Abstract:
A tubular casing section for a subterranean well has side wall apertures therein which are temporally sealed with plug structures. The plugged casing section is positioned within a wellbore at a selected subterranean formation and cemented in place within the wellbore. The plugs are removed, such as by melting, fracturing, shearing or corrosion, and a stimulation tool is lowered into the casing and operated to pierce through the cement lining at the unplugged casing side wall apertures and into the surrounding subterranean formation to establish communication between the formation and the interior of the casing.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to apparatus and methods for completing subterranean wells and, in a preferred embodiment thereof, more particularly relates to the completion of a subterranean well using a pre-drilled casing section having plugged side wall apertures therein which may be opened downhole in conjunction with the process of communicating an adjacent subterranean formation with the interior of the casing section via its side wall apertures. 
     In a conventional completion process for a subterranean well, a well bore is extended through a subterranean formation, and a tubular casing structure is coaxially positioned within the wellbore to maintain the integrity of the wellbore, and facilitate subsequent placement of various downhole tools in the well. After placement of the casing within the well bore, the casing is laterally enveloped within a cement liner structure by forcing cement downwardly through the casing, outwardly through its lower end, and back up the well bore along the exterior surface of the casing. The resulting external cement liner on the casing serves to prevent undesirable vertical communication between various formations via the space between the exterior side surface of the casing and the side surface of the well bore. 
     Next, an explosive-based perforating gun structure is lowered into the cemented-in casing to a position vertically adjacent the formation to be perforated. Firing of the perforating gun detonates various shaped explosive charges thereon, with each of the detonated shaped charges sequentially penetrating an adjacent portion of the casing side wall, the adjacent cement lining, and a portion of the formation extending outwardly from the cement lining. This explosive penetration of the formation operatively communicates it with the interior of the now perforated casing so that production fluid from the formation may enter the casing for appropriate retrieval and transport to the surface in a well known manner. 
     This conventional use of a perforating gun carries with it the usual risks, inconveniences and unreliability associated with dealing with highly explosive materials. Moreover, when utilizing a perforating gun the operator must often contend with the deploying, dropping and recovering the perforation equipment. 
     As can readily be seen from the foregoing, a need exists for improved apparatus and associated methods for communicating the interior of a well casing with a surrounding subterranean formation. It is to this need that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, a subterranean well is completed using a specially designed tubular casing section having side wall apertures which are sealingly covered by plug structures that are removable downhole to re-establish communication between the interior of the casing section and its outer side via the side wall apertures. 
     According to one illustrated method, the plug structures are of a eutectic material, and a casing assembly, having a longitudinal portion defined by the apertured casing section, is positioned within a wellbore with the plugged casing side wall apertures adjacent a portion of a preselected subterranean formation. A sealing layer is formed outwardly around the positioned casing assembly by sequentially flowing a sealing material, representatively a cement material, downwardly therethrough, outwardly therefrom, and then upwardly between the exterior of the casing assembly and the surface of the wellbore. 
     The plugged side wall apertures are then re-opened by introducing a source of heat into the apertured casing section. The heat source is representatively a heated liquid flowed into the casing assembly, but could be another type of heat source, such as steam or an ignited propellant material, if desired. Introduction of such heat source into the casing section melts the plug structures and re-establishes communication between inner and outer side portions of the casing sections through its now re-opened side wall apertures. (In cases where the casing assembly is not cemented-in along the wellbore surface, this communicates the subterranean formation with the interior of the casing assembly via the opened side wall apertures). 
     Next, fracture areas are created which extend outwardly from the re-opened side wall apertures, through adjacent portions of the sealing layer, and outwardly into the formation. Preferably, this step is performed using a propellant-based stimulation tool, such as a stimulating gun or stimulating stick, lowered into the casing assembly and actuated after the casing side wall apertures are re-opened. Alternatively, an explosive-based stimulation tool could be utilized. 
     While the plug structures are representatively of a eutectic material, they could be of a variety of other materials, and other techniques could be alternatively utilized to remove them, downhole, from the casing side wall apertures which they sealingly block. For example, the plug structures could be of a frangible material which could be broken downhole, could be of a corrodible material which could be eaten away downhole by an acidic or highly basic liquid introduced into the casing section, or could be of a combustible material which could be ignited and combusted downhole. 
