In hydrocarbon exploration and production, it is common to install permanent monitoring and/or control instrumentation downhole in a well. With an increase in the number of multi-zone and multi-lateral wells being developed together with multi-well subsea installations, downhole electronics apparatus consisting of multiple downhole electronic circuits for downhole monitoring and/or control all sharing a common downhole-to-surface power and/or communication cable are now being implemented. Instances of these systems are greatly increasing in downhole permanent monitoring and smart well control applications.

Downhole electronics apparatus which share resources are liable to complete failure in the event of a fault occurring in any one of the downhole electronic circuits.

For example, where a common voltage line is connected, via branch lines, to a number of downhole electronic circuits, if any electronic circuit develops a short circuit fault, the supply line voltage will be pulled down to zero volts and an expensive downhole network would be rendered inoperable.

One solution to this problem is in adding a protection circuit on each branch line between the shared cable and each downhole circuit. If a downhole electronic circuit fails and short circuits, then the protection circuit limits the current drawn by the faulty circuit thus allowing sufficient line voltage for the remaining downhole electronic circuits to function correctly, while limiting further damage to the short

circuited downhole electronics circuit. However, the major drawback to this arrangement is that the protection circuit electronics must be so reliable that network failure is extremely unlikely to occur as a result of an electronic circuit fault causing failure of the associated protection circuit. Protection circuit failures are likely during downhole electronic circuit fault conditions as the protection circuits may have to endure high voltages, high power dissipation and high temperature continuously as is the case with transistor based current limiting circuits or resistors.

It is an object of the present invention to provide a downhole electronics protection system which obviates or mitigates at least one of the aforementioned advantages.

This is achieved by inserting a suitable protection device into every branch supply line from the common shared resource line connected to a downhole electronic circuit, the protection device having a resistance which changes with the amount of current flowing through it.

According to a first aspect of the present invention, there is provided downhole electronics protection apparatus for protecting at least two downhole electronic circuit for downhole monitoring and/or control, the at least two downhole electronic circuit being located in an oil and/or gas well, the at least two downhole electronic circuit being coupled to a common supply line via at least two branch supply lines emanating from said common supply line wherein, at least two protection devices are disposed on the at least two branch supply lines between the common supply line and the at least two downhole electronic circuits, and the at least two protection devices each having a resistance which changes with the amount of current flowing through each of said at least two protection devices.

Preferably, the at least two protection devices are thermistors. Conveniently, the thermistors are Positive Temperature Coefficient (PTC) thermistors whose resistance increases with temperature, caused by the ambient temperature plus the self-heating due to the amount of current flowing through the thermister. More conveniently, the resistance increases logarithmically with current.

Preferably, at least two branch supply lines are terminated by said at least two protection devices.

This arrangement provides for redundancy in the apparatus wherein the at least two protection devices and downhole electronic circuits are physically in the same location performing the same function, and the inclusion of the at least two protection devices means that failure in one of the at least two downhole electronic circuits prevents that branch supply line acting as a short circuit on the common supply line and/or network communications.

Each downhole electronic circuit may be a control circuit. The control circuit may measure some instruments and/or control a tool or valve within the well. Additionally, the downhole electronic circuit may also be a communications transceiver to provide for data communication between the surface and the downhole electronic circuit.

According to a second aspect of the present invention, there is provided a method of protecting a downhole electronic circuit located in an oil and/or gas well, when a fault occurs in the downhole electronic circuit, said method comprising the steps of: disposing a first protection device on a first branch supply line from a common supply line in the well, disposing a second protection device on a second branch supply line from the common supply line in the well, coupling the first protection device to a first downhole electronic circuit,

coupling the second protection device to a second downhole electronic circuit, and in the event of a fault developing in the first downhole electronic circuit, a resistance of the first protection device increasing in response to the increase in current, power dissipation and internal temperature and limiting the current to the first downhole electronic circuit to protect the first downhole electronic circuit.

The method may include the further step of reducing current to the supply line after the fault has occurred to allow the protection device to re-set in the event that the fault was temporary and/or intermittent.

According to a third aspect of the present invention, there is provided a method of protecting a downhole electronic circuit located in an oil and/or gas well, so that the downhole electronic circuit continues to operate when a fault occurs in a second downhole electronics circuit sharing a common supply line, said method comprising the steps of: disposing a first protection device on a first branch supply line from the common supply line in the well, disposing a second protection device on a second branch supply line from the common supply line in the well, coupling the first protection device to a first downhole electronic circuit, coupling the second protection device to a second downhole electronic circuit, and in the event of a fault developing in the first downhole electronic circuit, increasing the resistance of the first protection device in response to the increase in current to limit the current drawn by the faulty first downhole electronic circuit and providing sufficient current to the second downhole electronics circuit for the second downhole electronic circuit to continue operating.

