ELECTRICAL DISCHARGE ACOUSTIC SOURCE WITH BANK OF CAPACITORS

Technical field

[0001]This invention relates to acoustic sources which use an electrical discharge to create the acoustic pressure wave in the surrounding environment. Such sources, such as spark or plasma sources find application as acoustic sources of the type used in the oil and gas industry as seismic signal sources or the like.

Background art

[0002]Acoustic sources are used in a number of different industries and fields of technology. A variety of acoustic sources have been developed that have a range of output energies and frequency bands. One such source, commonly referred to as a sparker, discharges electrical energy that has been stored in capacitors across one or more pairs of electrodes immersed in an electrolyte. The discharge creates an arc between the electrodes that is associated with an acoustic output. Sparker type sources have been employed in a wide variety of applications such as seismic exploration for natural resources, measuring the elastic moduli of rocks both in-situ for civil engineering site investigation and in the laboratory, and in medical imaging. They may also be used for sonic cleaning, killing organisms and in medical applications known as lithotripsy to break up unwanted material within the body.

[0003]Sparkers have a number of limitations that may be significant depending on their intended application, including: a. Limited output energy and frequency band. b. Rapid and erratic wear of the electrodes. c. Unpredictable failure of the energy storage capacitors. d. Pressure dependence of the output and spark formation. [0004] It has been proposed to use sparkers as marine acoustic sources for geological imaging beneath the seabed using the reflection seismic technique (e.g. US 3286226). Various designs using multiple electrodes have been proposed to improve the output and reduce variations due to erratic erosion of the electrodes, (e.g. US3613823). One of these uses a consumable wire as an electrode (WO 92/02926). Another (US5228011) contains the spark in a region that is isolated from the electrodes to prevent erosion of the electrode by the spark. None of these designs appears to have been entirely satisfactory.

[0005]There are a number of factors which can affect the efficient conversion of electrical to acoustic energy. Overall, the most efficient conversion of electrical to acoustic energy takes place if a damped electrical discharge is achieved. In this case most of the electrical energy is dissipated in the creation and expansion of the plasma arc rather than being lost in the oscillation of the electrical circuit. The damping depends on the initial conductivity of the electrolyte, circuit capacitance and inductance, exposed cathode area, the resistance of the arc, the electrode gap, the discharge voltage, the ambient temperature and the ambient pressure. Under optimal conditions the dominant acoustic frequency is similar to the resonant frequency of the circuit.

[0006] It is known that the amplitude of an acoustic source may be increased by firing a number of sources at the same time according to the principal of superposition, provided the sources are greater than one wavelength apart. Firing a number of sources in a pre-determined sequence can also be used to increase the overall frequency bandwidth of the source (US 4739858) without altering the frequency of the individual discharge circuits. Methods of shaping the source output, commonly referred to as tuning, are also known (e.g. US 5398217).

[0007]Sparkers and airguns are used for marine seismic reflection imaging in exploration for hydrocarbons. These sources are positioned in water and generate a bubble of air from an airgun or water vapour from a sparker which produces a pressure pulse as the bubble expands, oscillates and collapses. The acoustic efficiency of such bubble sources, including explosives, is highly dependent on the ambient pressure and so these sources are typically only operated at a few metres below the surface.

[0008] Under optimum electrical conditions and with a suitable electrode configuration, the acoustic output of a sparker is due to a shock wave generated by the formation of a plasma arc between the electrodes rather than vaporisation of the electrolyte. Under these optimum conditions sparkers do not produce a significant bubble and have been used effectively in boreholes at greater than 3km depth. This type of sparker that generates a high pressure plasma arc is referred to below as a Plasma Gun.

[0009] Borehole sparkers can suffer from poor and unpredictable capacitor reliability due to the very high charge densities that are required to provide sufficient energy from a small borehole tool volume. In addition the operating temperature range may have to be relatively high due to the elevated temperatures at depth. If a single capacitor fails the sparker must be brought back to the surface for repair. This could occur after a few shots or after many hundreds of shots making it impossible to plan the duration and so cost of a survey.

