Multi-stage propellant charge for downhole setting tools

1. An actuator for setting a downhole tool, the actuator comprising:

a first propellant configured to be ignited by an igniter, wherein the first propellant comprises a first carbon source; and

a second propellant configured to be ignited by combustion of the first propellant, the first propellant abutting the second propellant such that the first propellant covers an axial end of the second propellant, wherein the first propellant is configured to combust more quickly than the second propellant, wherein the second propellant comprises a second carbon source, wherein the first and second carbon sources are different, at least one of the first carbon source and the second carbon source are fibrous, and wherein the actuator is connected to the downhole tool.

2. The actuator of claim 1 , wherein the first propellant comprises a first weight percentage of an oxidizing agent, and wherein the second propellant comprises a second weight percentage of an oxidizing agent, the first weight percentage being greater than or equal to the second weight percentage.

3. The actuator of claim 2 , wherein the oxidizing agent of the first propellant is configured to ignite more rapidly than the oxidizing agent of the second propellant.

4. The actuator of claim 1 , further comprising a sleeve in which the first and second propellants are positioned, the first and second propellants being pressed axially together within the sleeve, the first and second propellants each extending to an inner diameter surface of the sleeve.

5. The actuator of claim 4 , wherein the first propellant is visible from a first end of the sleeve and the second propellant is not visible from the first end, and wherein the second propellant is visible from a second end of the sleeve, and the first propellant is not visible from the second end.

6. The actuator of claim 4 , wherein the first propellant is configured to block a flame generated by operation of the igniter from reaching the second propellant.

7. The actuator of claim 4 , further comprising a charge housing that is connected to the igniter, wherein the sleeve and the first and second propellants are positioned in the charge housing, and wherein, when initiated, the igniter fires a flame into the charge housing that ignites the first propellant.

8. The actuator of claim 4 , wherein the sleeve comprises a plurality of circumferentially-wrapped layers having axially-extending seams, the axially-extending seams of adjacent layers being circumferentially offset from one another.

9. The actuator of claim 1 , wherein the igniter comprises an electrically-resistive heating element that is in direct contact with the first propellant.

10. The actuator of claim 1 , wherein the first propellant and the second propellant occupy a volume, the second propellant occupying from about 80% to about 99% of the volume.

11. The actuator of claim 1 , further comprising a piston, wherein igniting the first and second propellants generates a pressure differential across the piston that causes the piston to move and set the downhole tool.

12. The actuator of claim 11 , further comprising an igniter comprising:

a sleeve at least partially filled with a charge;

a head threaded into connection with the sleeve and extending radially outward therefrom;

an insulator tube received through the head into the sleeve;

a conductor received through the insulator tube and including a shoulder that is too large to fit through the insulator tube;

a resistor positioned in the sleeve and electrically connected to the conductor and to the sleeve, so as to form an electrical path from the conductor to the sleeve, wherein current conducted to the sleeve is conducted therefrom to ground;

a seal positioned between the head and the shoulder;

a circlip coupled to the conductor, the circlip being sized such that the circlip bears against and does not proceed through the insulator tube; and

a cap positioned on an end of the sleeve, wherein at least the cap and the head cooperate to contain the charge within the sleeve.

13. The actuator of claim 1 , wherein an oxidizing agent in the first propellant has a greater percentage by weight than an oxidizing agent in the second propellant, wherein the oxidizing agent in the first propellant comprises from about 60 wt % to about 90 wt % potassium perchlorate, and wherein the oxidizing agent in the second propellant comprises from about 50 wt % to about 80 wt % potassium nitrate.

14. The actuator of claim 1 , further comprising a charge housing having the first propellant, the second propellant, or both therein, wherein the charge housing defines a radial pressure port having a plug therein, and wherein the plug is configured to be removed from the pressure port to relieve pressure within the charge housing after the first and second propellants have been combusted in a well and after the actuator is removed from a well.

15. The actuator of claim 1 , further comprising a charge housing having the first propellant, the second propellant, or both therein, wherein the charge housing defines a radial pressure port having a valve therein, and wherein the valve comprises a one-way valve that vents gas from within the charge housing in the well in response to combustion of the first and second propellants.

16. The actuator of claim 1 , wherein the first propellent entirely covers an axial end of the second propellant, such that a flame emitted by the igniter reaches the first propellant and is prevented from directly impinging upon the second propellent.

17. A method comprising:

igniting a first propellant by applying an electrical current to an igniter, wherein igniting the first propellant causes the first propellant to combust, which causes a second propellant to combust to actuate a downhole tool, wherein the first propellant covers an axial end of the second propellant wherein the first propellant is configured to combust more quickly than the second propellant, wherein the first propellant comprises a first carbon source, wherein the second propellant comprises a second carbon source, wherein the first and second carbon sources are different, and wherein the first carbon source, the second carbon source, or both are fibrous.

18. The method of claim 17 , wherein the igniter comprises an electrically-resistive heating element, the method further comprising connecting the heating element directly to the first propellant such that the first propellant is ignited without using a flame from the igniter to cause ignition, wherein applying the electrical current to the igniter causes the heating element to ignite the first propellant.

19. The method of claim 17 , wherein applying the electrical current to the igniter causes the igniter to emit a flame that impinges on the first propellant, wherein the first propellant covering the axial end of the second propellant blocks the flame from reaching the second propellant.

20. The method of claim 17 , wherein combusting the second propellant generates a pressure differential cross a piston, which causes the piston to move and thereby actuate the downhole tool.

21. The method of claim 17 , further comprising positioning the first and second propellants in a cylindrical sleeve, the first and second propellants both extending to an inner diameter of the sleeve.

22. The method of claim 21 , further comprising forming the sleeve by wrapping a plurality of layers circumferentially such that seams of the plurality of layers are axially extending, wherein the seams of radially-adjacent layers are circumferentially offset from one another.

23. The method of claim 21 , wherein the second propellant occupies between about 80% and about 95% of a volume of the sleeve, and wherein the first propellant occupies between about 5% and about 20% of the volume of the sleeve.

24. A setting tool for actuating a downhole tool, comprising:

a charge assembly comprising:

a charge housing;

a cylindrical sleeve positioned in the charge housing;

a first propellant positioned in the cylindrical sleeve and having an outer diameter surface that contacts an inner diameter surface of the cylindrical sleeve; and

a second propellant positioned in the cylindrical sleeve and having an outer diameter surface that contacts the inner diameter surface of the cylindrical sleeve, wherein the first propellant entirely covers an end surface of the second propellant, wherein the first propellant is positioned axially between an igniter and the second propellant wherein the second propellant occupies a greater volume than the first propellant occupies, and wherein the first propellant is configured to combust more rapidly than the second propellant wherein the first propellant comprises a first carbon source, wherein the second propellant comprises a second carbon source, wherein the first and second carbon sources are different, and wherein the first carbon source, the second carbon source, or both are fibrous; and

a piston assembly comprising a piston that is configured to slide in response to a pressure applied by combustion of at least the second propellant to actuate the downhole tool.

25. The setting tool of claim 24 , further comprising the igniter, the igniter comprising:

an igniter housing having a flame port, wherein the charge housing is coupled to the igniter housing; and

an igniter positioned in the igniter housing, the igniter configured to generate the flame that is directed out of the igniter housing via the flame port.