Patent Publication Number: US-6712148-B2

Title: Junction isolation apparatus and methods for use in multilateral well treatment operations

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides multilateral wellbore junction isolation apparatus and associated well stimulation methods. 
     Wellbore junctions are formed at intersections of wellbores in a subterranean well. For example, a main or parent wellbore may have a branch or lateral wellbore drilled extending outwardly from an intersection between the main and branch wellbores. Of course, the main wellbore may extend below the intersection with the branch wellbore, for example, to intersect a formation from which it is desired to produce hydrocarbons into the main wellbore. 
     Unfortunately, however, some wellbore junctions are not able to withstand substantial internal pressure applied thereto. For this reason, pressure within these wellbore junctions is limited to the fracture gradients of the respective formations in which the wellbore junctions are positioned. Thus, if stimulation operations, such as fracturing, must be performed for formations downhole of the wellbore junctions, expensive, time-consuming and/or complicated procedures must be used to prevent exceeding the fracture gradients of the formations at the wellbore junctions. Moreover, if an acid fracturing stimulation method is being employed the wellbore junctions are also susceptible to corrosion damage from the fracturing acid if care is not taken to shield the junctions from such corrosive material. 
     Therefore, it would be quite desirable to provide apparatus and s methods for isolating a wellbore junction which are convenient and easily utilized, and which isolate the wellbore junction from fluid pressure applied through the junction, as well as the corrosive effects Of a fluid creating such pressure. 
     SUMMARY OF THE INVENTION 
     In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, specially designed apparatus is provided for isolating a junction between first and second intersecting wellbores in a subterranean well. The apparatus is removably insertable in the well, in a single trip into the well, and is operative to create in the well a fluid flow passage sealingly straddling the junction and protecting the junction from a pressurized fluid, representatively a well treatment fluid such as a fracturing acid, forced into a portion of one of the first and second wellbores via the interior of a portion of the junction isolation apparatus. 
     In a preferred embodiment thereof, the junction isolation apparatus comprises an elongated generally tubular outer structure having first and second longitudinally spaced part upper and lower portions with first and second external sealing devices respectively disposed thereon, the second external sealing device having an outer surface through which a recess inwardly extends to the outer structure. An elongated generally tubular inner structure is coaxially, sealingly and removably received in the outer tubular structure, and a seal test fluid flow passage extends from the interior of the inner structure into the recess in the second external sealing device. 
     Preferably, the first external sealing device is a packer having unset and set orientations in which the packer respectively precludes and permits the removal of the inner tubular structure from the outer tubular structure, and the second external sealing device comprises a longitudinally spaced plurality of annular sealing members circumscribing a lower end portion of the outer tubular structure. The first sealing device may be of an alternative structure, such as a seal bore portion of the wellbore casing, if desired. Also, the packer could be replaced by a non-sealing type of support structure, such as a hanger, with the function of the first sealing device being performed by, for example, a bridge plug run prior to setting a whipstock used to deflect the isolation structure into the second wellbore, or a packer run in conjunction with the whipstock. 
     A lower end portion of the inner tubular structure is blocked by, for example, a plug structure or check valve, and upper and lower end portions of the inner tubular structure respectively carry third and fourth external sealing devices which slidingly seal against the inner side surface of the outer tubular structure and are positioned at the top and bottom of an annulus defined between the inner and outer tubular structures. A sidewall opening in the inner tubular structure, and a sidewall opening disposed in the outer tubular structure at the second seal device recess, communicate with the annulus. The annulus and these sidewall openings form the previously mentioned seal test fluid flow passage. 
     To ready the junction isolation apparatus for use it is lowered into the well, representatively on a suitable work string structure anchored to the inner tubular structure, in a manner sealingly engaging the second external sealing device with an interior area of a selected one of (1) a portion of the first wellbore downhole from the junction and (2) a portion of the second wellbore, and positioning the packer adjacent an interior area of the first wellbore uphole of the junction. By flowing a suitable pressurized test fluid downwardly through the work string and, via the test fluid flow passage, into the recess of the second sealing device the second sealing may be conveniently pressure tested before the packer is set. 