[0010] Due to the enduring problems of reliability, repeatability and the limited output of existing sparkers, the full potential of sparkers does not appear to have been realised, particularly in exploration applications. Here, means are revealed to obtain a consistent, optimal spark under all ambient temperature and pressure conditions. These means may be of benefit in all sparker applications including marine and borehole seismic applications, medical imaging and acoustic cleaning. In particular we propose a Plasma Gun for marine exploration that can be used over the full range of ocean depths. Disclosure of the invention

[001 I]A first aspect of this invention provides an electrical discharge acoustic source comprising:

- a bank of capacitors;

- a discharge electrode arrangement comprising pairs of electrodes;

- a power section for providing electrical charge to the capacitors; and

- a control section for controlling the connection of the capacitors to the pairs of electrodes; wherein the bank of capacitors further comprises a switching arrangement, configured so that any capacitor can be connected to any pair of electrodes, by which each capacitor in the bank is independently connectable to an electrode in the discharge electrode arrangement so as to provide a predetermined acoustic signal on operation of the discharge electrode arrangement.

[0012] In this way, a number of capacitors can be connected to a pair of discharge electrodes to obtain the desired output. Each pair of electrodes can be immersed in an electrolyte solution contained in a pressure compensating housing such as a flexible bladder or the like. In one embodiment, at least one electrode of the pair is moveable so as to allow the gap between the electrodes to be set to a predetermined level.

[0013] Each pair of discharge electrodes can be provided with a connection to the power section and a high voltage discharge switch by which electrical power from the connection can be discharged across the electrodes. The switching arrangement is configured such that each capacitor can be connected to each connection to the power section. Each capacitor can also be provided with an isolation switch that is separate from any switches connecting it to the power connections.

[0014]The control system can be operated to adjust a number of parameters associated with operation of the source, including: discharge voltage, circuit inductance, isolation of inoperative capacitors in the bank, number of discharge electrodes activated, the firing sequence and timing of the discharge, the gap between the electrodes in a pair, and the conductivity of electrolyte surrounding the electrodes. The control system can also include sensors for monitoring ambient and device temperature and pressure conditions, signal pressure and time, and source output.

[0015]One embodiment of the power section comprises a high voltage power supply and an array of batteries. The high voltage power supply can be connected to an external source of electric power.

[0016]A second aspect of the invention comprises a method of generating an acoustic signal using an electrical discharge acoustic source comprising:

- a bank of capacitors;

- a discharge electrode arrangement comprising pairs of electrodes;

- a power section for providing electrical charge to the capacitors; and

- a control section for controlling the connection of the capacitors to the pairs of electrodes; the method comprising selecting predetermined capacitors from any of the capacitors in the bank and independently configuring the connection of the predetermined capacitors to any of the pairs of electrodes in the discharge electrode arrangement, and discharging the connected capacitors through the discharge electrode arrangement so as to provide a predetermined acoustic output. [0017]One embodiment of the method comprises monitoring each capacitor in the bank and isolating any that are not functional. The functional capacitors can be connected in parallel to provide the desired output.

[0018]The invention also provides an electrode for use in a source according to the first aspect of the invention, comprising a discharge tip connected to a conductive tip holder that can be connected to a power supply, an insulating collar surrounding the connection between the tip and the holder and having an aperture through which the discharge tip projects. Formations are provided in the wall of the aperture for locating one or more sealing elements.

[0019] Other aspects of the invention will be apparent from the following description.

Brief description of the drawings

[0020] Figure 1 shows a schematic diagram of a source according to an embodiment of the invention;

Figure 2 shows detail of a switching arrangement for use in the embodiment of Figure 1;

Figure 3 shows one embodiment of an electrode assembly for use in the present invention;

Figure 4 shows detail of an electrode construction for use in the invention; and

Figures 5a and 5b show a three-electrode embodiment of the invention.

Mode(s) for carrying out the invention

[0021]This invention is based on the dynamic monitoring and control of factors that relate to spark formation in electrolytes, to enable the acoustic output to be controlled and optimised and to enable failed capacitors to be detected and remotely replaced in the circuit with spares. An electrode design is also described that has characteristics of slow and progressive erosion of the electrode tips and a consistent acoustic output. In addition the electrode gap may be remotely adjusted.

