Abstract:
Openings are created between a wellbore and a formation by firing a perforating gun adjacent a perforating zone in the formation and debris is removed. A tubing string extending to the formation is pressurized to a first pressure to actuate the perforating gun. A second higher pressure is applied to activate a downhole injection port. Substantially immediately thereafter fluids are injected into the wellbore near the openings and circulated to the surface for the removal of debris. An optional and uphole injection port can be used to adjust the hydrostatic head above the perforating gun with the removal or addition of fluid. The tubing string extends sufficiently above the wellbore at surface to enable lowering of the downhole injection port below the openings during fluid circulation for enhanced removal of debris.

Description:
FIELD OF THE INVENTION 
   This invention relates to a method and apparatus to perforate or re-perforate a well and then to substantially and immediately thereafter circulate a fluid for removal of solids and debris from an underground formation for an aggressive completion or stimulation. 
   BACKGROUND OF THE INVENTION 
   To recover hydrocarbons such as oil and natural gas from subterranean formations through a wellbore penetrating the earth to the hydrocarbon-bearing formation, it is common to perform a completion, including perforating, and in some circumstances to perform some type of stimulation procedure in order to enhance the recovery of the valuable hydrocarbons. 
   In order to recover the hydrocarbons, a well is drilled from the surface to the formation. Following drilling, the well is generally completed by installing a tubular well casing in the open borehole and cementing the casing in place. Because the casing and cement forms a continuous hollow column, no wellbore fluids are able to enter the well to be transported to and recovered at the surface. 
   For this reason, it is common to provide openings through the casing and cement annulus in the zone of interest by perforating the casing and cement into the surrounding formation to provide access from the formation into the wellbore for recovery of the formation fluids. In situations where existing perforations are deemed inadequate the formation can be stimulated using a variety of other techniques such as acidizing, fracturing, flushing, or re-perforating, any of which can produce debris. 
   Forms of debris include drilling or perforation debris, debris from cementing operations, and/or mud solids. Naturally occurring debris such as sand, silts or clays can also be present. In some formations shales and shale chunks, pyrite, coal and other fragmented sections of formations can be produced. This debris should be quickly removed from the wellbore or formation in order to prevent it from causing a blockage, or eroding or damaging production equipment. In some instances the removal of increased volume of debris can substantially enhance production. 
   Completion or stimulation methods include a method described in U.S. Pat. Re. No. 34,451 to Donovan et al wherein a perforating gun with an external auger is mounted to a tubing string to both aid in clean-up of the debris from the perforations as well as to facilitate the movement of the gun out of the debris. The auger flights create a tortuous path increasing the velocity of produced formation fluids and improves the ability of those fluids to carry debris. Hydrostatic kill fluid is circulated to remove debris and produced hydrocarbons. Thereafter, proppent is pumped down tubing and into the formation. The auger facilitates the removal of the gun packed in the sand. 
   In U.S. Pat. No. 4,560,000 to Upchurch a well perforating technique actuates a firing mechanism of a tubing-conveyed perforating gun using a pressure difference between at different points in the borehole. The technique obtains the benefit of underbalanced conditions to aid in creating a localized cleansing effect as the formation fluids enter the well casing. 
   Further, Applicant was part of the development of an aggressive perforating-while-foaming (PWF) production process to increase the production capability of a well. This process has gained wide usage over the last 4 years within the heavy oil industry, specifically wells drilled into unconsolidated sandstone formations. This method produced more sand in a shorter period of time than other more traditional methods. It is strongly suspected that this immediate removal of sand is linked to the superior performance of these wells. A perforating gun is tubing conveyed down an underbalanced well. The gun is detonated using a drop bar and remote trigger. Foam is almost simultaneously injected and continuously circulated through the wellbore, carrying with it debris from the formation. 
