Patent Publication Number: US-2016245062-A1

Title: Methods, strings and tools to enhance wellbore fracturing

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application No. 61/886,784, filed Oct. 4, 2013. 
    
    
     FIELD 
     The present invention relates to methods, strings and tools for fracturing a wellbore. 
     BACKGROUND 
     Wellbore treatments by fracturing have proven to be quite successful. 
     In a cemented well, that is where cement is placed in the annulus between the well liner and the wellbore wall, fracturing can be difficult. In particular, the cement blocks the annulus and, when ported tubulars are employed, the cement inhibits the fracture fluid from passing from the port of the ported tubular forming the well liner to the wellbore wall. As such, the fracture is often difficult to achieve. 
     To address these problems, Packers Plus Energy Services Inc., the present applicant, invented a cement diffuser to facilitate fracturing in cemented wells. For example, U.S. Pat. No. 7,798,226 and No. 8,033,331 describe the cement diffuser. The cement diffuser is installed over the port of a ported tubular. When the ported tubular is in place with the cement diffuser over its port, cement can be introduced to the annulus. When the cement is set, a path is created through the cement from the ported tubular to the wellbore wall. However, with such a cement diffuser, the fracture fluid reaches the formation in only one location: radially out from the port of the tubular. While this may facilitate fracturing over fracturing in a standard cemented well, the wellbore accessed may not be the best rock in which to have a fracture form and only a simple fracture is likely to form. 
     In fracturing, it has been found that fracturing can be enhanced by having fracture complexity and fracturing into natural weaknesses. Complexity is where plural fractures, including for example, both primary and secondary fractures, are generated along a single fracture site in a wellbore. Natural fractures are where a fracture opens via a natural weakness in the wellbore wall. Fracture complexity and natural fracturing has been difficult to achieve in cemented wells, even where a cement diffuser, as described in the above-noted US Patents, is employed. 
     SUMMARY 
     In accordance with a broad aspect of the present invention, there is provided a tubular installation in place in a borehole, the tubular installation creating an annular space between the tubular installation and a wall of the borehole, the tubular installation comprising: a tubular including a wall having an inner surface and an outer surface; a port extending through the wall, the port including an upper end wall and a lower end wall the distance between the upper end wall and the lower end wall defining the open axial length of the port; and an external structure carried on the outer surface, the external structure overlying at least a portion of the axial length and extending axially from the port beyond at least one of the upper end wall and the lower end wall and remaining in place overlying at least a portion of the axial length when the port is opened. 
     In accordance with another broad aspect of the invention, there is provided a wellbore tubular comprising: a wall having a first end, an opposite end, an inner surface and an outer surface; a port extending through the wall, the port including an upper end wall and a lower end wall, the distance between the upper end wall and the lower end wall defining the open axial length of the port; and an external structure carried on the outer surface, the external structure overlying at least a portion of the axial length and extending axially from the port beyond at least one of the upper end wall and the lower end wall, the external structure operable to create a pathway through hardened cement for passage of fracturing fluid from the port axially along the outer surface away from the port and toward the first end. 
     In accordance with another broad aspect of the present invention, there is provided a method for fracturing a wellbore, the method comprising: injecting fluid through a tubing string and out through a port into a cemented annulus between the tubing string and a wellbore wall, the fluid following a pathway through the cemented annulus, the pathway extending longitudinally away from the port and into contact with the wellbore wall. 
     It is to be understood that other aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein various embodiments of the invention are shown and described by way of illustration. As will be realized, the invention is capable for other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings, several aspects of the present invention are illustrated by way of example, and not by way of limitation, in detail in the figures, wherein: 
         FIG. 1A  is a schematic sectional view along a portion of a well bore with a ported tubular therein. 
         FIG. 1B  is a sectional view along line I-I of  FIG. 1A . 
         FIG. 2  is a plan view of a cement diffuser plate useful in the present invention. 
         FIG. 3  is a perspective view of a cement diffuser installed on a wellbore tubular. 
         FIG. 4  is a sectional view of a cement diffuser installed on a tubular. Reference may be made to line II-II of  FIG. 3  for orientation of the section through the cement diffuser and tubular wall. 
         FIG. 5A  is a schematic sectional view along a portion of another wellbore with a ported tubular therein. 
         FIGS. 5B and 5C  are perspective views of further wellbore tubulars including cement diffusers. 
         FIG. 6  is a schematic sectional view of a string installed in a well. 
     
    
    
     DESCRIPTION OF VARIOUS EMBODIMENTS 
     The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments contemplated by the inventor. The detailed description includes specific details for the purpose of providing a comprehensive understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. 