[0022]The embodiment of a Plasma Gun in accordance with the invention, shown schematically in Figure 1, comprises a computerised controller (the controller) 10, a surface power supply 12, a connecting cable 14 and a remote sparking device 16. The sparking device 16 typically comprises a separate housing that can be positioned under water or in a borehole extending underground in the formation to be investigated. The remote sparking device 16 comprises a power section 18 including one or more high voltage power supplies 20 and optional batteries 22, a bank 24 of energy storage capacitors connected via a switching arrangement 26 to a series of pairs of discharge electrodes 28, and a control system 30 including electronics and devices for monitoring and controlling the discharge of the pairs of electrodes 28.

[0023]An electrical charging circuit for a multiple electrode and capacitor arrangement is shown schematically in Figure 2, in which: a. p indicates the number of electrodes b. N indicates the number capacitors or groups of capacitors (for convenience each capacitor illustrated may be two or more capacitors connected in parallel). c. Gi to Gp are high voltage switches. These switches may be of the triggered spark gap type or other suitable design capable of holding off the required voltage and being remotely activated. d. Ei to Ep are electrode pairs across which the spark is formed e. Di to Dp are high voltage diodes to prevent voltage reversals on the capacitors. Alternatively, Di to D _N diodes can be connected in series with the individual capacitors. f. Ci to C _N are capacitors. The capacitance of each capacitor or group of capacitors may be different to obtain a range of acoustic frequencies to be combined in the output. g. Pi to P _N are isolator switches allowing each capacitor to be individually isolated from the rest of the circuit. The condition of the capacitors Ci to C _N is monitored individually. Prior to charging, any failed capacitors can be detected electronically and removed from the circuit by the switches Pi to P _N . h. HVi to HVp are high voltage negative supplies. These voltages may be variable to achieve the required firing voltage and discharge energy and are compatible with the voltage ratings of the capacitors, spark gaps and diodes connected to each of the HVi to HV _P supplies. i. Su to S _N p are high voltage switches connecting each of the Ci to C _N capacitors in parallel to one of the Ei to E _P electrode pairs. There should be no significant voltage across these switches when they are opened or closed. j. Ri to R _N are high voltage resistors to bleed off any residual charge remaining on the capacitors after firing. k. Hi to Hp are variable inductors. peration of this system is as follows: a. Wait for HVi to HVp to decline to a level at which the switches Su to S _N p can be activated. These voltages will decline due to the resistors Ri to R _N . b. Identify any failed capacitors. c. From the remaining capacitors, the controller selects which capacitors will be connected to each of the electrode pairs to obtain the required electrical discharge. d. The controller remotely operates each of the switches S _xi to SxP to connect capacitor x in parallel with the required power supply HVy where y is the electrode number. e. The high voltage supplies HVi to HV _P , that may have the same value or different values, are turned on by the controller. Depending on energy stored in each discharge circuit and the charging rates, HVi to HV _P may be turned on at different times to achieve full charge of the capacitors at approximately the same time. f. If a capacitor failure is detected during charging the controller switches off the corresponding HV supply, switches in an alternate capacitor and restarts charging of the new capacitor. g. When the controller detects that all the circuits are at the required discharge voltage, high sample rate monitoring of the discharge parameters is turned on. Subsequently the high voltage switches Gi to G _P are activated either synchronously or according to a pre-determined sequence to achieve a tuned output. h. The high sample rate monitoring continues for a sufficient period to cover the time in which the capacitors discharge. This period depends on the resonant frequencies of the circuits. For a frequency of 100Hz a period of from 10s of milliseconds before sparking to 50ms after sparking could be used. Based on the data collected during the discharge and the ambient conditions the operator may select alternate electrical parameters and revise the firing sequence to optimise the acoustic output. Over time, the acoustic output for an increasing range of ambient conditions and discharge parameters can be collected. By reference to this library of existing data it will be possible to select the electrical parameters to achieve a near optimum acoustic output that may be further refined for the specific conditions if necessary. [0026]A cross section through a typical electrode assembly for use in a source according to the invention is shown schematically in Figure 3. This figure illustrates the axial arrangement of the cathode and anode electrodes, 32 and 34 The anode 34 contains a large tungsten insert 36 that provides a low rate of erosion. The flexible bladder 38 containing the electrolyte, such as a copper sulphate or sodium chloride solution, surrounds the electrodes 32, 34. An electric motor or hydraulic means 40 is used to adjust the gap between the anode 34 and cathode 32 by moving the anode 34 axially. Connection between the anode 34 and ground, with a return path through the body of the tool, is provided by a flexible connection 42 The internal parts of the assembly may be filled with an insulating fluid, such as silicone oil. In this case the internal pressure and ambient pressure outside the tool may need to be balanced. This can be achieved by a piston 44 that is exposed on one side to ambient pressure and on the opposite side is exposed to internal tool pressure. A further piston arrangement 46 is revealed for continuously flushing electrolyte from a reservoir 48 through the bladder 38 and venting at the top of the bladder 50. The electrolyte reservoir 48 may be significantly larger than shown. Flushing the bladder 38 removes ionised electrolyte and electrode erosion products from the bladder in order to maintain a clean and consistent electrolyte for arcing.