   Although continuous circulation of foam effectively removes debris from the wellbore in the PWF process, the remote trigger can create un-safe work practices as a result. As well, drop-bars are not considered practical in highly deviated wells since the bar may not reach the bottom. Upchurch relies solely on formation pressure to clean out the wellbore, which can be insufficient in low pressurized formations and can prevent comprehensive elimination of debris from the wellbore. Donovan&#39;s method is also dependent on formation pressure to clean out the perforation debris from the wellbore, but is aided by the auger blades. Removal of wellbore debris is not a controlled factor in either case. If debris is not completely removed from the wellbore, it may block perforations, limit production, damage production equipment, or plug the outside or the inside of the production tubing reducing, partially or totally restricting production. In such instances, well clean-out procedures would be repeatedly required at a large expense. 
   SUMMARY OF THE INVENTION 
   A process is described for creating openings in a well casing and which substantially and immediately accommodates clean-up and production of debris. In a preferred embodiment, a pressure-actuated perforating gun is fired adjacent a zone in the formation to be perforated for forming openings. Substantially immediately thereafter, a fluid is continuously injected through a downhole pressure-actuated injection means or port near the openings and is circulated up through a wellbore at a sufficient velocity or elutriation rate overcome settling of debris and therefore to remove and lift debris from the formation. Optionally, an uphole foam injection means or port can aid in adjusting the hydrostatic head above the perforating gun. The tubing string extends sufficiently above the wellbore at surface to enable lowering of the tubing string and downhole injection means or port to below the openings for enhanced removal of debris. 
   In a broad aspect, a process for creating openings between a wellbore and a formation comprises running-in a tubing string into the wellbore to position a perforating gun adjacent a perforating zone, pressurizing to a first pressure to fire the perforating gun and produce openings between the wellbore and the formation, pressurizing to a second pressure to actuate a downhole injection means and injecting fluid therethrough at a sufficient velocity or elutriation rate to convey debris from the wellbore by circulating the fluid out through the downhole injection means of the wellbore to at surface. It is preferable to lower the tubing string during circulation so as to re-position the location of the downhole injection means to below the openings. Typically thereafter the tubing string is then removed. 
   In another broad aspect, an apparatus for creating openings between a wellbore and a formation comprises a tubing string in the casing and extending downhole from surface for positioning a perforating gun adjacent a perforating zone and forming an annulus between the tubing string and the casing, a downhole injection port located on the tubing string for injection of fluid at an elutriation rate so as to continuously remove debris from the wellbore, and means to pressurize the tubing for firing the perforating gun and opening the downhole injection means. An uphole foam injection means can be located on the tubing string for cleaning out the well and displacing wellbore fluid to create a desired fluid level. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1   a – 1   b  are a simplified cross-sections of a wellbore illustrating apparatus run-in on a tubing string for placement of a perforating gun adjacent a formation before firing and for injection fluids, respectively; 
       FIGS. 2   a – 2   g  are a series of schematics of stages of the methodology according to one embodiment of the invention; and 
       FIG. 3   a – 3   c  are flowcharts of some steps of an embodiment of the invention according to  FIGS. 2   a – 2   g  and illustrating some optional embodiments. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIG. 1   a , in a preferred embodiment, it is desirable to create openings  10  in a well casing  12  of a wellbore annulus  14  or wellbore  16  adjacent an underground formation  18 . Herein, the openings  10  are more conventionally referred to as perforations  20  which enable communication between the wellbore  16  and the formation  18  through the casing  12 . Generally, the perforations  20  are created by firing a perforating gun  22  in the wellbore  16 . Debris generally exists in the formation and in the casing which results from operations including drilling or perforation debris, debris from cementing operations, and from mud solids. Naturally occurring debris such as sand, silts or clays can also be present in the formation. In some formations shale, shale chunks, pyrite, coal and other fragmented particles of the formation can be produced. 
   As shown in  FIG. 1   b  and  FIGS. 2   c  and  2   d , debris is removed by substantially immediately commencing to inject and circulate a fluid  24  at sufficient velocities or rates so as to overcome settling velocities from some or substantially all of the debris for the fluid and lift this debris to surface  26 . Such rates are termed herein as elutriation rates. 
   Fluids  24  are chosen for their elutriation characteristics, such as density, viscosity, and flow velocities as well as how they interact with wellbore fluid  46  and formation fluids  66 . The possibility of formation damage should always be considered when choosing a fluid  24 . Fluid  24  options can include low density foams, gases, or liquids. 