     Upon a successful completion of this seal pressure test, the packer is set, thereby releasing the inner tubular structure from the outer tubular structure, and the work string is pulled out of the well, thereby also removing the inner tubular structure from the outer tubular structure and withdrawing the inner tubular structure from the well. The outer tubular member is thus left in place within the well, with the interior of the outer tubular member defining a fluid flow path that, at its upper end, communicates with substantially the entire cross-sectional area of an upwardly adjacent longitudinal portion of the first wellbore, and at its lower end communicates with the interior of the selected wellbore portion. This fluid flow path straddles and is sealingly isolated from the wellbore junction. 
     A wellbore treatment process, for example a fracturing/stimulation process, may then be carried out by pumping a pressurized well treatment fluid, such as a fracturing acid, downwardly through the full cross-sectional area of the first wellbore portion extending upwardly from the upper end of the remaining outer tubular member and, via the fluid flow path extending through the interior of the remaining outer tubular structure, into the selected wellbore portion. During this acid fracturing stimulation process the pressurized fracturing acid is isolated from the junction, to prevent pressure and/or corrosive damage thereto, and there is no return circulation flow of the stimulation fluid being forced into the selected wellbore portion. 
     The configuration and placement of the remaining outer tubular structure permits, as noted above, the well treatment to be downwardly flowed directly through the first wellbore portion disposed above the outer tubular member—i.e., through the entire cross-sectional area of such first wellbore portion. This advantageously reduces the pressure drop to which the flowing stimulation fluid is subjected and thus correspondingly facilitates higher stimulation fluid pumping rates. The configuration and construction of the overall isolation apparatus are quite simple, and the isolation apparatus may be installed in the well, and pressure-tested therein, with a single trip into the well. If the seal pressure test does not yield satisfactory results the entire isolation apparatus may be quickly and easily pulled out of the well for repair or refitting prior to the setting of the packer. After the stimulation or other well treatment process is completed, a suitable retrieval tool may be used to unset the packer and withdraw the outer tubular structure portion of the isolation apparatus from the well. Prior to its removal from the well the outer tubular member (when operatively extended into the second wellbore) may be conveniently utilized as a deployment tube through which a selected tool or other object may be lowered into the second wellbore to prevent interference between the lowered object and the junction area. 
     In an alternate embodiment thereof the junction isolation apparatus is provided with a modified outer tubular member having an enlarged upper longitudinal portion sized for coupling to a large diameter workstring which may be used to lower the junction isolation apparatus into the well, or be sealingly stabbed into the upper end of the outer tubular member after the junction isolation apparatus has been operatively positioned in the well by other means. During the stimulation process stimulation fluid is pumped downwardly through the workstring and operatively through the outer tubular member, thereby protecting the well casing from stimulation fluid pressure but still providing a substantially lowered stimulation fluid pumping pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic, longitudinally foreshortened cross-sectional view through a representative multilateral subterranean well illustrating the placement in one of its wellbores of a specially designed straddle stimulation structure embodying principles of the present invention and utilized to isolate a wellbore junction from fluid pressure and corrosion during an acid fracturing stimulation process; 
     FIG. 1A is an enlarged scale, longitudinally foreshortened cross-sectional view through the straddle stimulation structure during seal pressure testing thereof prior to initiation of the acid fracturing stimulation process; 
     FIG. 2 is a view similar to that of FIG. 1, but illustrating the performance of the acid fracturing stimulation process; 
     FIG. 2A is a view similar to that of FIG. 1A, but with fracturing acid being operatively forced through the interior of an outer tubular portion of the straddle stimulation structure; and 
     FIG. 3 is a schematic, longitudinally foreshortened cross-sectional view through an alternate embodiment of the straddle stimulation structure. 
    
    
     DETAILED DESCRIPTION 
     Schematically depicted in cross-section in FIG. 1 is a representative subterranean multilateral well  10  which has been readied for a stimulation operation, representatively an acid fracturing operation, utilizing a specially designed isolation assembly, representatively in the form of a straddle stimulation structure  12  embodying principles of the present invention and subsequently described in detail herein. 