[0027]The electrolyte contained in the flexible bladder 38 can be, for example, copper sulphate. Flexible bladders have been proposed previously for borehole tools and can be made of a flexible material such as rubber or Viton. A sensor can be provided to monitor the conductivity of the electrolyte. To achieve an optimum electrical discharge, the conductivity of the electrolyte can be increased or decreased by electrolysis (using separate electrodes to those used for sparking). The gap between the electrode tips is also variable as is discussed above and may be remotely set by the controller to optimise the electrical discharge.

[0028] Figure 4 shows an electrode suitable for use in the present invention. The cathode consists of a tungsten (or similar high melting point, hard metal, alloy or compound) electrode contained within a close fitting insulator made of the high temperature plastic such as PEEK (other, similar hard, high temperature insulating materials can be used).

[0029]The use of tungsten provides a high temperature capability for sparking and increased resistance to erosion compared to softer materials such as steel. The tungsten tip has a small diameter and small un-insulated length to minimise the exposed electrode area whilst retaining sufficient strength to withstand sparking without fracturing.

[003O]As shown in Figure 4, a tungsten electrode 52 is set within a metal holder 54 that contains a secondary sealing O ring 56. Depending on the specific tip material used, manufacturing requirements, or other considerations, the holder 54 can be produced in steel or an alternate metal such as brass. A primary sealing element (O ring seal) 58 is fitted over the tip 52 at the point where the tip 52 meets the holder 54. This eliminates the need to cut an O ring groove in the tungsten that would be a significant weakness and liable to shear during sparking. It has also been observed that sparking produces small spheres of melted metal on the tips of the electrodes. These spheres produce high current densities and so promote spark formation.

[003I]A machined insulator collar 60 having a central bore is provided around the tip 52 and holder 54 such that the point where the tip 52 meets the holder 54 is located within the bore of the collar 60. The inner wall of the bore in the collar 60 is provided with formations 59 defining an aperture through which the discharge tip 52 projects. The sealing element 58 is located between the end of the holder and the formations 59. Following machining of the insulator it is useful to relieve the stress in the material by following a heat treatment procedure. This greatly reduces the potential for fracturing of the insulator during sparking. The design of the cathode is such that the insulator can be a relatively simple piece without obvious stress concentrations that would weaken it. A sleeve 62 is provided behind the collar 60 and both parts are located in a housing 64.

[0032]The exposed area of the anode is not so critical, so the design of the anode can be very flexible. In this case a relatively large diameter tungsten insert, compared to the cathode, can be used to obtain a long life expectancy but many other designs could be employed.

