Electrical power supply arrangement for a downhole measurement assembly
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.
The present invention relates to downhole measurement assemblies and more particularly to the electrical power supply arrangement for such assemblies.
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.
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.
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.
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.
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:—
Referring now to
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.
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
As in the case of
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.
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.
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 (
The output from the circuit shown in
The operational cycle is as follows:
The charging path from the positive to capacitor 20a is from an input socket 51 to transformers T3 to inductor T1 to IC2/D2 to IC2/S2 to IC2/S1 to IC2/D1 to C20a.
The return path from the lower end of capacitor 20a is IC9/D1 to IC9/S1 to IC9/S2 to IC9/D1 to inductor T1 to socket 51.
Capacitor 20a is charged up when the MOSFETs in IC2 and IC9 are switched on.
When capacitor 20a is charging up, capacitor 20b is discharging into the load via IC4, IC6, inductor T4 and terminals 55a and 55b.
The discharge path for capacitor 20a is via IC5, IC8, inductor T4 and terminals 55a and 55b.
When capacitor 20a is discharging, capacitor 20b is being charged.
The charge path for capacitor 20b is similar to that for capacitor 20a but via IC3 and IC7.
The transformer secondaries switch the MOSFETs on and off in the correct sequence to charge and discharge the capacitors 20a and 20b. When the gate of a P-Channel MOSFET is negative with respect to its source, then that MOSFET is turned on. The upper MOSFETs, IC2, IC5, IC3 and IC4 are dual P-channel devices. When the gate of a N-Channel MOSFET is positive with respect to its source, then that MOSFET is turned on. The lower MOSFETs IC9, IC8, IC7 and IC4 are dual N-channel devices.
Consider the P-channel MOSFETs.
The output transformer 53 from the oscillator 50 has two centre tapped secondaries. One secondary controls the P-channel devices. The other secondary controls the N-channel devices. To save space, only the operation of the P-channel devices will be described.
Each of the sources is connected to the centre tap secondary through a 100 nF capacitor (C2, C5, C3 and C4). The capacitor gives DC isolation, but allows AC to pass through. If they were not there, then all the MOSFET sources would be connected together, rendering the circuit non-operational.
The upper half of the top secondary winding drives the gates of IC5 and IC3, while the lower half of the top secondary winding drives the gates of IC2 and IC4.
The gate drive to the MOSFETs is via drive circuits 62-69 each of which consists of a diode and resistor connected in parallel. The components compensate for a difference between the turn-on and turn-off delays in the MOSFETs. These delays, were they to occur, would mean that some MOSFETs are turned on for a short time when they should be turned off. The effect would be a small amount of ground current resulting in an imbalance in the currents in the twisted pair or co-axial cable.
This circuit has been designed to operate over a wide temperature range up to 165° C. and has been found to perform satisfactorily.
It is known that power may be passed up and down a downhole tool so as to allow communication between the tools and transmission of signals to the surface. The present invention preferably utilises the 1553 signal transmission and reception to supply power throughout the tool. However it will be appreciated that other communication standards may be utilised.
The type of communication we are dealing with is where a signal is superimposed on the supply line. It is fed into transmitter/receivers through a transformer. Referring to
Claims
1. 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.
2. An assembly according to claim 1 wherein the switching circuit includes a plurality of electrically controlled switches.
3. An assembly according to claim 2 wherein the electrically controlled switches are MOSFETs.
4. An assembly according to claim 1 wherein the control circuit is an oscillator.
5. An assembly according to claim 4 wherein a switch is provided for switching off the oscillator.
6. An assembly according to claim 4 wherein the oscillator operates in the range of 50-65 KHz.
7. An assembly according to claim 1 wherein there are two capacitors connected to the power supply by switches such that as one capacitor is connected to the power supply so as to be charged, the other capacitor is connected to said at least one tool section so as to provide power thereto.
8. An assembly according to claim 1 wherein the conductor arrangement comprises a twisted pair of conductors.
9. An assembly according to claim 1 wherein the conductor arrangement is a coaxial cable.
Type: Application
Filed: Feb 18, 2004
Publication Date: Oct 19, 2006
Inventor: Percival Williams (Gloucestershire)
Application Number: 10/546,056
International Classification: H01H 47/00 (20060101);