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CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of U.S. patent application Ser. No. 10/650,709 filed on Aug. 29, 2003, now abandoned the entirety of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to a method and apparatus to stimulate a well through ignition of a propellant in a well adjacent openings such as perforations and then to immediately thereafter circulate foam for removing blockage material from an underground formation. 
   BACKGROUND OF THE INVENTION 
   The primary bottlenecks to the production of hydrocarbons from a well is the inflow rate from the hydrocarbon formation into the wellbore. The inflow is affected by near wellbore condition and formation characteristics. The near wellbore conditions and the formations of damaged wells can be positively influenced, with increased hydrocarbon production, through stimulation treatment. Methods for well stimulation include, but are not limited to, treatments with various chemicals, hydraulic fracturing where liquids are injected under high pressure (usually with propping agents), methods in which explosives are detonated within the formations to effect mechanical fracture, and combinations of the above procedures. 
   Oil and gas wells are subject to many ailments, some of which are treatable. One such ailment is a blockage of perforations resulting in dramatic or catastrophic decline in production. Some formations, such as an unconsolidated formation contain fines, such as sand, which flow into the perforation and become trapped, creating a plug or blockage in the perforation. Other examples of blockages, or bridging, are perforation debris, clays, silts, asphaltenes, drilling damage, and foreign or manmade objects. It is therefore desirable to remove these blockages from the perforations. 
   One such method is described in U.S. Pat. No. 4,617,997 to Jennings, Jr. which teaches a method to create or enhance fractures in a formation and extending these fractures with foam generated downhole. A foaming agent is mixed with an aqueous fluid and placed into the wellbore fluid, the level of the wellbore fluid being above the perforations and productive interval of the formation. A propellant housed in a canister, which is attached to a retrievable wire line, is placed next to the fractures. The propellant is ignited creating heat, gas and pressure while simultaneously initiating the formation of foam. The foam enters the fractures under such increased pressure for extending the radial fractures. When the pressure decreases and the foam collapses, the decreased viscosity of the wellbore fluid causes any resultant fluid and debris which has accumulated in the fractures to return into the wellbore. It is not disclosed if or how resulting accumulated and recovered debris is removed from the wellbore. 
   Another method is taught by Mohaupt in U.S. Pat. No. 6,138,753. Mohaupt teaches a technique for treating hydrocarbon wells, using two separate propellant ignition phases. A gas generator comprising a propellant charge, housed in a carrier having many openings, is lowered into the well in-line with the perforated interval. The gas generator is ignited and produces sufficient energy to breakdown and clean-out all of the perforations and create micro-fractures originating from the perforations. This is followed by igniting a second gas generator to inject a treatment liquid into the formation with energy less than that required to fracture the formation. No removal of resulting debris is contemplated. 
   A technique to both remove blockage mechanisms, debris and fines from perforations and to ensure the complete removal of this debris from the wellbore is needed. Although blockage removal from perforations or fractures is a by-product of some fracturing procedures, the method and results vary. Jennings Jr. uses the foam primarily for a different purpose, to extend the fractures and is limited to the amount of foam produced by the foaming agent. Mohaupt breaks down debris and cleans-out perforations but does not remove the debris from the well. Mohaupt also does not use foaming techniques. If blockage debris and fines are not completely removed from the wellbore, the remaining debris can re-block perforations, erode production equipment and seals, or plug the outside or the inside of the production tubing reducing or totally restricting production. Well clean-out procedures would be repeatedly required at a large expense. 
   SUMMARY OF THE INVENTION 
   A process is described for formation treatment or stimulation and which accommodates clean-up of debris associated with the stimulation. In one embodiment, a propellant is ignited adjacent openings to the formation and, substantially immediately thereafter, foam is continuously injected adjacent the openings and circulated up through a wellbore to remove debris from the formation and convey the debris therefrom. The tubing string extends sufficiently above the wellbore at surface to enable lowering of the tubing string and foam discharge port to below the openings for enhanced removal of debris. 
