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  Electrical power supply arrangement for a downhole measurement assemblyUSPTO Application #: 20060232138Title: Electrical power supply arrangement for a downhole measurement assembly Abstract: A downhole measurement assembly comprising a metal casing housing a plurality of tool sections, one of which includes a detector sensitive to a magnetic field, and a DC power supply for said tool sections, the power supply and the tool sections being electrically connected to the casing, characterised in that the power supply is connected to at least one tool section via a switching circuit and a conductor arrangement, the switching circuit comprising a capacitor, a plurality of switches and control means operative alternately to charge the capacitor from the DC supply and discharge the capacitor through said at least one tool section using the conductor arrangement so as to inhibit the generation of an electromagnetic field in the conductor arrangement between the capacitor and the said at least one tool. (end of abstract)
Agent: Schwegman, Lundberg, Woessner & Kluth, P.A. - Minneapolis, MN, US Inventor: Percival F Williams USPTO Applicaton #: 20060232138 - Class: 307113000 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20060232138. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] The present invention relates to downhole measurement assemblies and more particularly to the electrical power supply arrangement for such assemblies. [0002] The oil and gas industry employs a variety of information gathering downhole tools during exploration and drilling of wells. These tools provide such information as hole inclination and azimuth, temperature, radioactivity, or resistivity. These tools are encased in non-magnetic stainless steel cylindrical pressure cases. By their nature, these tools have a large length to diameter ratio. The number of tools that have been introduced over the years has increased. Each tool is aligned serially down the drill string, and power is connected to each in parallel on a bus, which also allows communication between the tools and the transmission of signals to the surface. Traditionally, the pressure case of each tool is grounded and is the return path for DC current. Until recently, the directional measurement tool was always the one furthest down the hole and part of the directional measurement tool is an array of fluxgate magnetometers situated at the bottom of the tool in a module known as the stack. These magnetometers respond to the earth's magnetic field. Recently, other tools have been placed below the directional package and the power required by such tools results in current flowing in wire which passes the magnetometers and which in turn generates an electromagnetic field which causes errors in the readings produced by the magnetometers. [0003] Historically, the pressure cases of the downhole tools have always been grounded and connected to the system zero volts. This means that if conventional current carrying conductors are replaced in the region of the magnetometers by say a twisted pair or a coaxial cable, but otherwise the electrical arrangements remain unaltered, the current in the two conductors of the twisted pair or coaxial cable will be unbalanced as some current will flow through the parallel path of the pressure cases. In fact, most of the current will flow through the pressure case as it is a much better conductor. [0004] It is an object of the present invention to enable a fully grounded downhole measurement assembly to pass direct current in either direction without the current carrying conductors generating an electromagnetic field. [0005] The present invention provides a downhole measurement assembly comprising a metal casing housing a plurality of tool sections, one of which includes a detector sensitive to a magnetic field, and a DC power supply for said tool sections, the power supply and the tool sections being electrically connected to the casing, characterised in that the power supply is connected to at least one tool section via a switching circuit and a conductor arrangement, the switching circuit comprising a capacitor, a plurality of switches and control means operative to alternately charge the capacitor from the DC supply and discharge the capacitor through said at least one tool section using the conductor arrangement so as to inhibit the generation of an electromagnetic field in the conductor arrangement between the capacitor and the said at least one tool. [0006] In order that the present invention be more readily understood, an embodiment thereof will now be described by way of example with reference to the accompanying drawings in which:-- [0007] FIG. 1 is a diagrammatic representation of a downhole measurement assembly for explaining the present invention; [0008] FIG. 2 shows a basic circuit diagram for explaining the principle underlying the present invention; [0009] FIG. 3 shows a basic circuit diagram of a modification of the circuit shown in FIG. 2; [0010] FIG. 4 shows a more detailed circuit diagram of the circuit shown in FIG. 3; and [0011] FIGS. 5a and b show a block diagram of a practical circuit of the present invention; and [0012] FIG. 6 shows a more detailed circuit diagram of the circuit shown if FIG. 4. [0013] Referring now to FIG. 1, this shows a diagrammatic representation of a downhole measurement assembly 1 which comprises a plurality of individual tools 3 encased in non-magnetic stainless steel cylindrical pressure cases which are represented by the conductor 2 and which are mechanically and electrically connected together and also grounded. The tools 3 are supplied with DC power from a power supply 4 one side of which is also grounded. A stack 5 which includes an array of fluxgate magnetometers is located towards the bottom of the assembly 1 below the individual tools 3 but in this case other tools represented by the block 7 are shown in a position on the other side of the stack 5 from the power supply 4. This means that current for the other tools 7 has to be carried by conductors past the stack 5 and this in turn results in the generation of electromagnetic fields in the region of the stack 5 which causes in the output from the stack 5 to be inaccurate. [0014] It has been found that simply shielding the current carrying conductor in the region of the stack 