You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
This application claims the benefit of U.S. patent application Ser. No. 60/315,633, filed Aug. 29, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to apparatuses for perforating wells, such as oil and gas wells, and in particular to firing heads of perforating guns. 
     BACKGROUND OF THE INVENTION 
     Perforating guns have an array of explosive charges thereon. The explosive charges can fire projectiles or form a jet of liner material (such as copper). The guns are lowered inside of a cased well to a depth containing a pay zone of oil or gas. The explosive charges are detonated wherein the casing is perforated at the pay zone. Upon the completion of the well, oil and gas can then flow through the perforations into the casing and up to the surface. 
     Great care is taken with the explosive charges in the perforating guns in order to prevent their accidental detonation. An accidental detonation with the gun on the surface could result in the injury of a crew member. An accidental detonation in the well in an undesirable location could result in a loss of production of the well. Therefore, initiators are used to better control the detonation of the perforating guns. One type of initiator is known as a detonator, which is an electrical device. 
     Detonators are initiated by an electrical current. An electrical current heats a resistive element inside the detonator to a temperature that is sufficiently high to ignite a charge inside of the detonator. The detonator is located physically close to an end of a detonating cord so as to ignite the detonating cord. When ignited, the detonating cord propagates the detonation from the detonator to fire the explosive charges that are distributed along the length of the perforating gun. 
     One type of detonator has a spring loaded pin or button on one end and an explosive charge on the other end. For safety reasons, the detonator is internally grounded until the button is depressed. Thus, when internally grounded, the detonator is in a safe mode and is unable to detonate. This type of detonator is conventional and commercially available as part number DET-3050-008 from Owen Oil Tools of Fort Worth, Tex. 
     The detonator is located in a detonating block, which is located in a firing head at an end of the perforating gun. The detonator is contained within the detonating block and is adjacent to an end of the detonating cord in the perforating gun. A long rod, or arming and contact pin, is used to press the button on the detonator in order to arm it. Thus, the detonating block secures the detonator in place adjacent to the detonating cord and positions the detonator relative to the arming and contact rod. 
     Conventional detonating blocks function as collars to hold the detonator in place. Consequently, conventional detonating blocks have a passage extending from one end of the block to the other. Upon detonation, some of the hot gases from the detonator and the detonating cord blow back in the direction of the arming and contact pin, damaging the pin and its associated spring in the process. 
     When the perforating gun is brought back out of the hole to the surface after a detonation, such damage must be fixed before the gun can be reused. Often times, a well requires multiple perforations, requiring the perforating gun to make more than one trip downhole. Minimizing the damage to the detonating mechanism minimizes turnaround time for the perforating gun on the surface and equipment loss. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a firing head for a perforating gun that has minimum damage during a detonation. 
     It is another object of the present invention to provide a firing head for a perforating gun that can be reused with a minimum amount of turnaround time and equipment. 
     The present invention provides a firing head for a perforating gun for use in downhole applications. The firing head comprises a detonating block and a detonator. The detonating block has a first end and a second end, with the second end being structured and arranged so as to be located adjacent to a detonating material. A central passage extends through the detonating block from the first end to the second end. The central passage is structured and arranged to receive a detonator. At least one venting passage extends from the central passage through the detonating block to an exterior of the detonating block. 
     In accordance with one aspect of the present invention, the central passage further comprises a collar for receiving a detonator, the collar having an inside diameter that is smaller than an inside diameter of a portion of the central passage that is located between the collar and the second end. 
     In accordance with another aspect of the present invention, there is at least one venting passage between the collar and the first end of the detonating block and at least one venting passage between the collar and the second end of the detonating block. 
     In accordance with another aspect of the present invention, the venting passage between the collar and the second end of the detonating block is larger than the venting passage between the collar and the first end of the detonating block. 
     In accordance with still another aspect of the present invention, the firing head further comprises a detonator located and restrained in the central passage. 
     The present invention also provides a firing head for a perforating gun for use in downhole applications comprising a sub having a first end and a second end. A pin is located in the sub and is axially movable therein. The pin has a head located adjacent to the second end of the sub. A detonating block is removably coupled to the second end of the sub. The detonating block has a first chamber that receives the head of the pin and a detonating chamber that is structured and arranged to be adjacent to a detonating material in the perforating gun. The detonating block has a retainer located between the chamber and the detonating chamber. A detonator is located in the retainer and extends into the detonating chamber. The detonator has an arming mechanism that is located in the first chamber in selective contact with the head of the arming and contact pin. There is at least one venting passage in the detonating block extending from the detonating chamber to an exterior of the detonating block. 
     In accordance with another aspect of the present invention, the firing head further comprises at least one venting passage in the detonating block extending from the first chamber to the exterior of the detonating block. 
