GB2251686A - Downhole seismic source - Google Patents

Abstract

A tool comprises a drive unit (10) for generating seismic energy by the reciprocation of a mass (24) within a bore (22) in a housing (20) using two linear motors. Clamp units (8, 12) are fitted at either end of the tool and each has a piston (16) which is moveable between a deployed position in which it projects into engagement with the borehole wall and a retracted position flush with the tool housing. The clamp units (8, 12) secure the tool with a high degree of rigidity into the borehole so that seismic energy generated by the reciprocating mass (24) is transmitted into the surrounding geological strata. <IMAGE>

Description

DOWNIlOLE SEISMIC SOURCE The present invention relates to a downhole seismic source for use in geophysical exploration.

Information can be discovered about the geological structures in the ground by generating seismic signals and picking up the reflected and refracted signals at various points. Normally the weathered layer of ground near the surface absorbs much of the seismic energy. Therefore, better results are obtained if a borehole is first drilled and the seismic source placed down the borehole. Pickups can be provided at the surface and/or in the same and/or in an adjacent borehole depending on the type of survey configuration employed.

Most seismic sources are of the type which produce an impulse of seismic energy. This can readily be achieved by using a source which is an explosive charge. Other impulsive sources rely on the creation of an electric discharge or the release of a pulse of compressed air. The use of such impulsive sources gives rise to a number of technical problems. The source itself can damage the borehole as well as potentially creating significant environmental damage. Where explosive sources are employed there could be a considerable safety risk to the personnel operating the source.

Many of these impulsive sources also use energy which is stored either in the explosive or as a supply of compressed air or in capacitors. Any source which uses stored energy requires replacement of that energy if the source is to be activated again. Clearly this is impossible with an explosive source. Therefore, if seismic energy is to be repeatedly generated downhole, it is necessary to re-charge or replace the source each time. For deep boreholes, this can be a time consuming and expensive procedure. Even where the source can be recharged, such as a source relying on the energy stored in batteries or compressed air cylinders, it is still necessary to recover the source to the surface for occasional recharging.In many types of source, the creation of the seismic energy to be transmitted into the geological structures around the borehole is associated with the generation of other "noise". This noise may include sound energy which propagates along the borehole. This phenomena is known as tube waves. Where the source discharges a waste product during operation, such as the expanded air from a pneumatic source, this can give rise to cavitation noise. The generation of such noise means that the energy conversion into seismic signals is inefficient.

Moreover, the noise can interfere with the reception of the seismic energy which has been reflected or refracted from geological structures. Therefore, the received information can be difficult to interpret.

The precise energy spectrum created by an impulsive source is extremely irregular and generally irreproducable. This gives rise to considerable difficulties in interpreting the results since variations in the amounts of received energy from explosions that vary in depth may represent an unknown combination of variations in the geological strata, and variations in the amount of seismic energy generated during each test.

In order to overcome the various technical problems outlined above, the present invention provides a tool for use as a seismic source downhole comprising a housing containing a mass adapted to reciprocate in a bore within the housing, linear motor means for driving the mass in reciprocation, means for supplying electrical power to the motor means, and clamping means for rigidly clamping the housing to hold the tool tightly engaged against the wall of the borehole.

The use of a linear motor driven reciprocable mass to generate the seismic energy allows non-impulsive seismic energy to be transmitted into the geological strata surrounding the borehole. By controlling the frequency of the power supply to the linear motor means, a known energy spectrum can be applied to the geological strata, and precisely identical amounts of energy can be applied during different tests with the tool at varying locations within a single borehole or an array of boreholes.

Because the tool uses electrical power which can be supplied from the surface, for example by means of a standard seven core conductor logging cable connected to the source by means of an industry standard electrical interface, there are no waste products, save heat, generated by operation of the tool, as well as having the advantage of reducing damage to the borehole this means that the signal to noise ratio at the pickups is considerably enhanced relative to prior art impulsive sources. The use of digital signal recovery techniques is also particularly suited to enhancing the performance of the source, because of its excellent repeatability.

It will further be appreciated that the source of the invention has the advantage that it is much safer in use and leaves the borehole and the environment largely undamaged.

Preferably the clamp means provide tight contact between the wall of the borehole and the housing at at least two diametrically opposed regions at either end of the tool.

