Abstract:
A downhole tool assembly for use in a wellbore includes a tubular body carrying an explosive which is selectively detonated to create a dynamic underbalance or overbalance effect in the wellbore. The tubular body has opposite ends provided with plug assemblies including plug elements movable between a normally collapsed state and an actuable expanded state. The plug elements are adapted to be actuated to the expanded state between the tubular body and an outer extent of the wellbore before the creation of the dynamic underbalance or overbalance effect to isolate a discrete segment of the wellbore to which the dynamic underbalance or overbalance effect is confined.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) to U.S. Non-/Provisional Patent Application Ser. No. 61/183,102, entitled, “Device for Focus and Control of Dynamic Under Balance and Dynamic Over Balance in a Borehole,” filed on Jun. 2, 2009. This application is hereby incorporated by reference in its entirety. 
     
    
     FIELD 
       [0002]    The present disclosure generally relates to improving communication of formation fluids within a wellbore using dynamic underbalance or dynamic overbalance to effectively manipulate pressure conditions within a wellbore after perforation tunnels have been previously formed in the surrounding formation of a well. 
       BACKGROUND 
       [0003]    To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the wells and the guns fired to create openings in a casing and to extend perforation tunnels into the surrounding formation. 
         [0004]    The explosive nature of the formation of perforation tunnels shatters sand grains of the formation. A layer of “shock damaged region” having a permeability lower than that of the virgin formation matrix may be formed around each perforation tunnel. The process may also generate a tunnel full of rock debris mixed in with the perforator charge debris. The extent of the damage, and the amount of loose debris in the tunnel, may be dictated by a variety of factors including formation properties, explosive charge properties, pressure conditions, fluid properties and so forth. The shock damaged region and loose debris in the perforation tunnels may impair the productivity of production wells or the injectivity of injector wells. 
         [0005]    To address these issues, pressure in a wellbore interval is manipulated in relation to the reservoir or surrounding formation pore pressure to achieve removal of debris from perforation tunnels. The pressure manipulation includes creating a transient underbalance condition (the wellbore pressure being lower than a formation pore pressure) prior to detonation of a detonation cord or shaped charges of limited energy. Pressure manipulation also includes creating an overbalance pressure condition (when the wellbore pressure is higher than the formation pore pressure) prior to detonation or explosion of shaped charges of a perforating gun or a propellant. Creation of an underbalance condition can be accomplished in a number of different ways, such as by use of a low pressure chamber that is opened to create the transient underbalance condition, the use of empty space in a perforating gun or tube to draw pressure into the gun right after firing of shaped charges, and other techniques. The underbalanced condition results in a suction force that will extract debris out of the perforation tunnels and fluid from the wellbore into the tube enabling the well to flow more effectively or more efficient injection of fluids into the surrounding formation. Creation of an overbalance condition can be accomplished by use of a propellant (which when detonated causes high pressure gas buildup), a pressurized chamber, or other techniques. The burning of the propellant can cause pressure to increase to a sufficiently high level to fracture the formation. The fracturing allows for better communication of reservoir fluids from the formation into the wellbore or the injection of fluids into the surrounding formation. 
         [0006]    The manipulation of wellbore pressure conditions causes at least one of the following to be performed: (1) enhance transport of debris (such as sand, rock particles, etc.) from perforation tunnels; (2) achieve near-wellbore stimulation; and (3) perform fracturing of surrounding formation. 
         [0007]    During the manipulation of pressure, one or more packers or plugs are known to be positioned between the inside of the wellbore and the outside of the perforating gun or tube to isolate the interval over which the detonation or explosion takes place to achieve a quicker and amplified response for the underbalance or overbalance effect. 
         [0008]    It remains desirable to provide a device for confining the effects of a dynamic underbalance or dynamic overbalance in a defined region of the wellbore to enable removal of debris from the perforation tunnels and/or stimulation within the well. 
       SUMMARY 
       [0009]    The present application discloses a downhole tool assembly defining a transient plug arrangement which improves communication of formation fluids in the wellbore. In one example, a downhole tool assembly for use in a wellbore includes a tubular body carrying an explosive which is selectively detonated to create a dynamic underbalance or overbalance effect in the wellbore. The tubular body has opposite ends provided with plug assemblies including plug elements movable between a normally collapsed state and an actuable expanded state. The plug elements are adapted to be actuated to the expanded state between the tubular body and an outer extent of the wellbore before the creation of the dynamic underbalance or overbalance effect to isolate a discrete segment of the wellbore to which the dynamic underbalance or overbalance effect is confined. 