     In another illustrated embodiment of the completion method described above, a propellant-based stimulation tool lowered into the casing is used to simultaneously remove the casing side wall aperture plug structures and form the fracture areas that extend from the re-opened casing side wall apertures sequentially through the sealing layer and outwardly into the adjacent subterranean formation. The lowered propellant-based stimulation tool may be used to break the plug structures, or the plug structures may be formed from an ignitable propellant material which is ignited and combusted by the hot gases generated by activation of the stimulation tool. Alternatively, an explosive-based stimulation tool could be utilized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view through a pre-drilled casing section used in completing a subterranean well in accordance with principles of the present invention; 
     FIG. 2 is a view of the pre-drilled casing section after its side wall apertures have been sealed with plug structures which are representatively formed from a eutectic material; 
     FIG. 3 is a cross-sectional view through the plugged casing section operatively positioned within a well bore at a selected subterranean formation, and further illustrates in schematic form the cementing-in of the casing section and the introduction into its interior of a heated liquid used to melt the eutectic plug material; 
     FIG. 4 is a cross-sectional view similar to that in FIG. 3, but with the eutectic casing plugs having been melted by the heated liquid; 
     FIG. 5 is a cross-sectional view through the now unplugged casing section illustrating the use of a propellant-base stimulation tool to create communication between the formation and the interior of the casing section; 
     FIG. 6 is a cross-sectional view through an alternate embodiment of the pre-drilled, plugged casing section operatively installed in the well bore and illustrates the use of a propellant-based stimulation tool to both unplug the casing side wall apertures and pierce portions of the surrounding formation to create communication between the formation and the interior of the casing section; and 
     FIG. 6A is an enlarged scale cross-sectional view through a portion of the casing section illustrating the formation communication with the interior of the casing section via an unplugged casing side wall aperture and an adjacent stimulation intrusion into the formation. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 1, the present invention provides a tubular metal casing section  10  which is incorporated as a longitudinal portion of a tubular casing assembly operatively installed in a wellbore extending through a subterranean production formation as later described herein. By drilling or otherwise, a spaced series or representatively circular apertures  12  are formed in the side wall  14  of the casing section  10 , with each aperture  12  extending completely between the interior and exterior side surfaces of the casing side wall  14 . 
     After the casing side wall apertures  12  are formed, as shown in FIG. 2 they are sealed with schematically depicted plug structures  16 . Plug structures  16  are representatively formed from a eutectic material and may be operatively installed on the casing section in a variety of manners. For example, the plug structures  16  may be threaded into the apertures  12 , press-fitted into the apertures  12 , or molded into the apertures. The melting point of the eutectic material used to form the plugs  16  is selected so that the anticipated subterranean temperature to which the plugs will be subjected when the plugged casing section  10  is positioned within the selected underground formation will not melt the plugs. The specially designed plugged casing section  10 , as will now be described, is utilized in a unique well completion method that embodies principles of the present invention. 
     FIG. 3 illustrates the plugged casing section  10 , as a longitudinal part of an overall tubular metal casing assembly, having been operatively positioned within a subterranean wellbore  18  at a location therein extending through a selected production formation  20 . With the casing in place in the wellbore  18  as shown in FIG. 3, a conventional cementing-in operation is performed in which cement is forced downwardly through the casing (as indicated by the arrow  22 ), outwardly through its bottom end (not shown), and then upwardly through the space between the outer side surface of the casing and the side surface of the wellbore  18  to form the usual hardened cement liner  24  extending around the installed casing and radially extending between the casing and an adjacent portion  20   a  of the formation  20  which tends to be somewhat damaged as a result of the original wellbore drilling operation. 
