Thermal generator for downhole tools and methods of igniting and assembly
View Patent ↗An ignition device for a downhole tool includes a body with a cavity, a thermite material and a resistive element within the cavity. The resistive element, which is heated to the high temperature needed for ignition of the thermite material, has a non-galvanic outer surface at the ignition temperature. A large amount of electrical power is required to reach the ignition temperature, making the device safe and unlikely to accidentally ignite. The device has an electrical connector that is located within a sealed chamber when downhole.
1. An ignition device for a downhole tool, comprising:
a) a body having a cavity therein;
b) a thermite material in the cavity;
c) a resistive element located inside the cavity and having an outer surface in contact with the thermite material, the resistive element outer surface being non-galvanic at an ignition temperature of the thermite material, wherein the thermite material initiates an exothermic reaction when the resistive element is heated by a current of 0.75-1.5 amps.
2. The ignition device of claim 1 wherein the resistive element comprises a wire-wound resistor with a coating of ceramic.
3. The ignition device of claim 1 wherein the resistive element comprises a wire-wound resistor with a coating of enamel.
4. The ignition device of claim 1 wherein the resistive element comprises a length of nickel-chromium wire.
5. The ignition device of claim 1 wherein the thermite material comprises cupric oxide and aluminum.
6. The ignition device of claim 1 wherein the thermite material comprises iron oxide and aluminum.
7. The ignition device of claim 1 wherein the body has first and second ends, with the first end having an electrical plug and the second end having an opening that communicates with the cavity and the thermite material therein.
8. An ignition assembly for a downhole tool, comprising:
a) a first sub having first and second ends and a passage extending between the first and second ends;
b) an ignition device located in the passage, the ignition device having a thermite material and a resistive element located so as to be in contact with the thermite material, and having an electrical plug extending toward the first sub first end;
c) a second sub having an electrical receptacle, the second sub located in the passage so that the electrical receptacle receives the electrical plug to form an electrical connection;
d) a first seal between the ignition device and the first sub and second seal between the second sub and the first sub, the electrical connection located between the first and second seals.
9. The ignition assembly of claim 8 wherein the electrical plug is a banana plug.
10. The ignition device of claim 8 , further comprising:
a) a length of detonating cord located adjacent to the first sub second end;
b) at least one intermediate charge interposed between the detonating cord and the first sub second end.
11. A method of assembling an ignition assembly for use in a downhole tool, comprising the steps of:
a) providing an electrical ignition device having a first end with an electrical plug and a second end with an aperture for the exit of ignition products;
b) providing a seal in contact with and around the ignition device;
c) inserting the ignition device into a first sub so as to form a first seal between the ignition device and the sub;
d) securing the ignition device within the first sub;
e) providing a second sub with an electrical receptacle;
f) inserting the second sub into the first sub so that the receptacle receives the plug and so as to form a second seal between the second sub and the first sub.
12. The method of claim 11 wherein the step of inserting the second sub into the first sub further comprising the step of screwing the second sub into the first sub.
13. A method of igniting an ignition device on a downhole tool, comprising the steps of:
a) lowering a tool containing the ignition device downhole on a conductive wireline, the ignition device containing a thermite material having an ignition point of greater than 900° F.;
b) flowing, through the wireline, and into a resistive element in the ignition device, at least 25 watts of electrical power until the thermite material ignites.
14. An ignition device for use with a downhole tool, comprising:
a) a body having a cavity therein;
b) a thermite material in the cavity;
c) a high temperature resistive wire element located inside of the cavity and in contact with the thermite material;
d) a power supply electrically connected with the resistive wire element, the power supply providing a voltage that is regulated so as to maintain the continuity of the resistive wire element and providing an electrical power that is between 25 and 75 watts.