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FIELD OF THE INVENTION 
     The present invention relates to a well stimulation tool for oil and/or gas production. More specifically, the invention is a hydraulically-actuated propellant stimulation downhole tool for use in a hydrocarbon well. 
     BACKGROUND 
     In hydrocarbon wells, fracturing (or “fracing”) is a technique used by well operators to create and/or extend a fracture from the wellbore deeper into the surrounding formation, thus increasing the surface area for formation fluids to flow into the well. Fracing may be done by either injecting fluids at high pressure (hydraulic fracturing), injecting fluids laced with round granular material (proppant fracturing), or using explosives to generate a high pressure and high speed gas flow (TNT or PETN up to 1,900,000 psi) known as propellant stimulation. 
     Gas generating propellants have been utilized in lieu of hydraulic fracturing techniques as a more cost effective manner to create and propagate fractures in a subterranean formation. In accordance with conventional propellant stimulation techniques, a propellant is ignited to pressurize the perforated subterranean interval either simultaneous with or after the perforating step so as to propagate fractures therein. Typically, the propellant material is ignited due to shock, heat, and/or pressure generated from a detonated charge. Upon burning, the propellant material generates gases that clean perforations created in the formation by detonation of the shaped charge and which extend fluid communication between the formation and the wellbore. 
     SUMMARY 
     In one embodiment there is provided a downhole tool comprising a detonation section for stimulating a hydrocarbon-producing formation. The detonation section comprises a first end, a second end, a propellant volume located proximate to the second end, and a wall. The wall has an inner surface, an outer surface, a rupture disc and an actuating assembly. The inner surface defining a central bore extending from the first end to the second end. The outer surface is exposed to a well annulus during operation of the downhole tool. The actuating assembly comprises a detonator chamber, a detonator assembly, a firing pin and a flow passage. The detonator chamber has a first end positioned adjacent to the propellant volume and a second end having an inlet. The detonator assembly is located within the detonator chamber proximate to the first end of the detonator chamber. The firing pin is located within the detonation chamber. The firing pin is retained proximate to the inlet until an actuating pressure is applied through the inlet. The flow passage is contained between the inner surface and the outer surface and is in fluid flow communication with the detonation chamber through the inlet. The rupture disc is positioned between the flow passage and the central bore such that it prevents fluid flow communication between the flow passage and the central bore until ruptured by the application of the actuating pressure in the central bore. 
     Additionally, in the above-described downhole tool, the flow passage can be contained between the inner surface and the outer surface so as to be entirely interior to the wall. The firing pin can be retained proximate to the inlet by a shear pin such that the shear pin holds the firing pin back from the detonator until the actuating pressure is applied through the inlet. 
     In a further embodiment of the above-described downhole tool, the wall can have a plurality of actuating assemblies spaced about the circumference of the wall. The flow path of each actuating assembly can be in fluid flow communication with a circumferential chamber, which is in fluid flow communication with the central bore when the rupture disc is ruptured such that fluid is distributed to each flow path through the circumferential chamber. Additionally, there can be no more than one rupture disc associated with the circumferential chamber and the plurality of actuating assemblies. 
     In another embodiment of the above described downhole tool, there can be a plurality of detonation sections arranged sequentially such that the central bore of each section aligns to form a continuous central bore running through the plurality of sections. 
     In still yet another embodiment, there is a downhole tool comprising a detonation section for stimulating a hydrocarbon-producing formation. The detonation section comprises a first end, a second end, a propellant volume and a wall. The propellant volume is located proximate to the second end. The wall has an inner surface and an outer surface. The inner surface defines a central bore extending from the first end to the second end. The outer surface is exposed to a well annulus during operation of the downhole tool. The wall is comprised of a first wall element connected to a second wall element so as to form a circumferential chamber running circumferentially through the wall. The first wall element having a plurality of actuating assemblies. Each actuating assembly comprises a detonator, a detonator assembly, a firing pin and a flow path. The detonator chamber having a first end positioned adjacent to the propellant volume and a second end having an inlet. The detonator assembly is located within the detonator chamber proximate to the first end of the detonator chamber. The firing pin is located within the detonation chamber. The firing pin is retained proximate to the inlet until an actuating pressure is applied through the inlet. The first flow passage is contained between the inner surface and the outer surface and extends from the circumferential chamber to the inlet of the detonation chamber. The first flow path is in fluid flow communication with the detonation chamber through the inlet and is in fluid flow communication with the circumferential chamber. The second wall element has a second flow passage extending from the circumferential chamber to the inner surface so as to provide fluid flow communication between the central bore and the circumferential chamber. The rupture disc is positioned in the second flow passage such that the rupture disk prevents fluid flow communication between the circumferential chamber and the central bore until ruptured by the application of the actuating pressure in the central bore. 