     With reference to  FIGS. 1A and 1B , a ported tubular  12  is shown that facilitates wellbore fracturing in a cemented wellbore. In particular, tubular  12  facilitates fracturing by promoting complex fracturing and/or by facilitating access of fracturing fluid to natural weaknesses in a cemented installation. The ported tubular  12  can be cemented in place in a wellbore, as defined by wellbore wall  14 , wherein cement C resides and is allowed to set in an annular area  18  between the outer surface  12   a  of the tubular and wellbore wall  14 . 
     The ported tubular  12  includes an external structure that is a cement diffuser  10 . The cement diffuser is carried on outer surface  12   a  of the ported tubular. When in place in a cemented well, cement diffuser  10  creates a pathway P through the cement annulus both radially adjacent and extending axially of the port  16  of the tubular. In this embodiment, the pathway P is formed within the cement diffuser. To create the pathway, the external structure can create an area of the annulus generally free of set cement by blocking infiltration of the cement, by deforming or degrading to leave a space in the set cement and/or by preventing proper setting of the cement. The pathway follows the position of the cement diffuser. The cement diffuser is positioned both radially outwardly of the port and axially away from at least one of the upper and lower limits of the port and, as such, the pathway through the cement extends both radially outwardly and axially beyond the port towards one or both ends of the ported tubular. 
     Various external structures can form the cement diffuser. For example, materials that prevent infiltration of cement into their inner portions such as a hollow tubular structure, a collection of fibers (a brush, wool, twisted, woven, knit or compressed arrangements, etc.), foam such as sponge or closed cell foam such a styrofoam, degradable or deformable materials, straps, etc. are all useful to form a cement diffuser. 
     In  FIGS. 1A and 1B , cement diffuser  10  includes a collection of fibers secured over the port  16  at the outer surface and extending along at least an expanse of the outer surface of tubular  12  axially (aka longitudinally) adjacent port  16 . The fibers can be metal, synthetic such as of polymers or natural organic materials such as of cellulose, hemp, wood, cotton, etc. The collection of fibers is carried along with tubular  12  while running the tubular into a borehole. 
     The cement diffuser becomes useful when it is desired to cement the annular area  18  about the tubular. As will be appreciated, a cementing operation includes pumping liquid cement, arrows C, into the annular area between a tubular installation and a borehole wall. This is generally done by pumping cement from surface down through the inner diameter of the tubular installation and out into the annulus, either by pumping the cement out the bottom of the tubular installation or out through a port in the tubular wall. 
     The fibers of the cement diffuser are positioned to create pathway P through the cement, when it sets. The pathway is a cement-free space or weakened area of cement, through which fluids can flow more readily than through set cement. For example, to form pathway P, the fibers may substantially block clear access of the cement into the cement diffuser, as the cement moves through the annulus, thus the cement may tend not to infiltrate, or infiltrate only partially into, the spaces between the fibers of the collection of fibers due to fluid dynamics: leaving an open space within the cement diffuser which is free of cement. Alternately or in addition, the cement may tend not to infiltrate the fibers of the collection of fibers due to a chemical applied to block access into any voids between the fibers. Alternately, the cement may pass between the fibers of the cement diffuser, but the cement when set may be so thin, porous or unstable that the cement in that area is relatively weak. Alternately, the cement may pass between the fibers of the cement diffuser, but the fibers may degrade or be deformable (i.e. are able to be pushed aside), such that a space is formed in the set cement. Thus, in any event, a pathway is created by the cement diffuser through the set cement. 
     In one embodiment, the radially extended length of the collection of fibers is selected to span the annulus such that the collection of fibers at their outboard ends are at least closely adjacent or possibly touching the borehole wall  14 . In this way, the entire annular radial length outwardly of the port and the outer surface on which the cement diffuser is installed is either devoid of cement or includes only relatively weak deposits of cement. In such an embodiment, the outward extended length of fibers from the outer surface of the tubular may be selected at surface with consideration as to the expected annulus radial spacing between the tubular and the borehole wall, which will be known based on the drilling information and the tubular&#39;s known outer diameter. 
     So as not to interfere with the passage of cement through the annulus and the integrity of the annular cement seal above and below the cement diffuser, the cement diffuser may not extend fully about the circumference of the tubular. Thus, one or more open areas  19  are formed about the circumference of the tubular. For example, in the illustrated embodiment, where the tubular has a plurality of ports at one axial position and a cement diffuser over each port, the two cement diffusers may be spaced apart about the circumference of the tubular leaving open areas  19  therebetween through which the cement may flow through the annulus past the cement diffusers, when the tubular is positioned in a borehole. 
