Downhole stimulation tools and related methods of stimulating a producing formation

1. A downhole stimulation tool, comprising:

a substantially monolithic outer housing comprising:

two regions free of apertures extending therethrough; and

an additional region longitudinally intervening between the two regions and exhibiting apertures extending therethrough;

opposing propellant structures substantially longitudinally confined within the two regions of the outer housing and each individually comprising:

at least one higher combustion rate propellant region; and

at least one lower combustion rate propellant region longitudinally adjacent the at least one higher combustion rate propellant region; and

at least one initiator adjacent each of the opposing propellant structures,

wherein the additional region of the substantially monolithic outer housing is substantially free of propellant structures contained therein prior to the ignition of the opposing propellant structures.

2. The downhole stimulation tool of claim 1 , wherein the at least one higher combustion rate propellant region comprises a plurality of higher combustion rate propellant regions, and the at least one lower combustion rate propellant region comprises a plurality of lower combustion rate propellant regions.

3. The downhole stimulation tool of claim 2 , wherein at least one of the plurality of higher combustion rate propellant regions exhibits a different volume of propellant than at least one other of the plurality of higher combustion rate propellant regions, and at least one of the plurality of lower combustion rate propellant regions exhibits a different volume of another propellant than at least one other of the plurality of lower combustion rate propellant regions.

4. The downhole stimulation tool of claim 1 , wherein each of the opposing propellant structures exhibits substantially the same longitudinal sequence of the at least one higher combustion rate propellant region and the at least one lower combustion rate propellant region extending in opposite directions from locations proximate a lateral axis of the outer housing.

5. The downhole stimulation tool of claim 1 , wherein each of the opposing propellant structures exhibits an alternating sequence of the at least one higher combustion rate propellant region and the at least one lower combustion rate propellant region beginning with the at least one higher combustion rate propellant region at a location proximate a lateral axis of the outer housing.

6. The downhole stimulation tool of claim 1 , wherein each of the opposing propellant structures further comprises at least one additional propellant region exhibiting a combustion rate different than combustion rates of the at least one higher combustion rate propellant region and the at least one lower combustion rate propellant region.

7. The downhole stimulation tool of claim 1 , wherein the at least one higher combustion rate propellant region of each of the opposing propellant structures exhibits at least one different volume of propellant than the at least one lower combustion rate propellant region of each of the opposing propellant structures.

8. The downhole stimulation tool of claim 1 , wherein the at least one initiator comprises:

a first initiator adjacent an end of one of the opposing propellant structures proximate a lateral axis of the outer housing; and

a second initiator adjacent an opposing end of another of the opposing propellant structures proximate the lateral axis of the outer housing.

9. The downhole stimulation tool of claim 1 , wherein the at least one initiator comprises a plurality of initiators adjacent the at least one higher combustion rate propellant region and the at least one lower combustion rate propellant region of each of the opposing propellant structures.

10. The downhole stimulation tool of claim 1 , wherein:

the additional region is located proximate a lateral axis of the outer housing; and

the outer housing further comprises at least two other regions each more longitudinally distal from the lateral axis of the substantially monolithic outer housing than the two regions and each exhibiting additional apertures extending therethrough.

11. A downhole stimulation tool, comprising:

an outer housing comprising:

a first structure free of apertures laterally extending therethrough;

a second structure free of apertures laterally extending therethrough; and

a third structure connected to and longitudinally interposed between the first structure and the second structure and exhibiting apertures extending therethrough;

a propellant structure substantially longitudinally confined within the first structure of the outer housing and comprising:

at least one higher combustion rate propellant region; and

at least one lower combustion rate propellant region adjacent the at least one higher combustion rate propellant region;

another propellant structure opposing the propellant structure and substantially longitudinally confined within the second structure of the outer housing, the another propellant structure comprising:

at least one other higher combustion rate propellant region; and

at least one other lower combustion rate propellant region adjacent the at least one other higher combustion rate propellant region; and

initiators adjacent each of the propellant structure and the another propellant structure,

wherein the additional region of the outer housing is substantially free of propellant structures contained therein prior to the ignition of the propellant structure and the another propellant structure.