The clamping means may comprise clamp units each of which has a piston that can be extended and retracted into the housing along an axis transverse to the longitudinal axis of the tool and the borehole. This piston is preferably provided with á serrated contact face for engagement with the borehole wall. This type of clamp unit is more rigid than the type of fixing structures normally employed for downhole tools which are generally intended merely to locate a tool at a fixed axial position in the borehole. Many such fixing devices rely upon hinged lever mechanisms which have a low degree of rigidity.The use of such devices for fixing a seismic source into a borehole would result in a great deal of energy being dissipated by vibration of the tool relative to the wall of the borehole, whereas the high stiffness of the clamping proposed for use with the tool to be described allows much more efficient coupling of the seismic energy into the geological strata.

When the signals received by pickups during and after operation of the source are processed using advanced signal processing equipment now available, the present source, with its known and reproducible energy spectrum, can produce much more useful and informative results about the structure of the geological strata than has previously been possible.

An embodiment of a tool in accordance with the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a diagrammatic view partly in section showing an embodiment of the electromagnetic tool of the invention; Figure 2 is a longitudinal sectional view through a drive unit for use in the electromagnetic tool of Figure 1; Figure 3 is a section on the line 3-3 of Figure 2; Figure 4 is a longitudinal sectional view showing an alternative construction for the end of the drive unit of Figure 2; Figure 5 is a longitudinal sectional view through a first embodiment of an electromechanical clamp unit for use in the tool of Figure 1; Figure 6 is a partial section on the line 6-6 of Figure 5; Figure 7 is a partial section on the line 7-7 of Figure 5;; Figure 8 is a transverse section through a second embodiment of an hydraulic clamp unit for use in the tool of Figure 1; Figure 9 is a divided longitudinal sectional view on the line 9-9 of Figure 10 through a third embodiment of a clamp unit showing a clamping piston in a retracted position on the left hand side and in the deployed position on the right hand side; Figure 10 is a divided transverse section through the clamp unit shown in Figure 9; and Figure 11 is an external view showing the face of the piston of the toggle clamp unit of Figure 9.

The seismic source of the invention is a tool built up from three basic units as shown in Figure 1. At the upper end of the tool an electrical interface unit 4 is connected to a logging cable 6. The interface unit is coupled to an upper clamp unit 8. A drive unit 10 is connected between the upper clamp unit 8 and a lower clamp unit 12. Each unit is constructed as a separate component which can be coupled to its adjacent components by means of sealed screw fittings.

The construction of the tools from separate units is merely a preferred method of fabrication and is not essential to the invention. Each unit has a separate housing and these housings together comprise the housing of the tool. A power supply conduit 14 runs from the electrical interface unit 4 through each of the units in order to provide electrical power and, if required, hydraulic power by means of cables (not shown).

The interface unit 4 is preferably an industry standard unit for conveying the signals provided using conventional seven core logging cable into those necessary for operation of the tool. A suitable unit is made by Schlumberger.

The clamp units 8 and 12 fitted at either side of the drive unit 10 are each provided with a piston 16 which can reciprocate in a bore 18 having an axis transverse to the longitudinal axis of the tool. The pistons 16 are shown in Figure 1 as being in their deployed positions. The pistons may also be retracted into the tool housing so that a contact face of the piston is flush with the remainder of the tool housing so that the tool can be moved up and down the borehole. Various possible embodiments of the clamp unit will be described in more detail later with reference to Figures 5 to 11.

Between the clamp units 8,12 there is mounted a drive unit 10 which is the source of seismic energy. The unit 10 comprises an outer cylindrical housing 20 which defines within it a longitudinal bore 22 within which a mass 24 can reciprocate. Referring to Fig. 2, the mass 24 is supported for its linear reciprocating motion within the bore 22 on linear bearings carried by a rod 26 which extends longitudinally of the unit and is mounted to end caps 28, 30 at either end of the cylindrical housing 20. This rod 26 is hollow in order to define the power conduit 14. The mass 24 is made to reciprocate by means of a pair of linear motors each comprising a permanent magnet a pole piece and a coil.

These motors may be of the type described in UK Patent application No. 8801924. One linear motor is formed on either side of the mass 24 and they are arranged so that the interacting fluxes of the magnet and coil of each linear motor create a force driving the mass in the same direction.

The two linear motors are arranged to drive the mass 24 in alternating directions so that the effect of their operation is to drive the mass in reciprocating motion within the bore 22. The amplitude and frequency of the reciprocating motion of the mass are determined by the frequency of the alternating power signal applied to the coils of the two linear motors. The range of frequencies that can be produced in this way is 20-1000Hz although an upper limit of 500Hz is applicable in most circumstances since at higher frequencies the transmission of the seismic energy through geological strata is low. The particular frequency range of oscillation is chosen to suit the circumstances in which the source is being deployed and the type of strata expected.

The force that can be generated is of the order of 1000 Newtons for a source of 10cm diameter. Larger sources could produce greater forces.