     After the cement casing liner or sealing layer  24  has been formed, and has hardened, a heat source is applied to the eutectic plug structures  16  to melt them. Representatively, this heat source is in the form of a heated liquid, such as hot water  26  which is recirculated through the casing interior (for example, through a nonillustrated tubing string) to melt the eutectic plug structures  16 , as shown in FIG. 4, thereby re-opening the casing section side wall apertures  12 . It should be noted that, at this point, if the casing had not been cemented-in the formation  20  would have now been operatively communicated with the interior of the casing without the use of explosive perforating apparatus to create side wall holes in the casing. As will be appreciated, various other types of heat sources could be used to melt the eutectic plug structures  16  and thus provide communication between the outer side surface of the casing and its interior. For example, other hot liquids, steam, or various sources of dry heat could be utilized to melt the plug structures  16  and thus re-open the casing section side wall apertures  12 . 
     Turning now to FIG. 5, after the casing section side wall apertures  12  have been re-opened, the liquid  26  is left in the casing for use as a liquid tamp T therein. Next, a propellant-based stimulation tool, illustratively in the form of a schematically depicted stimulating gun  28 , is lowered into the now unplugged casing section  10 , representatively on a length of metal tubing  30 . Stimulating gun  28  may alternatively be lowered on a wireline if desired. The stimulating gun  28  is of a conventional construction, and includes a tubular metal body  32  filled with an ignitable propellant material  34  and having side wall perforations  36 . The propellant material is representatively potassium perchlorate mixed in a suitable epoxy binder material. 
     After the stimulating gun  28  is in place within the casing section  10  and the liquid tamp T, the propellant material  34  is ignited to thereby create high temperature, pressure and velocity propellant streams  38  which are discharged outwardly from the metal gun body  32  into the interior of the casing section  10 . The ignited propellant material thus discharged from the gun body  32  is forced outwardly through the now reopened casing section side wall apertures creates fracture areas  40  extending outwardly from the side wall apertures  12  sequentially through the cement liner  24  and into the adjacent formation  20 , thereby operatively communicating the formation  20  with the interior of the casing section  10 , and thus the balance of the casing assembly, without the use of explosive devices, such as a shaped charged perforating gun, within the casing. 
     A variation of this propellant-based completion method is illustrated in FIG. 6 in which a propellant-based stimulation tool, such as the representatively depicted stimulation gun  28 , is used to simultaneously remove the plug structures  16  and create the fracture areas  40 . In this method, the stimulating gun  28  is placed in the casing section  10 , within a suitable liquid tamp T extending therethrough, and fired by igniting the propellant  34  to create the previously described propellant streams  38 . The ignited propellant  34  is used in this method to simultaneously remove the plug structures  16 , thereby re-opening the casing side wall apertures  12  as shown in FIG. 6A, and form the fracture areas extending from the apertures  12  outwardly through the cement liner  24  and into the adjacent formation  20 . 
     The discharged propellant  38  may be used to break the plug structures  16  (which are correspondingly formed from a suitable frangible material), or the plug structures  16  may be formed from an ignitable propellant material (such as the propellant material used in the gun  28 ) and ignited and combusted by the heat generated by the hot discharged propellant streams  38 . 
     As can readily be seen from the foregoing, the present invention provides apparatus and methods for completing a subterranean well without the previous necessity of utilizing explosive material-based stimulation tools such as a shaped charge perforating gun. In addition to the various representatively illustrated techniques for removing the plug structures  16 , and thus re-opening the casing side wall apertures  12  downhole, other techniques could alternatively be utilized if desired. For example, when eutectic material plugs are utilized various other types of heat sources could be used to melt the plug structures—for example, hot oil or steam introduced into the pre-drilled casing section  10 . 
     Additionally, hollow plugs with closed ends projecting into the casing section interior could be used, with the plugs being openable by passing a suitable milling tool downwardly through the casing section  10  to cut off the inner ends of the plugs and thereby communicate the exterior of the casing section with its interior via the interiors of the now opened plug structures. Further, the plug structures could formed of a suitable material which would be corroded away by introducing an acidic or highly basic liquid into the casing. 
     While a propellant stimulating gun  28  has been illustrated as being used to create the fracture areas  40 , it will be readily appreciated by those of skill in this particular art that other types of propellant-based tools could alternatively be utilized if desired. As but one example, a stimulating stick device, in which a propellant material encapsulates a support member lowered into the casing on a wire line, could also be used if desired. 
     The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.