[0033]The spark gap between the cathode and anode affects the efficiency of the conversion of electrical to acoustic energy. As the cathode is at an elevated voltage it is most practical to adjust the electrode gap by moving the anode. As is discussed above in relation to Figure 3, this can be achieved by use of a motor. The electrode gap may be remotely controlled from the surface by shifting the anode. For example, the gap will increase during sparking due to electrode wear and so for prolonged operations the anode will need to be moved towards the cathode periodically to maintain the acoustic output. Alternatively, if the spark gap is moved to a region of lower pressure the anode may be moved away from the cathode to optimise the acoustic output.

[0034]The electrode arrangement shown in Figure 3 is scalable and can be smaller for high frequency/low energy discharges and larger for higher energy discharges. A multiple electrode source is illustrated in Figures 5a and 5b comprising three electrode pairs 66, 68, 70 mounted around a central core 72. An outer housing 74 with windows 76 can be provided around the electrode pairs 66-70. Other embodiments of multiple electrodes using the design shown in Figure 3 may be conceived depending on the application or other constraints.

[0035] In use, surface controller 10 receives data from the remote sparking device 16. These data are processed to determine the optimum parameters to achieve the desired acoustic output. The controller 10 sends instructions back to the sparking device 16 to update the electronic and physical parameters relating to sparking to optimise the output. The parameters that may be adjustable include:

Electronic parameters, such as

Discharge voltage ,

Circuit inductance, and

Switching failed capacitors out of circuit; and

Physical parameters, such as

Number of spark gaps fired,

Firing sequence and timing,

Electrode gap, and

Electrolyte conductivity.

[0036] Sensors exposed to the environment surrounding the sparking device 16 monitor the ambient temperature and pressure conditions. A high frequency pressure sensor, such as a hydrophone, can be provided to monitor the source pressure versus time signature. Within the sparking device further sensors monitor the temperature at critical points such as the capacitors and high voltage power supply to ensure the tool is operated within its specification. There may also be a motion detector, such as a geophone or a pressure detector such as a hydrophone, to monitor the source output.

[0037] Electrical power and communication with the sparking device 16 are transmitted via the connecting cable 14. The choice of connecting cable depends on the application and suitable cables for borehole or marine use, are well known. In some applications, such as borehole tools, the rate at which the tool can be charged and fired may be limited by the current and voltage ratings of the connecting cable. In this case, the optional batteries 22 can be included in the tool. The batteries 22 can be used to rapidly charge the capacitors 24 to minimise the time between shots. Whilst the tool is inactive, for example during movement to another tool depth and between shots, the batteries may be re-charged via the connecting cable.

[0038] Methods of digital communication of data via the connecting cable between the controller and sparking device are well known. During sparking relatively small data sample intervals of lOμs are useful to adequately resolve the high frequency parameters. High frequency parameters that can be monitored include the light output detected by a sensor within the bladder, the voltage across the electrodes, the current flow in the discharge circuit and the acoustic pressure or motion detector. Greater data sampling intervals of Is or more are sufficient for monitoring the ambient parameters of internal temperatures at various points within the tool, external temperature, external pressure, electrolyte conductivity, condition of critical electronic components, charging voltage and charging current.

[0039]The condition of each capacitor can be monitored electronically. If a capacitor failure is detected this is communicated to the surface controller 10. An instruction to the sparking device 16 to connect a spare capacitor into the circuit may be generated automatically or may be given manually by the operator and the failed capacitor is remotely removed from the circuit.

[0040]The controller monitors the charging voltage and current to determine when the required discharge voltage has been reached. At this time the controller can either automatically instruct the sparking device to fire or await a manual command to fire from the operator. As is discussed above, the sparking device 16 may contain a number of pairs of electrodes that can be fired together or sequentially. Firing a number of electrodes synchronously enhances the acoustic amplitude. Sequential firing at constant or variable intervals can be used to control the amplitude spectrum of the output.

[0041]The energy discharged across an individual electrode pair can be controlled by adjusting the discharge voltage and switching the number of capacitors connected to the electrodes. The acoustic frequency of each electrode pair is related to the circuit capacitance and inductance and can also be controlled remotely by changing the inductance. In seismic imaging applications the image resolution and range of investigation are related to the source frequency. Dynamically controlling the acoustic output enables a range of imaging applications to be covered with one source.