     In the above-described downhole tool, the rupture disc can be positioned adjacent to the inner surface. Also, the first flow passage and the second flow passage can be contained between the inner surface and the outer surface so as to be entirely interior to the wall. Additionally, there can be a plurality of detonation sections arranged sequentially such that the central bore of each section aligns to form a continuous central bore running through the plurality of sections. 
     In a further embodiment of the above-described downhole tool, the firing pin can be retained proximate to the inlet by a shear pin such that the shear pin holds the firing pin back from the detonator until the actuating pressure is applied through the inlet. 
     In still another embodiment, there is provided a method comprising:
         (a) introducing a casing string into a wellbore extending through at least one subterranean region having hydrocarbon deposits, wherein the casing string comprises a tubular wall defining an annular region between the tubular wall and the wellbore, and a central bore, which extends through at least one detonation section;   (b) increasing the pressure in the central bore such that rupture discs located within the tubular wall are ruptured thus detonating a propellant volume such that the subterranean region around the wellbore is fractured.       

     Further, the detonation can be accomplished by an increase in pressure carried out under substantially static downhole tool conditions to rupture the rupture disc. The method can further comprise after step (a) and prior to step (b), introducing cement into the annular region to thus cement the casing in the wellbore. Also, step (b) can further comprise perforating the cement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a detonation section of a downhole tool in accordance with an embodiment. 
         FIG. 2  is a sectional elevation through section line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a sectional elevation through section line  3 - 3  of  FIG. 1 . 
         FIG. 4  is an enlargement of the circumferential flow channel section of the embodiment of  FIG. 1 . 
         FIG. 5  is an enlargement of the rupture disc section of the embodiment of  FIG. 1 . 
         FIG. 6  is an enlargement of the firing pin retainment section of the embodiment of  FIG. 1   
         FIG. 7  is a sectional elevation of the pressure chamber and firing pin of the embodiment of  FIG. 1  prior to actuation of the firing pin. 
         FIG. 8  is a sectional elevation of the pressure chamber and firing pin of the embodiment of  FIG. 1  after actuation of the firing pin. 
         FIG. 9  is an illustration of a downhole tool comprising a casing string utilizing an embodiment of the invention; the downhole tool having been lowered into a wellbore. 
         FIG. 10  is an illustration of the downhole tool of  FIG. 9  after cementing of the casing string within the wellbore. 
         FIG. 11  is an illustration of the downhole tool of  FIGS. 9 and 10  after firing of the propellant. 
     
    
    
     DETAILED DESCRIPTION 
     In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawings are not necessarily to scale and the proportions of certain parts have been exaggerated to better illustrate details and features of the invention. In the following description, the terms “upper,” “upward,” “lower,” “below,” “downhole” and the like as used herein shall mean in relation to the bottom or furthest extent of the surrounding wellbore even though the well or portions of it may be deviated or horizontal. The terms “inwardly” and “outwardly” are directions toward and away from, respectively, the geometric center of a referenced object. Where components of relatively well-known designs are employed, their structure and operation will not be described in detail. 
     Turning now to  FIG. 1 , one embodiment of a detonation section  10  for a downhole tool is illustrated. Detonation section  10  is comprised of a wall  12 . Wall  12  typically is a cylindrical wall having an inner surface  14  and an outer surface  16 . Inner surface  14  defines a central bore  18 , typically a cylindrical bore, extending from a first end  20  to a second end  22  of detonation section  10 . As can be seen from  FIG. 1 , central bore  18  extends continuously through detonation section  10 . Outer surface  16  is exposed to the well annulus during operation of the downhole tool in a wellbore. The well annulus is the region between the downhole tool and the wellbore wall or the inner casing wall of the wellbore. Additionally, first end  20  is configured to connect to other components of the downhole tool or a casing string and second end  22  can be configured to connect to additional components of the downhole tool or a casing string. 