12. The downhole stimulation tool of claim 11 , wherein the at least one higher combustion rate propellant region of the propellant structure exhibits substantially the same combustion rate as the at least one other higher combustion rate propellant region of the another propellant structure, and the at least one lower combustion rate propellant region of the first propellant structure exhibits substantially the same combustion rate as the at least one other lower combustion rate propellant region of the another propellant structure.

13. The downhole stimulation tool of claim 11 , wherein the at least one higher combustion rate propellant region of the propellant structure and the at least one other higher combustion rate propellant region of the another propellant structure each comprise a first propellant, and the at least one lower combustion rate propellant region of the propellant structure and the at least one other lower combustion rate propellant region of the another propellant structure each comprise a second, different propellant.

14. The downhole stimulation tool of claim 11 , wherein the at least one higher combustion rate propellant region of the propellant structure and the at least one other higher combustion rate propellant region of the another propellant structure each comprise at least one mutually different propellant.

15. The downhole stimulation tool of claim 11 , wherein the at least one lower combustion rate propellant region of the propellant structure and the at least one other lower combustion rate propellant region of the another propellant structure each comprise at least one mutually different propellant.

16. The downhole stimulation tool of claim 11 , wherein a sequence of the at least one higher combustion rate propellant region and the at least one lower combustion rate propellant region exhibited by the propellant structure is substantially the same as a sequence of the at least one other higher combustion rate propellant region and the at least one other lower combustion rate propellant region exhibited by the another propellant structure.

17. The downhole stimulation tool of claim 11 , wherein the at least one higher combustion rate propellant region of the propellant structure and the at least one other higher combustion rate propellant region of the another propellant structure exhibit substantially the same volume of propellant and substantially the same transverse cross-sectional area as one another, and the at least one lower combustion rate propellant region of the propellant structure and the at least one other lower combustion rate propellant region of the another propellant structure exhibit substantially the same volume and substantially the same transverse cross-sectional area of another propellant as one another.

18. The downhole stimulation tool of claim 11 , wherein at least one of the initiators is adjacent at least one of end of the propellant structure, and at least one other of the initiators is adjacent at least one of end of the another propellant structure.

19. The downhole stimulation tool of claim 11 , wherein at least one of the initiators is located between adjacent propellant regions of the propellant structure, and at least one other of the initiators is located between adjacent propellant regions of the another propellant structure.

20. A method of stimulating a producing formation, the method comprising: positioning a downhole stimulation tool within a wellbore intersecting the producing formation, the downhole stimulation tool comprising:

a substantially monolithic outer housing comprising:

two regions free of apertures extending therethrough; and

an additional region longitudinally intervening between the at least two regions and exhibiting apertures extending therethrough;

opposing propellant structures substantially longitudinally confined within the at least two regions of the outer housing and each individually comprising:

at least one higher combustion rate propellant region; and

at least one lower combustion rate propellant region adjacent the at least one higher combustion rate propellant region; and

at least one initiator adjacent each of the opposing propellant structures; and

initiating each of the opposing propellant structures to combust the opposing propellant structures and vent produced combustion gases through the apertures in the outer housing to increase pressure adjacent to and within the producing formation,

wherein the additional region of the substantially monolithic outer housing is substantially free of propellant structures contained therein prior to the ignition of the opposing propellant structures.

21. The method of claim 20 , wherein initiating each of the opposing propellant structures comprises initiating at least one region of each of the opposing propellant structures in sequence with at least one other region of each of the opposing propellant structures.

22. The method of claim 20 , wherein initiating each of the opposing propellant structures comprises initiating at least one end of each of the opposing propellant structures.

23. The method of claim 20 , wherein initiating each of the opposing propellant structures comprises initiating the opposing propellant structures to produce a pressure profile in excess of hydrostatic wellbore pressure adjacent the producing formation to exhibit a plurality of pressure rises and a plurality of pressure falls over a period of time.

24. The method of claim 23 , further comprising producing the pressure profile for a duration of greater than or equal to about one second.

25. The method of claim 23 , further comprising producing the pressure profile for a duration of greater than or equal to about sixty seconds.

26. The method of claim 20 , further comprising anchoring the downhole stimulation tool into position within the wellbore.

27. The method of claim 20 , further comprising physically containing the increased pressure within an interval of the wellbore adjacent the producing formation.