         [0010]    In the particular example disclosed, the plug assemblies are responsive to detonation of the explosive such that the plug elements are actuated to the expanded state between the tubular body and an outer extent of the wellbore to isolate the discrete segment of the wellbore to which purging of the debris filled perforation tunnels or stimulation of wellbore is concentrated. In an alternative method, the plug assemblies could be actuated by an electrical, hydraulic or mechanical command. 
         [0011]    The present disclosure further contemplates an exemplary method for forming and controlling a dynamic underbalance or dynamic overbalance effect on a wellbore wherein the method includes the steps of (1) lowering a downhole tool assembly into a wellbore adjacent a formation zone of perforation tunnels previously formed in a formation surrounding the wellbore, the tool assembly carrying an explosive and having plug assemblies including expandable and collapsible plug elements provided at opposite ends thereof wherein the plug elements are normally in a collapsed state spaced from an outer extent of the wellbore and are actuable to an expanded state; (2) activating the plug elements to the expanded state such that the plug elements extend between the downhole tool assembly and the outer extent of the wellbore to isolate a discrete segment of the wellbore from a remainder of the wellbore; (3) detonating the explosive in the downhole tool assembly to create a dynamic underbalance or overbalance effect confined to the discrete segment of the wellbore for purging the perforation tunnels or stimulating the wellbores; and (4) deactivating the plug elements to the collapsed state upon termination of the dynamic underbalance or overbalance effect. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The best mode is described herein below with reference to the following drawing figures. 
           [0013]      FIG. 1  is a sectional view of a well formation having a wellbore provided with a downhole tool assembly according to the present disclosure; 
           [0014]      FIG. 2  is an enlarged fragmentary sectional view of a lower portion of  FIG. 1  in an unfired condition with certain portions of the structure surrounding the wellbore being omitted for simplicity; 
           [0015]      FIG. 3  is an enlarged fragmentary sectional view similar to  FIG. 2  showing the downhole tool assembly during a fired condition; 
           [0016]      FIG. 4  is a representation of the downhole tool assembly of  FIG. 1 ; 
           [0017]      FIG. 5  is a representation of the downhole tool assembly of  FIG. 3 ; and 
           [0018]      FIG. 6  is a further representation of the downhole tool assembly following a fired condition. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In the following description, certain terms have been used for brevity, clearness and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of prior art because such terms are used for descriptive purposes and are intended to be broadly construed. The different configurations and methods described herein may be used alone or in combination with other configurations, systems and methods. It is to be expected that various equivalents, alternatives and modifications are possible within the scope of the appended claims. 
         [0020]    Referring now to the drawings,  FIG. 1  illustrates a typical well installation  10  including a wellbore  12  normally containing borehole fluid  14 . As is well known, the wellbore  12  has a surrounding casing  16  and cement  18  disposed between the casing  16  and the surrounding surface formation  20 . A wellhead  22  is positioned at the top of the surface formation  20 , and is provided with an open bottom tubing  24  that extends downwardly into an upper portion of the wellbore  12 . In the well installation  10  illustrated, the surface formation  20  includes an area of caprock  26 , a damaged formation  28  and an undamaged formation  30 , all of which surround cement  18 . Perforation tunnels  32  extend through the casing  16  and cement  18  into the damage formation  28  at one or more desired formation zones  33 . 
         [0021]    The perforation tunnels  32  are previously formed using a perforating gun string to allow fluid flow from the formation zones  33  to flow into the well for production to the surface, or to allow stimulating injection fluids to be applied to the formation zones. The explosive nature of the formation of the perforation tunnels  32  shatters the sand grains in the damaged formation  28  and typically generates tunnels  32  full of rock debris mixed in with perforator charge debris. Such debris is known to impair the productivity of production wells and negatively impact upon the flow of formation fluids in the well. The present disclosure sets forth a device provided with a transient plug arrangement which is used to clean the debris from the plug perforation tunnels  32  or otherwise stimulate the surface formation  20  by focusing and controlling a dynamic underbalance or dynamic overbalance condition in a desired formation zone  33  so as to improve fluid communication in this zone  33  of the well. 
         [0022]    In accordance with the present disclosure, a downhole tool assembly  34  is lowered into the wellbore  12  in a zone of previously formed perforation tunnels  32 . The tool assembly is suspended in the wellbore  12  by a carrier structure such as by a cable  36  that extends through the wellhead  22 . The lower end of cable  36  is secured to a head  38  which, in turn, is connected to a casing collar locator  40  and a firing head  42 . A downhole tool  44  in the form of an elongated hollow gun or tube has an upper end that is connected to the firing head  42 , and a lower head attached to a connector  46  with a threaded end plug  48 . The downhole tool assembly  34  includes an upper plug assembly  50  positioned above and in communication with the downhole tool  44 , and a lower plug assembly  52  inverted with respect to, and similar in construction to plug assembly  50  and positioned below and in communication with the downhole tool assembly  44 . Because of the similarity and construction of the upper plug assembly  50  and the lower plug assembly  52 , only the description of the lower plug assembly  52  is set forth hereafter. 