   In a broad aspect, a process for treating a wellbore having openings in communication with a damaged formation comprises: running in a tubing string into the wellbore to position a propellant carrier adjacent the openings; overbalancing the wellbore to establish hydrostatic pressure on the formation; igniting the propellant so as to produce a pressure event and a volume of gas directed into the formation; injecting low density foam through the tubing string and into the wellbore at a location above the propellant carrier so as to reduce the hydrostatic pressure and produce at least some debris from the formation and into the wellbore; and conveying the debris from the wellbore by circulating the foam out of the wellbore at surface until sufficient debris is removed. Typically thereafter the tubing string is then removed. It is preferable to lower the tubing string during foam circulation so as to re-position the location of foam injection below the openings 
   In another broad aspect, novel apparatus for achieving this process comprises: a tubing string in the casing and extending downhole from surface for positioning a propellant in a propellant carrier adjacent the openings and forming an annulus between the tubing string and the casing; means for igniting the propellant; and means, such as a foam discharge port in the tubing string adjacent and above the propellant, for injecting and circulating foam from an injection location adjacent the openings, up the annulus and out of the wellbore. More preferably, the tubing string extends sufficiently above surface to enable lowering the foam discharge port below the openings for enhanced debris recovery. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is a simplified cross-section of a wellbore illustrating apparatus run in on a tubing string for placement of propellant carrier adjacent a formation before ignition; 
       FIG. 1   b  illustrates a partial cross-section of an optional arrangement according to  FIG. 1   a  without a lubricator; 
       FIG. 2   a  is a simplified cross-section of a wellbore illustrating actuation of the tubing string for ignition and foam circulation; 
       FIG. 2   b  illustrates a partial cross-section of an optional arrangement according to  FIG. 2   b  for actuating ignition and foam circulation using pressure-actuation; 
       FIGS. 3   a – 3   h  are a series of schematics of a sequence of events according to one embodiment of the invention; and 
       FIG. 4   a–c  are sequential flowcharts of some steps of an embodiment of the invention according to  FIGS. 3   a–h.    
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIG. 1   a , in a preferred embodiment of the invention, it is desirable to dislodge blockage mechanisms or debris from the wellbore area of a formerly productive interval of an underground formation  10  adjacent openings in a casing  12  of a wellbore annulus or wellbore  13 . Herein, openings are referred to as perforations  11  which are to include other alternate openings enabling communication between the wellbore  13  and formation through the casing  12  including screens, and slots for example. Generally, debris is removed by igniting a propellant  16  in the wellbore  13  and then substantially immediately commencing to inject and circulate low density foam to the surface  18  for the removal of resulting debris. 
   The formation  10  and wellbore  13 , which is no longer producing desired or even commercial rates, is prepared for a workover treatment using an embodiment of the present invention. A suitable wellhead configuration comprises a spool  15  having a foam and debris outlet  19  providing communication with the wellbore  13 , a blow-out preventor  21  and a pack-off  22  at a wellhead W, and a pup length of tubing  23  with a foam injection inlet  24 . 
   In one embodiment, propellant  16  is ignited with the assistance of a lubricator  30  further comprising lubricator tubing  31 , a drop bar  32  and a trigger  33  such as a mechanical release mechanism or valve for temporarily retaining and releasing the drop bar  32  on command. Alternatively, the propellant  16  may be pressure actuated, both embodiments being described in greater detail below. 
   With reference also to  FIGS. 3   a – 3   h  and  FIGS. 4   a – 4   c , a candidate well is selected  100  ( FIG. 4   a ) and a workover string is prepared comprising a tubing string  40  fit at its distal end with a propellant carrier  26  having a firing head (not shown) and a foam injection means  28  such as a foam discharge port  29  in the tubing string  40  adjacent to and uphole of the propellant carrier  26 . The tubing string  40  is made up with conventional components to assist in establishing a tubing tally and the like. 
   As shown at  FIGS. 3   a , 4   a  and at  101 , the tubing string  40  is lowered into wellbore  13  such that at  103  the propellant carrier  26  is located across from the existing perforations  11  communicating with the formation  10  to be treated. Of course, safe procedures must be used in a workover including proper tubing string entry techniques. The tubing string  40  is suspended in the wellbore  13  at the packoff  22 , the pup length of tubing  23  is installed, having sufficient length to manipulate the tubing string  40  from above the perforations to below the perforations. A lubricator  30  can be installed. The foam injection means  28  can further comprise a differential fill flow sub (not detailed), employed at the bottom of the tubing string  40  to exclude debris and the like during running in. 
   In  FIGS. 3   b , 4   a  and at  104 , In no particular order a conventional wellbore liquid  43  is rapidly added to the wellbore  13  for increasing a fluid level  20  and resulting hydrostatic head to about maximum, sufficiently above the perforations  11  or productive interval, maximizing the head which tends to place the well in an overbalanced condition. Also the tubing string  40  is filled with liquid, such as produced water, above the differential fill flow sub. At  FIGS. 3   c , 4   a , the propellant  16  is ignited and the foam discharge port  29  is opened, as described in process step  105 . The head of liquid in the tubing string  40  assists in directing the resulting high pressure event into the formation  10  rather than permitting the energy to escape uphole along the tubing string. 
