence, in the formation around the perforations when the housing is fixed to the wall. And a drive means for generating seismic waves propagated in a predetermined direction.