The method may include the further step of reducing current to the common supply line after the fault has occurred to allow the first protection device to re-set in the event that the fault was temporary or intermittent.

These and other aspects of the invention will become apparent from the following description when taken in combination with the accompanying drawings in which: Fig. 1 is a block diagram of a downhole electronics protection apparatus in accordance with an embodiment of the present invention; Fig. 2 is a block diagram of a downhole electronics protection apparatus in accordance with a preferred embodiment of the present invention, and Fig. 3 is a circuit diagram of a downhole electronic protection device arranged within a downhole electronics circuit in accordance with the preferred embodiment of the present invention.

Reference is first made to Fig. 1 of the drawings which depicts a downhole electronics protection apparatus generally indicated by reference numeral 10. A power supply unit (PSU) 12 is located outside a well at an accessible point for ease of maintenance. This is on the surface or, if the well was a subsea well, it would be positioned on the seabed. A common voltage supply line 14 is run from the PSU 12 into the well. At various locations on the common voltage supply line, branch supply lines 18a, 18b, 18c are connected at downhole nodes 16a, 16b, 16c. Three branch supply lines are shown in Fig. 1 but there could be any number incorporated.

Each branch supply line is connected to a current limiting protection device 20a, 20b, 20c respectively and in some cases the protection device 20a, 20b is coupled to a downhole electronic circuit 22a, 22b. Alternatively, the protection device 20c terminates the branch supply line 18c leaving a terminal 24 available for future downhole electronic circuits to be attached.

Reference is now made to Fig. 2 of the drawings which illustrates a downhole electronics protection apparatus, generally indicated by reference numeral 10, according to a preferred embodiment of the present invention. A power supply unit (PSU) and combined controller 112 are mounted on the surface of a well. A common voltage supply line 114 carries power and communication links to downhole tools 122a and 122b. The common voltage supply line 114 has branch supply lines 118a and 118b connected at nodes 116a, 116b. Each branch supply line 118a, 118b is connected through a respective PTC (Positive Temperature Coefficient) thermistor 120a, 120b to a respective downhole pressure tool 122a, 122b. The thermistors 120a, 120b are model number B59870-C160-A70 manufactured by Siemens.

The downhole pressure tool 122a, 122b includes a downhole electronic circuit, generally indicated by reference numeral 121, illustrated in Fig. 3. Common supply line 114 from power supply unit 112 is interrupted at node 116 to form branch supply line 118, which is connected to thermistor 120. The other side of thermistor 120 is connected to the input of circuit 121.

The output 123 of the circuit 121 forms the current return to the PSU 112. Circuit 121 draws its power through thermistor 120. The line voltage is regulated down to 5V by regulators 126 and 128. The pressure transducer outputs are fed into microcontroller 124 (type 87C51). The pressure and temperature are measured and the digital valves transmitted serially by the microcontroller 124. The data is capacitively coupled onto the supply line 118 through capacitor 130. At the power supply 112 the digital information is read by a reciprocal capacitive coupling circuit, not shown.

In use, the control circuit 121 draws the required current through the thermistor 120. In the event that a fault occurs within the control circuit 121, for instance the 87C51 chip 124 ceases to operate, heat is dissipated

in the chip 124 causing other components to fail, a short circuit develops in the circuit 121 pulling increased current through the thermistor 120. Thermistor 120 raises its resistance logarithmically as the current flow being drawn through it increases and ultimately limits the current flow at a predetermined level. This level is chosen to limit further damage occurring to the control circuit while leaving sufficient current in the common supply line 118 to keep other electronic circuits in other branch supply lines 114 in operation.

It will be appreciated that the principal advantage of the present invention is that failure of one downhole electronic circuit, within a downhole system sharing resources, will not cause complete failure of the system because current to the faulty circuit is limited, leaving sufficient supply for other circuits to draw sufficient current to operate normally. The use of a PTC thermistor as the protection device has advantages in that it is simple, with less fault possibilities, when compared with transistor based current limiting circuits; it dissipates low power when compared to current limiting resistors; it is fundamentally designed to cope with high continuous temperatures seen in oil wells and it is resettable, unlike thermal fuses, which means that a downhole circuit subject to intermittent faults can recover.

It will be appreciated that various modifications may be made to the network hereinbefore described without departing from the scope of the invention. The power supply unit and communications control may be located within the well. Further electronic circuits may be added and some circuits may activate valves as opposed to tools.