The moving mass aseembly is located in its mean position by a suitably calibrated soft spring 67 fitted to the end cap 14. Winding 38 is provided with a suitable annular end piece 69 to prevent damage to the windings from engagement by spring means 67.

The drive unit is rigidly coupled to the clamp units 8, 12 by means of screw threads 32 formed on annular projections on the outer faces of the end caps 28, 30, which screw threads each co-operate with a corresponding screw thread 34 formed on an internal cylindrical surface of an annular projection defined on the end of the housing of the clamp unit. The seismic energy generated by the reciprocating mass is transmitted through the rigid couplings to the clamp units and via the contact faces of the piston 16 and the regions of the housing of the tool which are in engagement with the borehole wall into the geological strata surrounding the borehole.In order to prevent dissipation of energy by vibration of the tool relative to the borehole, it will be appreciated that the stiffness of the clamping of the tool in the borehole has a considerable effect on the efficiency of energy transmission.

In the embodiment of the drive unit illustrated in Figures 2 and 3 the reciprocating mass 24 carries at either end the coils 36, 38 of the two linear motors and the coils reciprocate together with the mass 24. Annular permanent magnets 40, 42 are rigidly fixed to the rod 26. The magnets are arranged so that they have like poles facing each other so that current of the same phase can be applied to the coils 36, 38 which are wound in the same sense. It will, of course, be appreciated that the coils may be fixed and provided at the locations of the magnets 40, 42. In this case annular permanent magnets are employed which are fixed to the reciprocating mass 24. Similarly other variations that will be apparent to those skilled in the art can be made to the linear or other motors used in this tool provided the effect is to drive the mass in reciprocation motion.The power supply for the coils is taken from the power supply cable running through conduit 14 through an opening in the wall of rod 26 and then through further openings in the wall of the mass 24.

The mass 24 comprises two semi-cylindrical parts 24A, 24B which are joined at their top and bottom faces by annular walls 44, 46. Parts 24A, 24B are mounted by means of two linear bearings 48 and 54 which are rigidly fixed to a square section collar 50, which is integrally machined as part of the shaft 26. Linear bearings 48 and 54 are mounted on opposite faces of the square section collar 50 to guide the reciprocating mass. The collar 50 may also carry a projecting tongue 60 which is provided with a recess at its remote end. In this case a corresponding flange 64 projecting from the internal wall of the housing 20 is received in the recess. The flange and recess walls of the tongue are provided with co-operating holes to define a continuous bore through which a fastening bolt may be secured.In this way the bearing for the reciprocating mass 24 is secured against excessive radial vibration relative to the housing 20 of the tool. Rotation of the mass 24 relative to the housing 20 is prevented by locking of the threaded ends of rod 26.

The dissipation of heat generated by the operation of the linear motors is an important consideration in the design of the drive unit. The permanent magnets 40, 42 must not be allowed to approach their Curie temperature (Tc) at which their magnetism is lost. The permanent magnets may be made of a material with a high Curie temperature such as Samarium Cobalt or Neodymium Boron Iron. The magnets may also be provided with a highly reflective surface coating giving them a mirror finish. The windings 36, 38 and the inner surface of housing 20 can be provided with a black finish to maximise radiative heat exchange.An embodiment of the linear motor in which the windings are fixed to the cylindrical housing 20 and the permanent magnets are carried by the reciprocating mass 24 allows for somewhat improved heat dissipation since the heat generated in the windings can be dissipated by conduction through the housing and thence into the reservoir fluid surrounding the tool in the borehole. Since this fluid may be at a high ambient temperature in deep boreholes, the efficiency of heat dissipation is particularly important. In normal operation the tool will be operated for short periods and this allows time for cooling during which the tool may be moved from one test position in the borehole to another. For this reason, in operation, the tool is normally lowered to the bottom of the borehole before testing starts, and raised periodically during the cooling period between each test.Since the temperature conditions of the surroundings are likely to be greatest at the bottom of the borehole, and heat dissipation is more efficient when the tool is cold, this technique also produces better heat dissipation. It has been found in practice that four test periods of operation of approximately 20 seconds duration with short cooling periods in between can be undertaken in a time period of 12 minutes before the internal temperature change reaches 70"C, when the ambient temperature is 200"C. A cooling period of approximately 16 minutes is then necessary before the tool returns to ambient temperature.