         [0023]      FIG. 2  shows the downhole tool assembly  34  in an installed or unfired condition, while  FIG. 3  illustrates the downhole tool assembly  34  during a fired condition. 
         [0024]      FIGS. 1-6  depict the downhole tool assembly  34  as used to focus and control the effects of dynamic underbalance in a chosen area of the wellbore  12 . However, as will be understood hereafter, the downhole tool assembly  34  may also be employed to isolate the effects of dynamic overbalance, if desired. 
         [0025]    Referring now to  FIGS. 1-3 , the downhole tool  44  has an elongated tubular body  54  which is generally cylindrical in cross section. It can be appreciated from  FIG. 1 , that downhole tool  44  as well as head  38 , casing collar locator  40 , firing head  42 , the upper and lower plug assemblies  50 ,  52  and the connector  46  all have substantially similar cylindrical shape and outer diameters which will permit the insertion and extraction of assembly  34  relative to wellbore  12 . The tubular body  54 , when positioned in the downhole tool assembly  34 , defines a sealed internal underbalance chamber  56  ( FIGS. 2 and 3 ) which typically contains only air at atmospheric pressure such as that set at the well surface for insertion into the wellbore  12 . Air at atmospheric pressure provides an internal chamber pressure which is significantly less than the wellbore pressure encountered at a formation zone  33  or the formation pore pressure. 
         [0026]    As seen in  FIG. 2 , the tubular body  54  has a trunk  58  which is threadedly connected to an upper end  60  of elongated hollow cylinder  62  that extends from the body  54 . An elongated hollow piston  64  is disposed for sliding movement back and forth inside the cylinder  62 . The piston  64  has an enlarged upper end  66  that normally is positioned against a lower end  68  of the cylinder  62  when the assembly  34  is in the unfired condition in the wellbore  12 . A pair of annular O-rings or seals  70  is provided between the inner surface of cylinder  62  and the outer surface of the piston upper end  66 . A lower end  72  of the piston  64  is formed with a central recess  74 , and is normally disposed upon the top of connector  46  when the assembly  34  is in the unfired condition. 
         [0027]    The piston  64  slides back and forth upon an elongated hollow mandrel  76  that has a top end  78  threadably secured to a neck portion  80  of a cylinder  62  such that the mandrel  76  extends through the center of the cylinder  62  and lies inwardly of the piston  64 . As seen from  FIG. 3 , a lower end  82  of the mandrel  76  is threadably attached to the connector  46 . The mandrel  76  is formed with a vertically extending passageway  84  ( FIG. 3 ) which opens into tubular body  54 , and is designed to hold a detonating or primer cord  86  that extends between the firing head  42  and the lower end  72  of piston  64  when assembly  34  is in the unfired condition. If a non-explosive device is required, the passageway  84  would contain electrical connections leading to an electrical release system. 
         [0028]    An upper portion of mandrel  76  is constructed with a vent  88  that communicates with an interior of cylinder  62 . A lower end  90  of the mandrel  76  is provided with an opening  92  for retaining a rupture element, electrical release or shear disk  94  that normally extends radially into the piston recess  74  when the assembly  34  is in the unfired condition. An annular O-ring or seal  96  is provided between the lower end  90  of mandrel  76  and the lower end  72  of piston  64 . A coil spring  98  surrounds the mandrel  76  and lies inwardly of the inner surface of cylinder  62 . The spring has a top end  100  engaged against the neck portion  80  of the cylinder  62 , and a bottom end  102  engaged against the upper end  66  of piston  64 . 
         [0029]    The lower plug assembly  52  (as well as the upper plug assembly  50 ) typically includes a flexible, elastomeric production packer or plug element  104  which is expandable and collapsible. The plug element  104  is generally designed to be temperature, chemical and tear resistant as well as extremely elastic. As seen in  FIG. 2 , the plug element  104  surrounds the piston  64  and extends between the cylinder  62  and the piston  64 . More particularly, a top end  106  of the plug element  104  is attached to a recessed portion at the lower end  68  of cylinder  62 . A bottom end  108  of the plug element  104  is secured to a recessed portion at the lower end  72  of piston  64 . In the example shown, the plug element  104  has an inner layer  110  and an outer layer  112 . 
         [0030]    As will be explained in greater detail below, the foregoing construction generally provides that each plug element  104  is movable between collapsed and expanded states or positions relative to the inside of casing  16  by virtue of sliding movement of piston  64  relative to the cylinder  62  and the mandrel  76 . 