   As shown in  FIGS. 1   a , 1   b , the formation  18  and wellbore  16  are prepared for an aggressive completion or stimulation techniques using a preferred embodiment of the present invention. A suitable wellhead configuration comprises a spool  28  having a fluid and debris outlet  30  providing communication with the wellbore  16 , a blow-out preventor  32  and a pack-off  34  at a wellhead  36 , and a fluid injection inlet  38 . 
   With reference also to  FIGS. 2   a – 2   g  and  FIGS. 3   a – 3   c , a completion is prepared comprising a tubing string  40  fit at its distal end with the pressure-actuated perforating gun  22  set to fire at a first pressure, and a downhole injection means or port  42  set to open or burst at a second pressure. The downhole injection port  42  is located uphole of the perforating gun  22 . The tubing string  40  is made up with conventional components to assist in establishing a tubing tally and the like. 
   The apparatus enables injection of fluid  24  for lifting debris from the wellbore  16  such as when there is not sufficient formation production volume or pressure to remove the debris or where the debris has a high enough density to be unaffected by usual formation production. Circulation of a suitable fluid  24  can be implemented providing enhanced lift. Such fluid  24  is circulated at sufficient velocity, viscosity and density or elutriation conditions and rates to remove the debris. 
   Generally, a fluid level  62  is established above the perforating gun  22 . Circulation of fluid  24  is established through the fluid injection inlet  38  at the surface  26  and wellbore fluid  46  and fluid  24  are recovered through the spool  28  at the surface  26 . Additionally, the downhole injection port  42  is preferably a conventional pressure-activated ‘S’ drain or burst plug  50 . 
   At  FIG. 2   a  and step  100  of  FIG. 3   a  ( FIG. 3   a , 100 ), if the well is a good candidate for the operation, the tubing string  40  is run in  FIG. 3   a , 101  and preferably positioned  FIG. 3   a , 102  in the wellbore  16  such that the perforating gun  22  is located across from a zone  60  to be perforated and is covered by some wellbore fluid  46 . Of course, safe procedures must be used in a completions operation or stimulation technique including proper tubing string entry techniques. The tubing string  40  is packed off above the wellbore  16 , as shown in  FIGS. 1   a , 1   b.    
   As shown in  FIG. 2   b  and  FIG. 3   b  at A, if the desired fluid level  62  exists  FIG. 3   a , 103 , the tubing string  40  is pressurized using pressurizing means and the perforating gun is actuated. The fluid level  62  creates a minimum hydrostatic pressure above the perforating gun  22  allowing maximum inflow from the formation  18  once the formation  18  is perforated, but covers the perforating gun  22  to keep it from splitting. 
   The tubing string  40  is pressurized  FIG. 3   b , 104  to the first pressure for actuating a firing head  54  of the perforating gun  22  and forming perforations  20 . Activation of the perforating gun  22  is not affected by its orientation in the well casing  12 . An explosion  64  creating perforations  20  in the well casing  12  between the wellbore  16  and the reservoir or formation  18  for recovery of formation fluids  66 . 
   At  FIG. 3   b , 105  if a misfire occurs and the drain  50  is blown or opened, the tubing string  40  needs to removed and the problem diagnosed  FIG. 3   b , 106 . If required, the ‘S’ drain, burst plug  50  and/or firing head  54  are serviced or replaced. The tubing string  40  is run-in hole and the process starts again. 
   As shown in  FIG. 2   c , and if there was no misfire, as soon as physically possible, substantially immediately after firing the perforating gun  22 , fluid  24  is continued to be pumped into the tubing string  40 , applying further pressure  FIG. 3   b , 107  to a second pressure, greater than the first pressure, for actuating the pressure-actuated “S” drain or burst plug  50 , at the downhole injection port  42  enabling fluid communication therethrough with the wellbore  16 . A pump, or optionally, pressurized gas may be used to apply pressure in the tubing string  40 . 
   Circulation of the fluid  24  conveys or aides the conveyance of the debris up the wellbore  16  to the surface  26  for removal of substantially all debris. 