     Generally, detonation section  10  and wall  12  will be made up of one or more wall elements or sleeves. As illustrated, detonation section  10  has first wall element or first sleeve  26 , and second wall element or second sleeve  28 . First sleeve  26  and second sleeve  28  are configured such that when connected they form circumferential flow channel  30 , which can better be seen with reference to  FIGS. 3 and 4 . O-rings  29  provide a fluid tight seal between first sleeve  26  and second sleeve  28 . As can be seen from  FIG. 3 , circumferential flow channel  30  extends circumferentially around the interior of wall  12  such that it is entirely interior to wall  12 . Circumferential flow channel  30  is in fluid flow connection via flow passage  32  to a rupture disc chamber  34 . Flow passage  32  is entirely interior to wall  12 . As used herein, “entirely interior to wall  12 ” means residing within wall  12  so as not to have a flow passage or channel wall in addition to the wall  12  wherein such separate flow passage or channel wall would be exposed to the interior central bore  10  or the annular region  74  (see  FIG. 9 ). Thus, “entirely interior to wall  12 ” excludes tubes or passages running along inner surface  14  or outer surface  16  of wall  12 . 
     Rupture disc chamber  34 , which can be better seen with reference to  FIG. 5 , can be accessed through a plug  36  accessible from and forming a part of outer surface  16 . In operation of the downhole tool, rupture disc chamber  34  will be sealed by plug  36  such that rupture disc chamber  34  is entirely interior to wall  12 . Rupture disc  38  can be positioned adjacent to inner surface  14  of wall  12 . Rupture disc  38  provides a second seal for rupture disc chamber  34  such that, when in place, rupture disc  38  prevents fluid flow communication between flow passage  32  and central bore  18  through rupture disc chamber  34 . When rupture disc  38  is ruptured by a predetermined pressure within central bore  18 , fluid flow communication is established between flow passage  32  and central bore  18 . In an additional embodiment, rupture disc chamber  34  and flow passage  32  are not used, and the rupture disc is located in the first wall element  26  at the circumferential flow channel so that the rupture disc is directly between the circumferential flow channel  30  and central bore  18 . In another embodiment, multiple rupture discs are associated with circumferential flow channel  30 ; typically, with a flow passage and rupture disc chamber also associated with each rupture disc. However, it is presently preferred and considered advantageous that there is no more than one rupture disc associated with the circumferential flow channel  30 . 
     Returning to  FIG. 1 , a propellant region  40  of wall  12  comprises a ported sleeve  48  and a portion of wall  12  which serves as an internal sidewall  42  of the propellant region  40 . A cylindrical propellant volume  44  is adjacent to and between the internal sidewall  42  and ported sleeve  48 . Ported sleeve  48  has a plurality of circular pressure ports  46  (shown in  FIGS. 9, 10 and 11 ) therein to direct and shape the gases and emissions generated during detonation of the propellant volume  44 . Typically ports  46  are spaced equally radially around ported sleeve  48 . 
     As can be seen with reference to  FIGS. 1, 2, 6, 7 and 8 , one or more actuating assemblies  50  are contained at least partially and generally entirely within wall  12 . As best seen from  FIG. 7 , each actuating assembly  50  comprises a detonator chamber  52  having a first end  51  positioned adjacent to a propellant volume  44 . Each actuating assembly  50  also has a second end  53 , which has an inlet  54 . Within detonator chamber  52  are detonator assembly  56  and firing pin  58 . Detonator assembly  56  is located proximate to first end  51  so as to be able to detonate propellant volume  44  when activated by firing pin  58 . Firing pin  58  is retained proximate to inlet  54  by a shear pin  60 . 
     A flow passage  62  extends from inlet  54  to circumferential flow channel  30  and can be entirely interior to wall  12 . Flow passage  62  places inlet  54  in fluid flow communication with circumferential flow channel  30  such, when rupture disc  38  is ruptured, inlet  54  is in fluid flow communication with central bore  18 . Prior to the rupturing, rupture disc  38  prevents fluid flow communication with central bore  18 . 
     The detonator assembly  56  includes a primer  80 , primer case  82 , shaped charge  84  and an isolation bulkhead  86 . The primer  80  is spaced from the firing pin  58  within the primer case  82 . The shaped charge  84  is positioned adjacent to the primer case  82  opposite from primer  80 . The isolation bulkhead  86  is positioned adjacent the shaped charge  84  and proximate to the propellant volume  44 . In this position, detonation of the shaped charge  84  will cause corresponding ignition of the propellant volume  44 . 