     In the following description of the well  10 , and other apparatus and methods described herein, directional terms, such as “a bove”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Specifically, the term “ab ove” is used herein to designate a direction toward the earth&#39;s surface (i.e., “uphole”), and the term “below” is used herein to designate a direction away from the earth&#39;s surface along a wellbore (i.e., “do wnhole”), even though the wellbore may not be substantially vertical. Additionally, it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. 
     The representative multilateral well  10  illustrated in FIG. 1 has been constructed in a suitable conventional manner and has an illustratively vertical main wellbore section  14  with a tubular metal casing  16  cemented into the wellbore  14  as at  18 . Forming a continuation of the lower end of the main wellbore  14  is a first lateral or branch wellbore  20  which turns outwardly in a generally horizontally direction and extends through a subterranean zone or formation  22  in which it is desired to perform a stimulation operation, such as acid fracturing, to thereby increase production of hydrocarbons therefrom. The wellbores  14  and  20  combinatively define a first wellbore portion of the multilateral well  10 . 
     Extending through the branch wellbore  20  is a tubular liner  24  having an open upper end portion  26  sealed within a lower end portion of the main wellbore casing  16  by a schematically depicted annular seal structure  28 , a polish bore portion  29  just beneath the upper end portion  26 , and a horizontal lower end portion  30  extending through the formation  22  and having a suitable plug (not shown) at its outer end. Liner  24  is cemented into the branch wellbore  20  with cement  32  which is representatively an acid soluble cement. To facilitate a subsequent acid fracturing or other stimulation or treatment operation in the formation  22 , perforations  34  have been formed through the liner portion  30  and the cement  32  into the formation  22  by, for example, utilizing a perforating gun (not shown) lowered into the liner section  30 , detonated, and then withdrawn from the well  10 . 
     Intersecting the main wellbore  14  at a junction area  36  disposed above the upper end of the first branch wellbore  20  is a second lateral or branch wellbore  38  that turns outwardly from the main wellbore  14  in a generally horizontal direction and extends through a subterranean zone or formation  40  in which it is desired to perform a stimulation operation, such as acid fracturing, to thereby increase production of hydrocarbons therefrom. 
     Extending outwardly through the junction area  36  is a tubular transition joint  42  outwardly circumscribing a polish bore portion  44  of a tubular liner  46  extending through the second branch wellbore  38  and having a horizontal lower end portion  48  passing through the formation  40  and having a suitable plug (not shown) at its outer end. Liner  46  and the transition joint  42  are cemented into the branch wellbore  38  with cement  32 . To facilitate a subsequent acid fracturing stimulation operation in the formation  40 , perforations  34  have been formed through the liner portion  48  and the cement  32  into the formation  40  by, for example, utilizing a perforating gun (not shown) lowered into the liner section  48 , detonated, and withdrawn from the well  10 . 
     With the multilateral well  10  constructed in this conventional manner and representatively readied for an acid fracturing type stimulation operation, the specially designed straddle stimulation structure  12  is utilized in a manner which will now be described to isolate and protect the junction area  36  from damage from the high pressure and corrosiveness of the stimulation fluid. Referring now to FIGS. 1 and 1A, the straddle stimulation structure  12  is operatively deployed in the well  10 , in a single trip down the main wellbore  14 , by lowering it through the main wellbore  14  on a suitable tubular work string  50 . For purposes of initial discussion it will be assumed that the straddle stimulation structure  12  is to be utilized to carry out an acid fracturing stimulation operation in the lower formation  22 . 
     Still referring to FIGS. 1 and 1A, the straddle stimulation structure  12  includes an elongated, open-ended outer tubular member  52 , and an elongated, open-ended inner tubular member  54  coaxially extending through the outer member  52  and forming therewith an annular space  56  positioned therebetween. AS illustrated, a laterally enlarged upper end portion  58  of the inner tubular member  54  overlies the open upper end of the outer tubular member  52  and is suitably anchored to the lower end of the work string  50 . Externally carried respectively on upper and lower end portions of the inner tubular member  54  are annular seal members  60  and  62  (see FIG. 1A) which slidingly and sealingly engage the interior side surface of the outer tubular member  52  and thereby sealingly block off upper and lower ends of the annular space  56 . 