5 is not possible, as the shield itself will cause disruption of the earth's magnetic field and also, as mentioned above, simply using a twisted pair or coaxial cable is also not sufficient if one wishes to retain the basic structure of the downhole assembly utilising the grounded pressure cases of the individual tools. [0015] The present invention proposes to use a twisted pair or coaxial cable for the current carrying conductors but which allows the pressure cases to be electrically connected together in such a manner that they carry no current. This is achieved by utilising a capacitor connected by a switching network as will be explained in relation to FIG. 2. [0016] As in the case of FIG. 1, one side of the DC supply 4 is connected to the pressure casing 1a of the downhole measurement assembly which is in turn grounded. A capacitor 20 is connected between the poles of a two-pole electronic changeover switch which in one position of the switch accumulates charge from the supply and, when the switch is changed over to its other position, allows the capacitor to transfer its charge to one or more downhole tools which are represented by the load 24. It can be seen that although the grounds are connected, no current can flow between them. Thus, all the current delivered to the downhole tools represented by the load 24 must flow in the conductors 25 and 26 connected between the electronic changeover switch 22 and the downhole tools. These conductors 25 and 26 thus carry the same current but in opposite directions and can be a twisted pair or coaxial cable which can be routed past the magnetometers in the stack without causing electromagnetic interference. When the changeover switch position is reversed quickly, the effect is enhanced and it is preferred to reverse the switch 22 several thousand times a second. [0017] FIG. 3 shows an improved arrangement of the circuit shown in FIG. 2. The improved arrangement employs two capacitors 20a and 20b each provided with its own two-pole electronic changeover switch 22a and 22b respectively. With this arrangement, when one capacitor is charging, the other is discharging. This arrangement is more efficient and reduces the ripple voltage between the conductors. This is an important consideration as the conductor 25 is also used as a communications bus in many downhole tool arrangements. [0018] FIG. 4 shows a more practical circuit diagram of the circuit shown in FIG. 3 and in FIG. 4 the two-pole changeover switches have been replaced by semiconductor switches in the form of MOSFETs. It is preferred to use P-channel MOSFETs for switches SW1 to SW4 and N-channel MOSFETs for switches SW5 to SW8. Although it is not shown in FIG. 4, an oscillator is provided in order to drive the MOSFET switches. The positive pole of the DC power supply 4 to the capacitor 20a is via MOSFET switch SW1 when closed and the other side of capacitor 20a is connected to the negative terminal of the DC power supply 4 via the MOSFET switch SW5. In this way, capacitor 20a charges. At the same time, capacitor 20b is connected to and discharges into the load via the closed MOSFET switches SW4 and SW8. On changeover, the switches SW1, SW5, SW4, and SW8 are opened and the switches SW2, SW6 close which allows the capacitor 20a to discharge through the load while the MOSFET switches SW3 and SW7 are also closed to allow the capacitor 20b to be charged from the power supply 4. [0019] FIGS. 5a and 5b when combined at conductors A-J show a block diagram of a practical circuit diagram for implementing the arrangement shown in FIG. 4. The same component numbers and designations are used in FIGS. 5a and 5b as are used in FIG. 4 and consequently a detailed description of the operation of FIG. 5 will not be given. Suffice to say that in FIG. 5a, the oscillator is now shown and represented by the reference numeral 50. In the present example, it operates at about 53 KHz and controls the MOSFETs SW1 to SW8. The frequency is not particularly critical but operation has been found to be optimal with a frequency in the region 50 KHz to 65 KHz. [0020] When the circuit is operating, the current taken by it is approximately 16 mA. This is due to the current required by the oscillator and amplifier and the current required to switch the MOSFETs, some of which have a gate capacitance of up to 1.5 nF. When it is merely required to pass current past the stack without the need for preventing magnetic interference, current can be saved by turning off the oscillator, in which case the upper and lower switches are short circuit. An on/off switch 51 in the form of a transistor is provided for this purpose. When a logic 1 from a control circuit 52 is applied to the gate of the transistor it is turned on and provides power to the oscillator. A logic 0 turns the transistor off and oscillation ceases. [0021] The output of the oscillator 50 is fed to a primary winding of a transformer 53 which has two centre-tapped secondary windings. The upper secondary of the transformer 53 then simply acts as a short circuit to connect the gates of the upper MOSFETS IC2-IC5 through their networks to ground via a resistor (R21) connected to the centre tap turning them on. Similarly, the lower MOSFETs IC6-IC9 are turned on by the lower secondary. [0022] In normal forward operation, that is with the power supply located above the tools in the downhole measurement assembly and providing current to the lower tools, current flows through transformer T3 (FIG. 5b) and inductor T1 (FIG. 5a) to R7 and R5. Should it be required for current to flow in the reverse direction, clearly the upper MOSFETs have to be turned on to provide current to R5 and R7. In the circuit shown in FIG. 5, this will always happen as the gates of the upper MOSFETs will be connected to the lower supply via R21. [0023] The output from the circuit shown in FIG. 5 is taken from the output terminals 55a and 55b which can be suitable for connection to a twisted pair of wires or a coaxial cable. [0024] The operational cycle is as follows: Continue reading... Full patent description for Electrical power supply arrangement for a downhole measurement assembly Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Electrical power supply arrangement for a downhole measurement assembly patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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