     The cement diffuser is installed with a port-located portion thereof overlying at least a portion, for example herein illustrated as fully, over the open axial length L of its port  16  and an end portion of the cement diffuser is installed on the outer surface of the tubular positioned axially beyond the port. The cement diffuser is continuous with the port-located portion and the end portion directly adjacent each other. Together the port-located portion and the end portion ensure the pathway through the cement extends axially along the tubular body beyond the open axial length L of its port and longitudinally towards one or both of the ends of the tubular body. The cement diffuser end portions are installed on the outer surface axially below and/or above the opening of the port. With respect to the port-located portion, which is the portion overlying the port open axial length L, fluid from within the tubular can pass up through the port and through that portion of the cement diffuser. Additionally, the fluid can continue into and pass through the extending ends of the cement diffuser. While the fluid is supplied through the port, it travels along the tubular outer surface through the diffuser axially away from the port. In this way, the fluid can move through the pathway created by the cement diffuser to access a length of the wellbore, as determined by the length of the end portions. This facilitates the fracturing operation by accessing a length of the wellbore beyond the axial length of the port and increases the chances of the fracturing fluid locating a natural weakness in the borehole wall and of generating complexity in the fracture. It is likely that the breakdown pressure to achieve a successful fracture will be reduced over the breakdown pressure for fracturing in a standard cemented well and in a well with a fracturing cement diffuser only at the port. 
     The position of the end portion of the cement diffuser as axially beyond the port means that the end portion extends longitudinally, along the long axis x, and generally toward the ends of the tubular. While, for example, cement diffuser  10  is shown in  FIGS. 1A  and  1 B as parallel with the long axis x of the tool, this need not be the orientation. For example, the cement diffuser can be straight or curved. For example, in one embodiment, the cement diffuser is curved: installed in a spiral fashion along the outer surface of the tubular (see for example  FIG. 5B , where cement diffuser  210 ′ is mounted on a tubular overlying a port  216 ′ through the tubular&#39;s wall and cement diffuser  210 ′ spirals about the surface of the tubular as it extends axially away from the port  216 ′). 
     Additionally, the cement diffuser can have a portion extending circumferentially beyond the side edges of the port. 
     Fillers such as chemicals, other fibers, hollow, degradable or frangible components, etc. can be positioned in the voids formed between the fibers of the collections, such fillers being selected to prevent the solidification of cement in the voids. 
     In use, the cement diffuser either directly provides a path for the injected fluids to pass therethrough, or the cement diffuser can be pushed aside, expelled or broken down immediately or over time to create the pathway or cement that infiltrates the cement diffuser, if any, is unstable, thin or weakly set to readily create a pathway when injected fluids enter the pathway. Injected fluids can be passed through the tubular and out through the port over which a cement diffuser has been installed. The injected fluids pass outwardly though the port and into the pathway. The injected fluids pass through the pathway, including that extending away from the port to access a length of the wellbore greater than the axial open length of the port to facilitate fracturing of the wellbore by creating complex fractures and/or forming fractures at naturally weak rock. 
     The cement diffusers can be secured on the exterior of the tubular in various ways. With reference to  FIGS. 2 to 4 , in one embodiment, the cement diffuser includes a plate  120  with a plurality of holes  122   a,    122   b  therethrough that can be secured on the outside of a tubular  112  over a port  116  and along the outer surface of the tubular. The holes serve various purposes and may have various sizes and shapes, as desired. For example, in the presently illustrated embodiment of  FIG. 2 , larger holes  122   a,  in this illustrated case formed as slots, are included on the plate, where the plate may be installed over a port  116  and greater volume flows may be passed through the slots. Smaller holes  122   b  are formed over another area of the plate. The plate may take various forms. This plate shown fits over port  116  and has end portions  120   a  for mounting over a portion of the tubular outer surface axially beyond the axial length L, between upper end wall  116   a  and lower end wall  116   b,  of its port. However, the plate may be further elongated or more than one such plate may be installed to form a longer length cement diffuser extending axially away from the axial length L of the port  116 . 
     Fibers  124  may be threaded through the holes  122   a,    122   b.  For example, the holes may be stuffed with fibers and the fibers may extend outwardly therefrom. The fibers may be linearly twisted in bundles, as shown. Alternately, the fibers may be individually extending or in the form of bunches, interengaged bundles, plugs, randomly arranged, linearly arranged, parallel, etc. The fibers together form a collection that extends out from the plate into the annulus about the tubular. In the illustrated embodiment, for example, fibers extend out substantially radially from the ports, relative to the circular dimension of the tubular. Fibers  124  may be selected to be long enough to touch the borehole wall of a borehole in which they are to be used. The fibers in this embodiment, form a brush like structure that can engage and ride along the borehole wall, but are threaded through the holes of the plate  120  such that they are substantially not dislodged by such engagement. 