     Disposed within a lower end portion of the inner tubular member  54  is a schematically depicted blocking structure  64  which is representatively a fixed plug member, but may alternatively be, for example, a velocity check valve structure or a removable plug member. Somewhat above the blocking structure  64  are a circumferentially spaced plurality of sidewall outlet ports  66  formed in the inner tubular member  54  and positioned below a circumferentially spaced plurality Of sidewall outlet ports  68  formed in the outer tubular member  52 . 
     An annular upper external sealing device  70  is externally carried on an upper end portion of the outer tubular member  52 , and an annular lower external sealing device  72  is externally carried on a lower end portion of the outer tubular member  52 . Illustratively, the upper sealing device  70  is a VERSA-TRIEVE® packer as manufactured by Halliburton Energy Services, Inc. of Duncan, Okla. The packer  70 , when in an unset orientation (as shown in FIGS. 1 and 1A) is used in a conventional, well known manner to prevent the removal of the inner tubular member  54  from the outer tubular member  52 . However, when the packer  70  is subsequently set (as schematically depicted in FIGS. 2 and 2A) within the main wellbore  14 , the packer  70  releases the inner tubular member  54  from the outer tubular member  52 . A no-go sub structure  74  (see FIG. 1) is carried by the outer tubular member  52  somewhat above the external annular sealing device  72 . 
     The lower external sealing device  72 , as illustrated in FIGS. 1 and 1A, representatively comprises a plurality of axially spaced annular resilient seal members  72   a , 72   b , 72   c , 72   d  externally carried on a lower end portion of the outer tubular member  52 , with the sidewall outlet ports  68  in the outer tubular member  52  being disposed between the annular seal member pair  72   b , 72   c.    
     With continuing reference to FIGS. 1 and 1A, to stimulate the subterranean formation  22  representatively using an acid fracturing process, the straddle stimulation structure  12 , with its packer  70  in an unset orientation, is lowered through the main wellbore  14  on the work string  50  until the seal structure  72  sealingly stabs into the liner seal bore portion  29  and the no-go structure  74  abuts the upper end of the liner portion  26 . As can be best seen in FIG. 1A, this communicates the annular space  56  within the straddle stimulation structure  12  with a sealed-off annular space  76  bounded by the outer tubular member  52 , the liner seal bore portion  29 , and the annular seal elements  72   b , 72   c.    
     According to one aspect of the present invention, this permits the lower seal structure  72  to be pressure tested prior to the setting of the packer  70 . Thus, if leakage around the seal structure  72  is detected, the straddle stimulation structure  12  may simply be pulled out of the well  10  on the work string  50  in a simple and rapid manner and repaired or refitted as necessary. To test the in-place lower seal structure  72  prior to carrying out an acid fracturing stimulation process in the formation  22 , a seal test fluid, representatively water  78 , is pumped downwardly through the interiors of the work string  50  and the inner tubular member  54 . The water  78  is forced outwardly through the inner tubular member sidewall ports  66  and into the seal annulus  76  via the sealed-off annulus  56  between the outer and inner tubular members  52 , 54 . The water  78  is brought to a predetermined seal test pressure, and a predetermined seal test time is permitted to elapse. 
     If the pressure of the water  78  appreciably diminishes during the seal test period, leakage around the lower seal structure  72  is accordingly detected, and the straddle stimulation structure  12  may be rapidly and easily removed from the well  10  as described above for seal repair or refitting. On the other hand, if the pressure of the water  78  does not appreciably drop during the seal test period, the lower seal structure  72  passes its pressure test, and the acid fracturing stimulation of the formation  22  is initiated as will now be described in conjunction with FIGS. 2 and 2A. 