         [0031]    The operation of the downhole tool assembly  34  of the present disclosure will now be described, with initial reference to  FIGS. 1 and 4  which show the tool  44  suspended in the wellbore  12  containing borehole fluid  14  and positioned adjacent a formation zone  33  having a series of previously formed perforation tunnels  32  filled with damage and debris. The tool  44  is in the installed or unfired condition as described above with internal chamber  56  ( FIG. 1 ) of the tool  44  being at atmospheric pressure which is significantly lower than the pressure in the surrounding wellbore  12  and the pore pressure of surrounding formation  20 . The lower pressure in internal chamber  56  is in communication with the top of each piston  64  via the mandrel passageway  84  and the vent  88 . Each piston  64  is prevented from slidably moving along its mandrel  76  towards the low pressure in chamber  56  by the engagement of the ruptured disk  94  in the mandrel  76  and, to some extent, by the spring  98  which is normally biased against the top of piston  64 . 
         [0032]    When it is desired to focus an underbalance event in a desired formation zone  33 , a well operator actuates the firing head  42  and detonates the primer cord  86  causing an extremely rapid explosion along the entire length thereof. The firing of primer cord  86  causes rupturing  112  of the tubular body  54 , as shown in  FIG. 5 , and also ruptures the shear disks  94  which frees the pistons  64  to slide along the mandrels  76 . Rupturing the tubular body  54  creates a pressure differential between the higher pressure in wellbore  12  and the lower pressure in the internal chamber  56 . This causes the pistons  64  to move quickly along mandrels  76  towards each other in the direction of arrows A shown in  FIG. 5  against the relatively weak force of springs  98  which are compressed. At the same time, flexible plug elements  104  are rapidly squeezed or compressed adjacent the ends  68  of the cylinders  62  ( FIG. 3 ) so as to instantaneously deploy and expand the plug elements  104  into temporary plugging engagement with the inside of casing  16 . The existing pressure forces maintain the pistons  64  and plug elements  104  in position. 
         [0033]    Upon instantaneous deployment of the plug elements  104 , a dynamic underbalance effect created by the pressure differential is initiated resulting in a suction flow of the fluid from the wellbore  12  and debris from the perforation tunnels  32  only from the isolated wellbore zone  114  ( FIG. 5 ) defined by and between the expanded plug elements  104 . In the meantime, the low pressure sides of the pistons  64  are flooded with borehole fluid  14  which flows through the exposed ruptured openings  116  ( FIG. 3 ) and the passageways  84  in mandrels  76  equalizing the pressure and allowing the plug elements  104  to turn to their original collapsed shape and dimensions. The equalized pressure also allows the compressed springs  98  to assist in returning the plug elements  104  to their original shape as shown in  FIG. 6 . Upon restoration of the plug elements  104  to their initial condition, the tool  44  filled with fluid and debris is extracted from wellbore  12  such that the cleaned material deposited in the tubular body  54  may be analyzed, if desired. Thereafter, the fractured tool  44  including the plug elements  104  may be disposed of. 
         [0034]    It should be understood from the above exemplary embodiment that the downhole tool assembly  34  creates a transient mechanical plug arrangement that is utilized to focus and control the effect of dynamic underbalance in the wellbore zone  114  temporarily defined by the expanded plug elements  104 . Such arrangement disrupts the movement and pressure effects of the borehole fluids outside the wellbore zone  114  towards the area of dynamic underbalance so as to maximize the effect of cleaning of debris from the perforation tunnels  32  in the zone  114 . In addition, the transient plug arrangement confines the effect of the explosion occurring in the tubular body  54  to the defined wellbore zone  114 . 
         [0035]    While the exemplary embodiment set forth above is described for a dynamic underbalance effect, it should be appreciated that the present disclosure can also be used to focus and control the effects of dynamic overbalance, if desired. In such case, plug elements  104  would again be positioned above and below a dynamic overbalance chamber defined by tool  44 , and tubes having low pressure chambers would be positioned above and below plug elements  104 . 
         [0036]    In the present disclosure, the plug elements  104  are self-deployed by the pressure differential created by the detonation before the transient pressure event (dynamic underbalance or dynamic overbalance) occurs. However, it should be realized that the plug deployment may be independent of the event that causes the underbalance or overbalance condition. That is, it is not essential that the plug deployment be triggered by the primer cord explosion. Plug deployment, as well as rupturing of the tubular body  54 , could otherwise be actuated, such as, for example, by an electrical solenoid or other electromechanical or hydraulic device before the underbalance or overbalance effect takes place. 
         [0037]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
         [0038]    Various alternatives and embodiments are contemplated as being with in the scope of the following claims, particularly pointing out and distinctly claiming the subject matter regarded as the invention.