The end caps 28, 30 of the drive unit housing may have various constructions. In Figure 2, the end cap 30 is shown as being integral with the cylindrical housing wall. Both end caps may have the construction shown for the end cap 28 which is machined as a separate element. The inner face of the end plate 28 is provided with an annular flange 62 having an internal screw thread which engages with the external screw thread on an annular projection of a support member 63 which is fixedly secured to an end face of the permanent magnet 42 and the rod 26 to provide central support for the rod 26. One or more O-ring seals 65 are provided between the abutting surfaces of the end cap 28 and the cylindrical housing wall. It will be noted that the power conduit 14 extends through the entire drive unit including the end caps 28, 30 and the rod 26.

In the alternative end fitting shown in Figure 4 a support member 66 is screwed directly to the end of the rod 26. One face of the support member 66 engages against an inwardly projecting flange 68 formed on the cylindrical inner surface of the housing wall 20. This face of the support member is also recessed to support the permanent magnet 42. An annular end cap 70 is in turn screw threaded to an external thread formed on the annular projection from the support member 66 in order to provide a connecting projection of standard size with an external screw thread 32 to co-operate with a corresponding internal screw thread 34 on the end of a clamp unit to which the drive unit is to be secured. The abutting faces of the end cap 70 and housing wall 20 are sealed by means of an O-ring seal 65 as in the previous embodiment.

Various embodiments of clamp units 8,12 will now be described. These constructions vary in the mechanism employed to drive the piston 16 between its retracted position and its deployed position where its contact face is in tight engagement with the wall of the borehole.

The embodiment shown in Figures 5 to 7 is an electromechanical system driven by means of an electric motor 72 which is accommodated in a cylindrical bore within an upper part (as seen in Figure 5) of a housing 74 of the unit. This electric motor is powered by means of current supplied by means through an electrical power supply cable passing through the conduit 14. The output from the motor 72 is connected via a gearbox 82 to an output shaft 76 which is, in turn, connected by means of a flexible coupling 78 to a bevelled drive pinion 80. This drive pinion 80 drives a worm gear 82, to which the piston 16 is secured, in rotation via a drive chain consisting of the gears 84 to 90. The bevelled gear 84 is directly engaged with the bevelled gear pinion 80 and is fixedly secured to the coaxial spur gear 86 which drives the spur gear 90 by means of an intermediate spur gear 88.The spur gear 90 is fixedly secured to a projecting shaft of the worm 82. The worm 82 co-operates with an internal thread formed on the body of the cylinder 16 so that when the shaft 76 rotates the worm 82 rotates and drives the piston 16 either transversely outwardly of the housing or retracts it into the housing depending on the sense of rotation of shaft 76. Control signals for controlling whether the motor is to operate and also the setting of the gear box, which determines the sense of the rotation of shaft 76, are provided from a cable running in the conduit 14. Borehole ambient pressure is equalised on each side of the piston 16 by a bleed hole 17, which is suitably filtered to prevent the entry of debris.

An alternative embodiment of a mechanism for driving the piston 16 between its deployed and retracted positions is shown in Figure 8. This system is hydraulically driven and the piston 16 is secured to an operating piston 92 which moves in sealing engagement in the bore 18 so that application of hydraulic fluid to one side of the piston 92 and exhaustion of it from the other side allows the piston to be moved. A supply of hydraulic fluid is provided from a hydraulic cable carried through the conduit 14 and an appropriate valve and bore system 94 is provided in the housing of the clamp unit in order to control the supply and exhaustion of hydraulic fluid in order to control the position of the piston 92 and therefore the piston 16.

It will be seen from Figure 8 that the face 96 of the piston which is intended to come into contact with the wall 98 of the borehole is provided by means of an insert which is screw threaded into a shaft 99 carried by and passing through the operating piston 92 into a guide bore 100 formed in the housing behind bore 18. An annular flange 102 or 102' as shown by dotted lines in Figure 8 is fitted to the shaft 99 adjacent the contact face 96. The flange 102, 102' is provided with O-ring seals 103 to seal the abutment between the flange and the shaft and also the outer, flattened wall of the housing of the clamp unit. Flanges 102, 102' of various sizes may be provided in order to allow the tool to be used in boreholes of varying diameter so that a close fit between the contact face 96 of the piston and the borehole wall can always by maintained in order to achieve maximum rigidity of the clamping of the tool in the borehole. The rear of shaft 99 is also exposed to the borehole environment, equalising the pressure on each side.

This avoids the hydraulic system having to drive the piston assembly against the borehole ambient pressure.