     Upon successful completion of the lower seal pressure test, the packer  70  is set to thereby sealingly engage with the interior side surface of the casing  16 , thereby locking the upper end of the outer tubular member  52  within the casing  16  and releasing the inner tubular member  54  from the outer tubular member  52 . AS indicated by the arrow  80  in FIG. 1A, the work string  50  is then pulled upwardly out of the main wellbore  14  bringing the now freed inner tubular member  54  with it. This leaves the outer tubular member portion  52  of the straddle stimulation structure  12  in place within the main wellbore  14 , with the lower seal structure  72  still sealingly engaged with the polish bore portion  29  of the liner  24 . 
     Fracturing acid  82  (see FIGS. 2 and 2A) is then downwardly pumped directly through the casing  16  and into the liner  24  via the interior of the outer tubular member  52 . Pressurized acid  82  entering the liner  24  is forced outwardly through the perforations  34  into the formation  22  to fracture it and thereby stimulate its subsequent production rate. During this formation stimulation process there is no return flow of the stimulating fluid. 
     The ability, provided by the unique configuration and operation of the straddle stimulation structure  12  described above, to pump the fracturing acid  82  (or other stimulation or well treatment fluid as the case may be) directly through the casing (i.e., utilizing the full interior cross-sectional area of the main wellbore  14  as a stimulation fluid flow area), as opposed to having to pump stimulation fluid downwardly through smaller diameter auxiliary tubing extending through the main wellbore  14 , desirably provides lower stimulation fluid pressure drops and permits higher stimulation fluid flow rates. 
     According to a key aspect of the present invention, during this downflow of pressurized fracturing acid  82 , the wellbore junction area  36  is sealingly isolated and protected from contact by such acid flow and damage thereby from either its pressure or its corrosiveness. AS can best be seen in FIG. 2, the outer tubular member  52  defines an acid flow path which sealingly straddles and is isolated from the junction area  36 . 
     AS will be readily be appreciated by those of ordinary skill in this particular art, leakage in the lower seal structure  72  could permit pressurized acid  82  to move upwardly through the casing  16 , around the tubular member  52  and contact the junction area  36 . However, the previously described method for testing the lower seal structure  72  substantially eliminates the possibility of this undesirable contact with the junction area  36  in a quick and easy manner. 
     After the acid fracturing stimulation of the formation  22  is carried out as described above, the packer  70  can be unset, and the in-place balance of the straddle stimulation structure  12  (i.e., the remaining outer tubular member  52 ) can be pulled out of the well  10  and the well  10  prepared for production in a suitable conventional manner. 
     The illustrated packer  70  could alternatively be one of a variety of other types of sealing devices such as, for example, a seal bore portion of the casing  16 , or could be a non-sealing type of support structure such as a hanger device. In this latter case the provision of a sealing structure between the outer tubular member  52  and the casing  16  above the junction  36  could be effected using a sealing device which is not carried by the member  52  such as, for example, a bridge plug run prior to setting a whipstock (not shown) used to divert the member  52  into the lateral wellbore  38 , or a packer rung in conjunction with the whipstock. 
     While the straddle stimulation structure  12  has been illustrated and described herein as being utilized in the acid fracturing stimulation of the formation  22  associated with the lower branch wellbore  20 , it can of course also be used in conjunction with the acid fracturing stimulation of the upper formation  40  associated with the upper branch wellbore  38 , while at the same time isolating the junction area  36  from contact by the pressurized acid. This alternate use of the straddle stimulation structure  12  is effected by simply lowering the structure  12  into the main wellbore  14  and then, instead of stabbing the lower seal portion  72  of the structure  12  into the lower liner  24  as previously described herein, suitably deflecting the structure  12  into sealing engagement with the seal bore portion  44  of the upper liner  46  as indicated in phantom in FIG.  2 . The acid fracturing of the formation  40  may then carried out in a manner previously described herein for the formation  22 . Alternatively, of course, the fracturing or other treatment of the formation  40  may be carried out before the fracturing or other treatment of the formation  22  if desired. 
     Referring again to FIG. 2, after the stimulation of zone  40 , the outer tubular member  52  may be conveniently be used as a deployment tube structure through which an object, such as the tool  84 , may be lowered through the outer tubular structure  52  into the lateral wellbore  38  using a suitable lowering structure such as a wireline  86 , tubing string or the like. To facilitate the entry of the tool  84  into the open upper end of the tubular structure  52  such upper end may be provided with a funnel-like configuration as at  88 . 