[0042]The design of the electrodes can be important in obtaining a consistent and efficient discharge. A large range of electrode designs have been proposed. Dyer B.C. and Baria R., 1996 Development of the CSMA mark II high temperature borehole sparker source. Scientific Drilling, VoI 5, No 6, pp243-248 describe electrode parameters that affect the efficiency of the acoustic output. These include minimisation of the exposed cathode area and the electrode gap that is set to achieve a damped electrical discharge. Further, the electrolyte conductivity and/or the discharge voltage must be increased with increasing ambient pressure to allow a spark to form.

[0043]A wide variety of applications of sources according to the invention exist, including: a. Velocity calibration shots for example check shots in microseismic velocity estimation b. Sonic cleaning of pipes c. Killing aquatic organisms d. Crosshole seismic imaging e. Reverse Vertical Seismic Profiling f. Single well surveying g. Very long spaced sonic logging

[0044]Three particular applications of the plasma gun are proposed : deep marine seismic imaging at source depths of 10s to 1000s of metres, generating directional waves from within a borehole and back off shooting in drilling.

[0045] Conventional marine seismic reflection surveying and VSP surveys are typically performed using an airgun or other source at a few metres depths. For deep water surveys in water depths of 10s to a few 1000 metres the source amplitude decreases in proportion to the distance the source energy is transmitted through the water. Increasing water depth also reduces the potential lateral resolution of the seismic image. Conventional seismic sources become increasingly inefficient with increasing depth and so are typically operated within a few metres of the surface only. In comparison the Plasma Gun can be operated at depths in excess of 3000m. Using an array of Plasma Guns, fired in a timed sequence or together, a suitably large acoustic pulse may be generated for seismic imaging beneath the sea bed. The seismic receivers can be deployed in any of the conventional configurations such as a linear, horizontal array in the water, a sea bed array or a string of receivers within a borehole, depending on the imaging objectives.

[0046] In a borehole, a Plasma Gun with a number of vertically spaced electrodes as illustrated in Figures 5a and 5b may be contained within a single tool. These electrodes may be fired in a timed sequence or simultaneously to generate elastic waves in the rock. Using acoustic monopole sources Che X-H and Qiao W-X., 2004. Acoustic Field in Formation Generated by Linear Phased Array of Transmitters in Fluid-Filled Boreholes. Chinese Journal of Geophysics, Vol. 47, No. 4, 2004, pp:830-836, describe how the source energy may be steered into preferred directions in the rock by firing the sources in a suitable sequence. The Plasma Gun can be controlled in the same way to generate energy propagating in a preferred direction that is optimal for the survey configuration and the imaging target.

[0047]Whilst drilling deep boreholes, the drill bit may become stuck such that it is not possible to withdraw the drill bit and drill pipe from the borehole. In this instance it may be necessary to separate the drill pipe as close as possible to the drilling bit and leave the drill bit downhole by a method referred to as backoff shooting. The backoff shot is an explosive charge that is fired as close as possible to the drilling pipe joint at which the drill pipe is to be separated. This may take a number of backoff shots to achieve which is very time consuming as only one shot can be fired at a time and to recharge the backoff tool it must be brought back to the surface. Using the Plasma Gun instead of the backoff shot it would be possible to fire many shots in a single deployment until the pipe string separates.

[0048]As the electrical energy discharged by a Plasma Gun may not be primarily dissipated in producing a bubble, the acoustic output does not necessarily decline due to the effects of pressure on the bubble in the same way as for an airgun. This also means that there may effectively be no secondary acoustic output from the Plasma Gun due to oscillation and collapse of the bubble. There are also very significant advantages in conducting reflection surveys using a suitable Plasma Gun source close to the seabed, particularly if the receivers are also towed close to the seabed or are laid on the seabed. These advantages include reduced energy losses from transmission through the water column, reduced interference from the sea surface reflection, commonly referred to as the surface ghost, improved lateral resolution due to a reduction in the Fresnel radius and reduced acoustic interference with near surface swimming marine mammals. Other variants and uses within the scope of the invention are envisaged.