   As shown in  FIG. 1   a , in one embodiment the lubricator  30  temporarily houses the drop bar  32  and is used to cooperate with the firing head to initiate ignition of the propellant  16 . The fill sub remains sealed from the wellbore  13 , excluding liquids therefrom, until actuated by the falling drop bar  32 . As shown in  FIG. 2   a , in the context of a lubricator  30 , the trigger  33  is actuated for releasing the drop bar  32 . The drop bar  32  actuates a firing head which ignites the propellant  16 . In  FIG. 4   b  and at  105  and  106 , should a misfire occur, the drop bar  32  is fished out and re-set to repeat at  104 . As well as igniting the propellant  16 , the drop bar  32  also actuates the fill sub for opening the foam discharge port  29 . In an alternate embodiment, the firing head is pressure actuated. Accordingly, there is no need for a drop bar nor a lubricator. Additionally, the foam injection means  28  comprises the foam discharge port  29  fit with a pressure-actuated plug. In  FIG. 2   b , in the context of a pressure-actuated firing head, a pump  44  is employed to pressurize the tubing string  40  to a first pressure for initiating a pressure-actuated firing head. Unless the pressure-actuated plug is already opened due to the propellant ignition, further pumping is applied and pressure increase releases the pressure-actuated plug at the foam discharge port  29  enabling communication with the wellbore  13 . 
   In  FIGS. 3   c , 4   a , and at  104 , hydrostatic pressure of the liquid  43  in the wellbore  13  as well as that of the liquid in the tubing  40  assists in directing the resulting high pressure event into the formation  10  rather than wasting the energy uphole. Rapidly expanding gas and pressure  45  assists in removing blockages from the formation  10  about the perforations  11 . 
   At  FIGS. 3   d , 4   b  and at  107  and substantially immediately after igniting the propellant  16 , conventional low density foam  46  is injected into the wellbore  13  through the foam discharge port  29 . The circulation of foam  46  is established through the injection inlet  24  at the pup length of tubing  23  at surface and wellbore liquid  43  and foam  46  are recovered from the wellbore  13  through the spool  15  at surface. The foam  46  dramatically lowers the hydrostatic head on the formation  10  stimulating production of formation fluids. The wellbore  13  is now exposed to larger formation pressure and inflow. As a result, debris is produced into the wellbore  13 . Additionally, circulation of the foam  46  and its relatively high viscosity aid in conveying the produced debris up the wellbore  13  to the surface. The foam  46  is circulated and transports wellbore liquid  43  and debris to the surface  18  where it is removed with the foam  46 . Circulation of foam  46  ensures the capture and removal of substantially all produced debris, as the low density foam  46  rises to the surface  18 . 
   At  FIGS. 3   e , 4   b  and at  108 , when circulating foam  46  and for more effective removal of debris, the tubing string  40  is slowly lowered so that foam discharge port  29  is below the perforations  11 . The ability to lower the tubing string  40  and the depth it can be lowered is predetermined by the pup length of tubing  23  above the packoff seal  22 . In  FIG. 4   c  and at  109 , it can be desirable in some instances to stroke, or lower and raise, the tubing string  40  periodically to prevent lodging of the debris and sand flowing into the wellbore  13  between the tubing string  40  and well casing  12 . This action is recommended to continue until sufficient debris has been successfully removed. 
   At  FIGS. 3   f , 4   c  once sufficient debris has been removed, the formation  10  is sufficiently rejuvenated so as to re-establish useful inflow. At  110 , the tubing string  40  then raised to elevate the propellant carrier  26  above the perforations  11  and, at  111 , one of a variety of techniques can be used to apply sufficient hydrostatic head to kill the well before safely pulling the tubing string  40  from the wellbore  13  at  FIGS. 3   g , 4   c . Typically the methodology for killing the well is tailored to the particular well and can include simply diminishing foam circulation or circulating air to allow formation fluid  47  production to fill the annulus  13  and kill the well or more aggressively to load up with a suitable wellbore liquid  43 . 
   At  FIGS. 3   h , 4   c , and as an objective of rehabilitating the formation  10 , a production string  50  with pump  51  can be run in to re-establish production from the treated well. 
   Note that propellant carriers and foam formulations are known and include those set forth in Jennings Jr. U.S. Pat. No. 4,617,997. 
   As suggested in  FIG. 4   a  at  100 , some wells are better candidates than others for this process, and while this process was developed for the criteria described below, is not limited to these applications:
         The well would have a shut-in fluid level, or low cumulative production, to indicate some recoverable reserves are still in place;   The well would have exhibited a dramatic, or catastrophic, decline in production, indicating a blockage mechanism has occurred and the decline rate is not natural depletion;   Offset wells where previous re-perforating, and propellant stimulation operation has provided incremental production, even briefly, where the increased production may sustain due to the increased depth of stimulation from the propellant or removal of the debris by the stable foam operation;   Wells with diagnosed shale collapse are excellent candidates due to suspicion of the presence of large particulate debris and suspicions that such deposits are a distance from the wellbore; and   This method is further recommended in cases where less aggressive work over techniques have failed, or have failed to sustain increased production.

Summary:
A damaged formation is stimulated by igniting a propellant adjacent openings in the wellbore in communication with the damaged formation. Substantially immediately thereafter, low density foam is injected adjacent the openings and circulated to the surface for the removal of debris released from the formation. A tubing string has a foam discharge port at a distal end and a foam injection port at surface. The tubing string extends sufficiently above the wellbore at surface to enable lowering of the tubing string and foam discharge port to below the openings for enhanced removal of debris.