     As previously described herein, an advantage provided by the use of the straddle stimulation structure  12  is the ability to pump fracturing or other well stimulation or treatment fluid downwardly through the entire cross-sectional area of the casing  16 . However, in some instances it may be desirable or necessary not to pump pressurized fluid directly through the casing, but to pump the fluid through the straddle stimulation structure via an alternate flow route which protects the casing  16  from the pressure of the treatment or stimulation fluid being downwardly pumped. 
     To accommodate this situation, while at the same time providing desirably lowered pumping pressure drops for the stimulation or other treatment fluid, the present invention provides, as schematically depicted in FIG. 3, an alternate embodiment  12   a  of the previously described straddle stimulation structure  12 . For the purpose of facilitating comparison of the structures  12  and  12   a , components in the structure  12   a  similar to those in structure  12  have been given identical reference numerals having the subscripts “a”. 
     Turning now to FIG. 3, the straddle stimulation structure  12   a  has a modified outer tubular member  52   a  which has a lower longitudinal portion  90  with a diameter identical to the diameter of the previously described outer tubular member  52 , and an upper longitudinal portion  92  having a substaritially larger diameter. As an example, but not by way of limitation, the casing  16  has a 7″ diameter, the lower longitudinal portion  90  has a 3.688″ diameter, and the upper longitudinal portion  92  has a 4.5″ diameter and has an open upper end sized to sealingly receive a lower end portion of a similarly sized tubular workstring  94  which is shown in phantom in FIG.  3 . 
     To use the modified straddle stimulation structure  12   a , it is suitably positioned within the well (representatively extending into the lateral wellbore  38 ) and has its lower seal structure  72  pressure tested as previously described in conjunction with the straddle stimulation structure  12 . The inner tubular member  54   a  is then removed from the outer tubular member  52   a  as indicated by the arrow  96  in FIG.  3 . The workstring  94  is then lowered downwardly through the wellbore  14  and sealingly stabbed into the open upper end of the remaining outer tubular member  52   a , and pressurized stimulation fluid, such as the fracturing acid  82 , is pumped downwardly through the workstring  94  and into the lateral wellbore  38  via the interior of the outer tubular member  52   a . The workstring  94  and the outer tubular member  52   a  can then be removed from the well. 
     As an alternative to stabbing the workstring  94  into the upper end of the outer tubular member  52   a  after the straddle stimulation structure  12   a  has been set in the well, its lower seal structure  72  pressure tested, and its inner tubular structure  54   a  removed, the modified straddle stimulation structure  12   a  can simply be lowered into place on the lower end of the workstring  94 . The lower seal structure  72  can then be pressure tested by flowing a seal test fluid downwardly through the workstring  94 . The inner tubular structure  54   a  can then be removed upwardly through the interior of the workstring  94 , and stimulation fluid  82  pumped downwardly through the workstring  94 . The workstring  94  and the remaining outer tubular member  52   a  can then be lifted out of the well. As will be appreciated by those of skill in this particular art, it is not mandatory that the straddle stimulation structure  12   a  have a lower seal structure, or to test such lower seal structure, when the straddle stimulation structure is used only to deploy a tool into the lateral bore  38  as previously described herein. 
     While the straddle stimulation structures  12  and  12   a  have been representatively described herein as being utilized in conjunction with an acid fracturing stimulation process, it will readily be appreciated by those of ordinary skill in this particular art that they could also be used to advantage with other well treatment or stimulation fluids, Such as water. Additionally, the various wellbore portions  14 ,  20  and  38  have been representatively depicted herein as being cased or lined, but it is to be clearly understood that the principles of the invention may be incorporated into other methods performed in uncased or unlined wellbores. Furthermore, the principles of the invention are not limited to wellbore junctions formed between main and branch wellbores. Also, while the drawings representatively depict a TAML level  4  junction construction, the junction isolation apparatus and methods illustrated and described herein could also be utilized in conjunction with a TAML level  2 ,  3 ,  5  or  6  junction construction 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.