       FIG. 8  illustrates the actuating assembly after detonation. By applying a predetermined pressure, rupture disc  38  is ruptured and fluid flow communication is established between inlet  54  and central bore  18 . Prior to the rupturing, firing pin  58  is in a first position proximate to inlet  54 . Upon the rupturing, the fluid introduced to inlet  54  at the predetermined pressure causes firing pin  58  to move towards detonator assembly  56  because of the pressure differential established across firing pin  58 . The pressure differential is maintained by seal rings  61 . In other words, the portion of detonation chamber  52  adjacent to first end  57  of firing pin  58  is at a first pressure, which is equal to or greater than the pressure at inlet  54  prior to rupturing of rupture disc  38 . After rupturing of the rupture disc  38 , the pressure at the inlet  54  increases to the predetermined pressure, which is greater than the first pressure. The pressure differential is great enough to move firing pin  58  and, thus, shear the shear pin  60 , which allows firing pin  58  to move to a second position contacting and detonate primer  80 . Detonation of primer  80  is contained by primer case  82  and causes detonation of the adjacent shaped charge  84 , which transfers explosive energy to the propellant volume  44 , causing ignition thereof. The explosive energy is directed radially outwardly in the form of pressure waves through ports  46  (see  FIGS. 9 to 11 ) and into the surrounding subterranean formation. 
     As can be best seen from  FIG. 2 , there can be a plurality of actuating assemblies associated with circumferential flow channel  30 . In  FIG. 2 , firing pin  58  can be seen within a plurality of detonation chambers  52 . Each detonation chamber  52  would be in fluid flow communication with the same circumferential flow channel  30  by separate flow passages  62  as described above. Each detonation chamber  52  and associated flow passage  62  would generally be spaced symmetrically around the interior of wall  12 . 
     Also, as can best be seen from  FIG. 9 , there can be a plurality of detonation sections  10  on a downhole tool or casing string. In  FIG. 9 , a casing string  70  comprises casing  71  and at least two detonation sections  10   a  and  10   b . Additionally, the casing string  70  can have tools  72   a  and  72   b , which, for example, can be a packer such as used during cementing operations or other similar tools. As will be realized from  FIG. 9 , casing  71 , tools  72   a  and  72  and detonation sections  10   a  and  10   b  can each have central bores  18 , which can be aligned sequentially so that the central bores  18  of each form a continuous central bore running through downhole tool or casing string  70 . 
     With reference now to  FIGS. 9, 10 and 11 , a process using an embodiment of the downhole tool will now be described. In  FIG. 9  a casing string  70  is introduced into wellbore  64  having a wall  66 . Wellbore  64  extends through at least one subterranean region  68  having hydrocarbon deposits. As shown, the wellbore  64  extends through at least two such subterranean regions  68   a  and  68   b . The casing string comprises a tubular wall  12  defining an annular region  74  between tubular wall  12  and wellbore wall  66 . The casing string also comprises a central bore  18 . The central bore  18  extends continuously through detonation sections  10   a  and  10   b  and can extend continuously through the length of the casing string  70 . As shown, the detonation sections  10   a  and  10   b  of casing string  70  are placed adjacent to subterranean regions  68   a  and  68   b , respectively. Each detonation section is located adjacent to a subterranean region having hydrocarbon deposits. It will be appreciated for some applications, more than one detonation section will be adjacent the same subterranean region. 
     After introducing of casing string  70  into wellbore  64 , casing string  70  can be cemented in wellbore  64  as shown in  FIG. 10 . Cement  76  can be introduced into annular region  74  to thus cement the casing string  70  in the wellbore  64 . Cement  76  can be introduced in accordance with methods known in the art. 
     After cementing operations, if any, are completed, perforation and/or fracing can be performed as illustrated in  FIG. 11 . The fluid pressure in the central bore  18  is increased to a predetermined pressure or greater such that rupture discs, located within tubular wall  12  and exposed to the central bore  18 , are ruptured. By rupturing the rupture discs, inlet  54  to detonation chamber  52  is exposed to the predetermined fluid pressure, thus, moving the firing pin and detonating the propellant volume  44 , as described above. The detonation of the propellant volume is such that the cement located adjacent to the detonation sections  10   a  and  10   b  is perforated  90 , and/or subterranean regions adjacent to wellbore  64  is fractured  92 . As will be appreciated, the detonation is accomplished by an increase in pressure carried out under substantially static downhole tool conditions to rupture said rupture disc. By “static downhole tool conditions” it is meant the rupturing of the disc and movement of firing pin by increased fluid pressure actuates the detonation without the necessity of further mechanical or electrical movement or actuating of the downhole tool such as by movement of sleeves, valves or other mechanical apparatuses. 
     While various embodiments have been shown and described herein, modifications may be made by one skilled in the art without departing from the spirit and the teachings herein. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations, combinations, and modifications are possible. Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow, that scope including all equivalents of the subject matter of the claims.

Summary:
A hydraulically-actuated propellant stimulation of a downhole tool for use in hydrocarbon wells, which comprises a rupture disc that allows a predetermined pressure in the central bore of the tool to actuate a detonator assembly and, thereby, detonating a propellant volume.