     Fibers  124  may be secured to the plate such that they are forced out of the way by fluid flows through the port. In particular, the fibers over the ports may be forced out of holes  122   a,    122   b  of the plate when fluid injection occurs through the port  116  and plate  120 . Alternately, the fibers may be installed or formed such that there remain fluid flow passages between the fibers, when they remain in the holes. In another possible embodiment, fibers  124  may be formed of erodible or degradable materials/construction such that they break down at some point after cementing, for example, by the erosive power of the injected fluids. 
     Fillers, here shown as further fibers  126  of similar or, as shown, different construction/materials, may be engaged between fibers  124  in the holes. In the illustrated embodiment, for example, more delicate polymeric batting is placed between the tufts formed by the bundles of fibers extending from the holes  122   a,    122   b  of plate  120 . 
     As noted hereinabove, other fillers can be positioned in the voids formed between the fibers of the collection of fibers, such fillers being selected to prevent the entry or solidification of cement in the voids between fibers. Other fillers include for example, one or more of hollow balls, sponge, sytrofoam, or chemicals such as, for example, one or more of grease, sugar, salt, cement retarder, etc. 
     Plate  120  can be secured over the port and along the surface in various ways, such as by fasteners  130  in apertures  132 , welding, plastic deformation, etc. A recess  134  may be provided on the outer surface of the tubular such that the plate can be positioned below the tubular&#39;s outer surface contour. 
     Fillers can also be positioned inwardly of plate  120  to act against passage of or setting of cement in port  116  and in the inner diameter of the tubular. 
     When tubular  112  with cement diffuser  110  of  FIG. 4  is placed within the confines of wellbore wall  114  and cement is introduced to cement the well, cement C surrounds the cement diffuser but cannot readily infiltrate the fibers  124  and filler  126  of the cement diffuser. Any cement that does infiltrate the cement diffuser is weak. As such, a pathway is formed through cement diffuser  110  that is open to port  116 . 
     Cement diffuser  110  remains in place over port  116  when the port is opened for fluid injection therethrough. Thus, while port  116  may have a closure (not shown), cement diffuser  110  does not in the illustrated embodiment act as a closure for the port. In particular, even after cementing fluid can exit port  116  while cement diffuser is in place or cement diffuser, or portions thereof, are pushed out of the way or degrade after use to create the pathway. 
     When fluid is injected to fracture the well, that fluid, arrows F, may pass from the inner diameter ID of the tubular  112  through the port  116 . Fluid, arrows F, may then pass along the pathway in the cement created by fibers  124  and filler  126 . Because the cement diffuser extends axially beyond the axial length of the port, the weakened pathway does so as well. Thus, the fluid may contact the wellbore wall  114  and create a complex fracture or locate an area of weakness in the rock, which may be both radially out from port  116  and/or axially spaced from the upper and lower limits  116   a ,  116   b  of port  116  and enhance the fracture results by reducing break down pressure and creating more than one fissure into the formation. 
     Another embodiment of a cement diffuser is shown in  FIG. 5A . That external structure includes a rope type structure  224 , wherein the rope fibers are twisted and extend axially along the length of the rope. In this drawing two structures  224  are shown secured to the external surface  112   a  of the tubular  112 . Each structure  224  extends over a plurality of ports  216  in the wall of the tubular and axially beyond the axial lengths L of the ports. 
     The external structure can take other forms. For example, the external structure may include anything that can be positioned on the external surface of a wellbore tubular, holds to withstand the rigors of being run into a well, past which cement can flow and which creates a immediate or formable pathway in the cement, when the cement sets. Some structures of interest are centralizers (like a bow spring centralizer or an open vane centralizer), hollow tubes (like hollow tubes with frangible burst members installed in ports thereof), axially extending deformable structures (like rubber vanes that can deform to let a pressurized fluid pass), etc. These structures are mounted adjacent a port and positioned to receive a fluid supply from a port and to create a weakened pathway in the cement from the port, axially along the tubular away from the axial length of the port and in the annular area. 