A further embodiment of the clamp unit is shown in Figures 9-11. This type of clamp unit uses a toggle mechanism to deploy the piston 16. The piston may be driven by means of a motor which may be similar to that described with reference to Figure 5 or by hydraulic means. However the gear chain is adapted to drive an operating rod 104 in a reciprocating motion in order to produce a corresponding reciprocating motion of the piston between its retracted and deployed positions. The rod 104 carries a support member 106 to which a first lever 108 is pivotally mounted. The other end of the lever 108 is pivotally mounted to the base of the piston 16 and also carries a toothed gear 110 which engages with a corresponding gear 112. The gear 112 is carried by a second lever 114 also pivotally mounted in the base of the piston.The other end of lever 114 is pivotally connected to a fixed mounting in the housing of the clamp unit. The manner in which reciprocation of the operating rod 104 drives the piston by means of the toggle mechanism of levers 108,114 and gears 110, 112 is clearly seen by a comparison of the left and right hand sides in Figure 9.

The contact face 96 of the piston shown in Figure 9 is formed as a separate, renewable member. Three threaded bores 116 are provided in the body of the piston to allow the contact face plate to be secured to the body by means of plugs 118 with threaded shafts. The plugs 118 may be formed of a different material from the remainder of the face plate to ensure efficient engagement between the borehole wall and the contact face of the piston. Where the borehole is cased the surface of at least part of the contact face of the piston is preferably serrated.

In all the embodiments of the clamp unit described it is important that there should be the minimum scope for vibration of the piston 16 while it is in its deployed position, since the function of the tool is to transmit all the seismic energy generated by the drive unit through the contact face of the piston and the regions of the housing which are in engagement with the borehole wall through into the geological strata.

The control signals for the operation of the various motors, gear box 82 or hydraulic valves used to operate the drive and clamp units are provided from the surface in one of the conductors of the logging cable 6. The required control signals are generated at the surface and are transmitted via the logging cable 6 and through a control cable which passes through the conduit 14 to each of the clamp units and drive unit of the tool so that the entire operation of the tool can be completely controlled from the surface.

A typical size for the tool described is a diameter of 10 centimetres and a length of 1.5 meters. The drive unit itself would represent only a third of that length. A suitable weight for the mass 24 is 3kg. By increasing the diameter of the drive unit to maximise the magnetic volume of the two linear motors greater force can be generated by the same amount of power.

It will be appreciated that the source of seismic energy for the tool is electrical power supplied from the surface. The amount of seismic energy transmitted into the surrounding geological strata of the borehole can be accurately reproduced during each test carried out with the tool. The frequency of the energy transmitted can be swept by controlling the power supply frequency in a known manner.

Although the variation in frequency of the reciprocation of the mass within the drive unit, may not precisely follow the signal transmitted to the geological strata, any deviation will be constant and this "mechanical signature" of the tool can be measured and taken into account when generating the power supply frequency or processing the signals received by the pickups.

Maximum use of the available primary electrical power can be made by increasing the minimum sweep frequency. This reduces the maximum amplitude of axial movement of the mass 24, thereby allowing a shorter pair of coils 36,38 to be used. Hence there will be less length of coil which is not overlapping the magnets 40,42 and not contributing to the magnetic circuit. Such excess coil length merely dissipates heat without contributing to the force on the reciprocating mass.

Claims (10)

1. A tool for use as a seismic source downhole comprising a housing containing a mass adapted to reciprocate in a bore within the housing, linear motor means for driving the mass in reciprocation, means for supplying electrical power to the motor means, and clamping means for rigidly clamping the housing to hold the tool tightly engaged against the wall of the borehole.

2. A tool according to claim 1, wherein the linear motor means comprises two linear motors each comprising a coaxially mounted coil and permanent magnet, the coil or permanent magnet of each motor being secured to the mass which is to reciprocate.

3. A tool according to claim 2, wherein the mass is mounted for reciprocation on linear bearing means on a rod extending axially of the bore.

4. A tool according to claim 3, wherein the coil or permanent magnet of each linear motor is rigidly fixed to said rod via the linear bearing means.

5. A tool according to any one of the preceding claims, wherein said clamping means comprises two clamp units one at either side of a unit comprising the mass and linear motor means, each clamp unit having a piston that can be'extended and retracted into the tool housing along an axis transverse to the longitudinal axis of the tool.

6. A tool according to claim 5, wherein each piston is provided with a contact surface shaped for engagement with the borehole wall.

7. A tool according to claim 6, where at least a portion of said contact surface is serrated.

8. A tool according to any one of claims 5 to 7, wherein each clamp unit comprises an electric motor and drive means for driving the piston between its extended and retracted positions.

9. A tool according to any one of claims 5 to 7 wherein each clamp unit comprises hydraulic means for driving the piston between its extended and retracted positions.

10. A tool for use as a seismic source substantially as herein described with reference to any of the accompanying drawings.