     The present invention also provides a method of detonating explosive charges in a downhole perforating gun. An arming mechanism for a detonator is provided. The detonator is provided in proximity to a detonating material. The arming mechanism, the detonator and the detonating material are all located along a longitudinal axis. The detonator is detonated. Gases from the detonator are vented laterally of the longitudinal axis so as to minimize damage to the arming mechanism. 
     In accordance with one aspect of the present invention, the step of providing the detonator in proximity to a detonating material further comprises the step of providing the detonator in a holder. The step of venting gases from the detonator laterally further comprises the step of venting the gases through the holder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross-sectional view of a firing head of a perforating gun of the present invention, in accordance with a preferred embodiment, shown with the detonator in the armed position. 
     FIG. 2 is a longitudinal cross-sectional view of a prior art detonating block. 
     FIG. 3 is a longitudinal cross-sectional view of the detonating block of the present invention, in accordance with a preferred embodiment. 
     FIG. 4 is an isometric view of the detonating block of FIG.  3 . 
     FIG. 5 is a longitudinal cross-sectional view of the detonating block, in accordance with another embodiment. 
     FIG. 6 is an isometric view of the detonating block of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a firing head  11 , or detonating arrangement, for a top-fire perforating gun  13 . The perforating gun  13  is designed to be lowered into an oil or gas well inside of casing. The perforating gun  13  has a number of shaped charges (not shown) located below the firing head. Detonating cord  15  extends from the bottom of the firing head to each of the shaped charges. 
     The firing head  11  includes a detonator  17  aligned with the end of the detonating cord  15 . The detonator  17  is maintained in alignment by a detonating block  19 , which block is contained within a spinning collar  21 . 
     FIG. 2 shows a prior art detonating block  19 A. The detonating block  19 A is cylindrical, having first and second ends  21 A,  23 A. A central, cylindrical passage  25  extends through the block  19 A, from the first end  21 A to the second end  23 A. The passage  25  has a first portion  27  extending from the first end  21 A to about midway of the block, and a second portion  29 , extending from about midway to the second end  23 A. The second portion  29  of the passage is narrow, having a diameter that is slightly larger than the detonator  17 . The first portion  27  of the passage is of a larger diameter. The first portion  27  has a first bore  31  that is smooth walled and of a diameter that is sufficiently large to receive a head  33  of an arming and contact pin  35  (see FIG.  1 ). The first portion of the passage also has a threaded counterbore  37  coupled to an end of an arming and contact pin sub  39 . A shoulder  41  is formed at the junction of the first and second portions of the passage. The shoulder  41  serves as a stop surface for the detonator  17 . 
     The conventional and commercially available detonator  17  is cylindrical with a stop shoulder  43  (see FIG.  1 ). In the preferred embodiment, the detonator has a button  45  at one end. The button must be depressed to arm the detonator. Once armed, electrical current is sent through the button and out via the casing. The detonator has an explosive charge  18  therein. 
     The present invention improves the detonating block  19 A by providing venting passages from the central passage containing the detonator to the outside of the detonating block. In addition, the portion of the central passage near the second end of the detonating block is enlarged. Furthermore, the outside diameter of the block is reduced. 
     To describe the detonating block  19 , terms such as “upper” and “lower” will be used with reference to the orientation of FIGS. 1,  3 - 6 . Referring to FIG. 3, the detonating block  19  of the present invention has first and second ends  21 ,  23  (upper and lower ends), a smooth bore  31  and a threaded counterbore  37  adjacent to the upper end. There is also a shoulder  41  functioning as a stop surface for the detonator  17 . A narrow central bore  47  or passage extends from the shoulder  41  towards the lower end  23 . A counterbore  49  extends from the lower end to the narrow bore  41 . The counterbore  49  is of a larger diameter than the central bore  47 . The counterbore  49  forms a detonating chamber, while the bore  31  forms an upper chamber. Between the bores  31 ,  49 , a collar or retainer  50  is formed, through which the central bore  47  extends. The collar  50  is about midway between the first and second ends  21 ,  23 . The bore  47  has a diameter that is slightly larger than the diameter of the detonator  17 . 
     The upper chamber  31  has a diameter that is sufficiently large to receive the head  33  of the arming and contact pin  35 . 
     Venting passages  51 ,  53  extend radially outward from the upper and detonating chambers  31 ,  49  to the outside of the detonating block. There are upper venting passages  51  that vent the upper chamber  31  and lower venting passages  53  that vent the detonating chamber  49 . The lower venting passages  53  are larger in diameter than the upper venting passages because most of the gases escape through the lower venting passages. Some gas does pass through the collar and out through the upper venting passages  51 . In the preferred embodiment shown in FIGS. 3 and 4, there are four lower venting passages  53  spaced 90° apart around the circumference of the detonating block. Likewise, there are four upper venting passages  51  spaced 90° apart around the circumference of the detonating block. 