       FIG. 5B , as noted previously, includes a spirally oriented cement diffuser  210 ′. This is formed of a foam that can be installed on the tubular&#39;s outer surface but eventually breaks down after cementing. Since the cement diffuser  210 ′ remains in place during cementing, the helical wraps of cement diffuser  210 ′ are installed to leave open flow areas  219 ′ therebetween through which cement, arrows C, can pass. After installation, the fracture fluid can move through ports  216 ′ and along the spiral pathway to contact an axial length of the wellbore well beyond the axial open lengths of the ports. As such, the chance of the fracture fluid contacting a natural weakness along the spiral path is increased over a situation where the fracture fluid passed radially out from the ports through the cement and into contact with the wellbore wall radially outwardly of the ports and only along a length substantially the same as the axial lengths of the ports. 
       FIG. 5C  shows another cement diffuser  210 ″ mounted on a tubular  212 ″ overlying a plurality of ports  216 ″ opened through the tubular&#39;s wall. This cement diffuser  210 ″ is formed as a sleeve-type structure on the outer surface. The cement diffuser extends axially beyond the upper and lower limits of ports  216 ″. In fact, the cement diffuser extends substantially the full length of the tubular leaving only the end connections exposed. Additionally, cement diffuser  210 ″ extends circumferentially about the tubular beyond the side edges that limit the circumferential open width of ports  216 ″. While the cement diffuser may be formed of various materials, in the illustrated embodiment cement diffuser  210 ″ is formed of a resilient material such as sponge that is durable to withstand the rigors of being run into a wellbore and is strong enough to hold its shape during cementing. Since the cement diffuser  210 ″ resists deformation by cement passing thereby, an open flow area  219 ″ is provided through an axial length of the cement diffuser. 
     The sponge forming cement diffuser  210 ″ can, however, be pushed aside (i.e. cut into and/or compressed) when fluid at fracturing pressures is pumped through ports  216 ″. As such, fracture fluid can pass through cement diffuser  210 ″ and contact the wellbore along a length much greater than the axial open length and open width of the individual ports and about a substantial portion of the circumference of the tubular. Fracturing fluid is much more likely to find an area of weakness and/or to form complex fractures along the substantial portions, both axially and circumferentially, about the ports. In particular, almost the full length and circumference of the wellbore that relates to the length and circumference of the cement diffuser is free of set cement and can be accessed by fracture fluid, except that area of flow channel  219 ″ which contains set cement. 
     As shown in  FIG. 6 , a string  312  of ported tubulars with external cement diffusing structures  324   a,    324   b  may be installed in a wellbore  314 . Here the external structures  324   a  are in the form of bow spring centralizers and external structures  324   b  are in the form of spirally extending cement diffuser formed of a collection of fibers. The ported tubulars may be connected into a string with an inner diameter ID through which fluids may be conveyed to treat the well and from production of the well. The external structures are in place on the external surface  312   a  of the ported tubulars  312  as the string is installed in the well and when the string is in place in the well, the external structures are already positioned or can be activated and are adjacent a port  316 . The external structures are also positioned to receive a fluid supply from a port  316  into a pathway formed as a result of the external structure though annular cement. 
     As the wellbore is cemented, cement C surrounds the external cement diffuser structures, but the structure of the external structures, wherein they include hollow sections, sections filled with fillers, etc., each create a pathway in the set cement. These pathways extend from the port at which they are positioned, axially in the annular area along the tubular outer surface longitudinally away from the open area of the port. In one embodiment, the external structures extend from the port open area and have a radial thickness such that they come close to touching or touch the wellbore wall. As such, the pathways likewise extend from the port open area towards the wellbore wall, while extending axially away from the port. The pathway may be open to the wellbore wall or only a thin sheathe of cement may be present between the pathway and the wellbore wall. 
     When fluid is injected to treat the wellbore, the fluid may pass through the inner diameter ID of the wellbore string, out through the ports  316  and along the paths to create complexity in the wellbore fracture and/or to create fractures in weak rock. These fractures may be axially spaced from the locations of the ports since the injected fluid can follow the axially extending pathways formed by the external cement diffusing structures. This is different than the wellbore treatment that can be effected through a port D without an external cement diffusing structure as described herein, wherein the annular cement prevents axial flow of the fracturing fluid and often only a simple fracture may be created, that being directly radially out from the port. The chance of that fracture being in a natural area of weak rock is unlikely, it being dependant entirely on the exact location of the port in the wellbore. The complex fracture causes increased contact at the wellbore wall compared to a simple fracture. 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to those embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the claims, wherein reference to an element in the singular, such as by use of the article “a” or “an” is not intended to mean “one and only one” unless specifically so stated, but rather “one or more”. All structural and functional equivalents to the elements of the various embodiments described throughout the disclosure that are known or later come to be known to those of ordinary skill in the art are intended to be encompassed by the elements of the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 USC 112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or “step for”.