   Turning to  FIG. 2   d  and to  FIG. 3   c , 108 , when circulating fluid  24  and for more effective removal of the debris, the tubing string  40  is slowly lowered so that downhole injection port  42  is below the perforations  20 . At  FIG. 2   e  and  FIG. 3   c , 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  16  between the tubing string  40  and well casing  12 . This action can continue until sufficient debris has been successfully removed. 
   Once the operation is complete and sufficient debris has been removed from the wellbore  16 , the well&#39;s productivity thereafter is increased. 
   At  FIG. 2   e  and  FIG. 3   c , 110  the tubing string  40  is then raised to elevate the perforating gun  22  above the perforations  20 . At  FIG. 2   f  and  FIG. 3   c , 111 , one of a variety of techniques can be used to apply sufficient hydrostatic head to kill the well before safely pulling  FIG. 3   c , 112  the tubing string  40  from the wellbore  16 . Typically the methodology for killing the well is tailored to the particular well and can include simply diminishing fluid  24  circulation to allow formation fluid  66  production to fill the annulus  14  and kill the well or more aggressively load up with suitable wellbore fluid  46 . 
   At  FIG. 2   g , and as an objective of rehabilitating the formation  18 , a production string  68  with a production pump  70  can be run in to re-establish production from the treated well. 
   In an alternate embodiment, and returning at  FIG. 3   a , 103  if the fluid level  62  is deemed inappropriate, and as shown in  FIG. 2   b  the hydrostatic head may be adjusted. If the fluid level is too low  FIG. 3   a , 103 ,B, conventional wellbore fluid  46  can be added  FIG. 3   b , 200  to the wellbore  16  for increasing or creating an optimal fluid level  62  by adding wellbore fluid  46  down the annulus. 
   In another embodiment of the invention, at  FIGS. 2   a , 2   b  and  FIG. 3   a , 103 ,C it may be desirable to reduce the hydrostatic head above the perforating gun  22 . An optional uphole injection means or port  44  is located uphole of the downhole injection port  42 . The uphole injection port  44  is preferably a conventional rotational valve  48 . The rotational valve  48  is strategically located to establish the desired fluid level  62  uphole of the downhole injection port  42  and the perforating gun  22 . 
   In  FIGS. 2   a  and  FIG. 3   a , 101 , the tubing string  40  is lowered into the wellbore  16  with the rotational valve  48  in the open position. If the well has not been previously cleaned out, or if too much hydrostatic pressure exists, at  FIG. 3   a , 102  a well depth  56  is tagged and low density foam or suitable fluid can be circulated through the rotational valve  48  to displace any wellbore fluid  46  to create the desired fluid level  62 . The rotational valve  48  can be positioned at other locations in the wellbore  16  and fluid  24  circulated  FIG. 3   b , 300  to remove wellbore fluid  46  above the rotational valve  48 , resulting in the desired fluid level  62 . Thereafter, the perforating gun  22  may need to be re-positioned to align with the zone  60  to be perforated. Accordingly, at  FIG. 2   b  and  FIG. 3   b , 301 , the tubing is rotated to close the rotational valve  48 , discontinuing any foam injection and creating a continuously sealed tubing string  40  for pressurizing. 
   The preferred fluid  24  is low density foam. Inherently, foam has a high viscosity at low shear rates making it extremely useful as a circulating medium in low pressure reservoirs. These properties minimize fluid loss to the formation and reduce needed annular velocities yet provide sufficient debris elutriation with high lifting capability at minimum circulating pressures. Circulation conditions including foam generated with natural gas or nitrogen instead of air can be used to clean out higher pressure wells. 
   Alternatively, production fluids can also be used. A variety of natural and process additives or polymers are available to increase the lifting, carrying and suspending capability of the fluid. 
   It will be readily apparent to those skilled in the art that many variations, application, modifications and extensions of the basic principles involved in the disclosed embodiments may be made without departing from its spirit or scope. 
   As suggested in  FIG. 3   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 which include:
         Sand production initiation in stubborn sand formations for cold heavy oil production with sand,   Known drilling damage completions,   Enhanced and rapid drainage geometry development, and   Enhanced initial and cumulative production.