     The size and number of venting passages can vary. For example, the lower venting passages can be smaller in size while greater in number or larger in size while fewer in number. Also, the passages need not be circular bores as shown. Circular bores are easy to machine with the use of drill bits. However, the detonating block  19  can be cast, wherein the venting passages need not be circular. Also, as shown in FIG. 3, the venting passages are purely radial in direction having no axial or circumferential component. However, the venting passages can be inclined so as to have an axial and/or circumferential component. 
     The outside diameter of the detonating block  19  is smaller than the outside diameter of the lower end portion  40  of the sub  39 , which lower end portion is received by the collar  21 . This creates an annulus  69  around the detonating block  19 . 
     FIGS. 5 and 6 show the detonating block  71  in accordance with another embodiment. The block  71  is substantially similar to the block  19  of FIGS. 3 and 4 except that additional upper venting passages  52  are provided. Thus, the upper venting passages  51 ,  52  are spaced 45 degrees apart around the circumference of the block. In addition, the upper venting passages  52  are offset longitudinally from the upper venting passages  51 . In the embodiment shown, the passages  52  are located closer to the collar  50 . 
     The firing head  11  is assembled in accordance with normal procedures; the detonating block  19  of the present invention does not alter the assembly. The assembly will be briefly described with reference to FIG.  1 . The detonator  17  is inserted into the opening  47  of the collar  50 . The stop shoulder  43  of the detonator  17  bears on the shoulder  41  of the collar  50  and the button  45  on the detonator is nearest the upper end  21  of the detonating block. The detonating block is threaded onto the lower end of the arming and contact pin sub  39 , such that the head  33  of the arming and contact pin  35  is located adjacent to the button  33 . Once the detonating block  19  is threaded onto the sub  39 , the button  45  is depressed, thereby arming the detonator  17  (not shown). A cap (not shown) may be used on the upper end of the sub  39  and a plug (not shown) may be used on the lower end of the spinning collar  31  during assembly as safety devices. A cable head  59  (shown schematically in FIG. 1) is screwed onto the upper end of the sub  39 , followed by threading the spinning collar  21  onto the upper end of the perforating gun  13 . The inside of the spinning collar is sealed so as to prevent fluid from reaching the fluid sensitive detonator  17 . 
     In operation, the detonator  17  is unarmed as long as the button  45  is extended. The perforating gun is lowered downhole to its desired depth. When ready to perforate, an electrical current is passed through the pin in the detonator. The detonator detonates, igniting the detonating cord and explosive charges contained in the perforating gun. 
     The detonating detonator produces hot gases in the detonating chamber  49 . Without the venting passages  51 ,  53  these gases create an overpressure along the longitudinal axis of the tool that bends and distorts the arming and contact pin  35 . However, the venting passages  51 ,  53  allow the gases to escape transversely to the annulus  69  around the detonating block  19  and flow away from the pin  35 . Some of the gases flow into the upper passage  31  and through the upper venting passages  31  to the annulus  69 . Thus, the gases are unable to create an overpressure that is sufficient to damage the pin  35  and its spring  36 . 
     To rearm the perforating gun, the gun is retrieved to the surface. The firing head  11  is disassembled and a new detonator  17  is installed. In a typical operation, only the detonator  17  need be replaced, thus reducing turnaround time of the firing head and the perforating gun. The delicate firing head mechanism with the pin  35  remains unharmed and can be reused again and again. 
     An isolator seal  61  is provided around the head  33  of the arming and contact pin  35 , in order to prevent the hot explosive gases from impregnating a pin isolator  63 . The pin isolator  63  is located on the opposite side of the head  33  from the detonator  17 . The isolator seal  61  also prevents high-pressure borehole and formation fluids from leaking past the o-ring seal and invading the annulus space between the contact pin and the sub. This eliminates the need to rebuild and clean the entire firing head assembly. 
     With the embodiment shown in FIGS. 5 and 6, the additional upper venting passages  52  improve the venting of the gases to the annulus  69 . Staggering the additional upper venting passages  52  closer to the detonator appears to vent the gases more effectively. 
     The foregoing disclosure and showings made in the drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense.

Summary:
A firing head for a perforating gun includes a detonating block having a first end and a second end, with the second end being structured and arranged so as to be located adjacent to a detonation material. A central passage extends through the detonating block from the first end to the second end. A detonator is located and restrained in the passage so as to be adjacent to the detonating material. At least one venting passage extends from the central passage to an exterior of the detonating block.