Patent Publication Number: US-2010126722-A1

Title: Wellbore system and method of completing a wellbore

Description:
PRIORITY CLAIM 
     The present application claims priority from PCT/EP2008/053625, filed 27 Mar. 2008, which claims priority from EP Application 07105070.2, filed 28 Mar. 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     In the industry of hydrocarbon fluid production from a wellbore it is common practice to complete a lower section of the wellbore, extending into the hydrocarbon fluid-bearing formation, with a completion that stabilises the wellbore wall and/or reduces sand production from the wellbore. For example, screens or gravel packs are generally placed in open-hole wellbore sections to support the wellbore wall and prevent caving-in of loose material, and to restrain sand from flowing with the formation fluids to surface. Basically, gravel packing includes the steps of installing a production liner provided with small inlet openings, e.g. in the form of slots or screens, in the wellbore and then filling the annular space between the production liner and the wellbore wall with particulate material such as sand and gravel. The resulting gravel pack maintains structural integrity of the wellbore in the absence of a casing, while still allowing flow of fluid from the reservoir into the wellbore. Screens and gravel packs also control the migration of formation sands into production tubulars and surface equipment, which can cause washouts and other problems, particularly from unconsolidated sand formations. After a flow path is made, acids and fracturing fluids can be pumped into the wellbore to fracture, clean, or otherwise prepare and stimulate the reservoir rock to optimally produce hydrocarbons into the wellbore. Finally the wellbore is sealed-off above the reservoir section, inside the casing, and connected to the surface via one or more production tubings. 
     In the description and claims hereinafter the terms “wellbore” and “borehole” will be used interchangably, and without intended difference of the meaning of such terms. 
     Many wellbores are drilled such that a lower wellbore section extends inclined or horizontally into the reservoir formation to increase the contact length of the wellbore with the reservoir formation. For example, wells that are drilled from an offshore platform all deviate in different directions so that hydrocarbon fluid can be produced from a large surface area of the reservoir formation. Although deviated and horizontal wellbore sections significantly enhance the production potential of a wellbore, particularly when compared to vertical wellbores, it has been experienced that problems may occur in properly installing completions in such deviated or horizontal wellbore sections. One such problem relates to the proper placement of a gravel pack. Generally, gravel packs are installed using a liner provided with a cross-over sub assembly to allow a slurry of particulate material and viscous fluid to be pumped through the liner and the cross-over sub assembly into the annulus of a lower wellbore section where the particulate material settles out of the slurry. The viscous fluid is then circulated back via the cross-over sub assembly and the annulus between the liner and the wellbore wall (or casing), to surface. Experience has shown that in an inclined or horizontal section it is difficult, if not impossible, to fill the entire annular space between the liner and the wellbore wall with the gravel pack particulate material. This is due to the particulate material that settles out of the slurry, tending to fall to the bottom of the inclined or horizontal wellbore section so that an upper portion of the wellbore section remains uncovered with particulate material. 
     As a result, an undesired flow passage remains above the gravel pack, which allows fluid to flow in longitudinal direction through the wellbore section thereby bypassing the gravel pack. This can lead to several problems such as, for example, the ability of locally produced sand from the formation to spread along the length of the gravel pack thereby potentially negatively affecting the permeability of the entire gravel pack. Another problem becomes apparent if a treatment fluid needs to be injected via the liner into the open-hole section. The treatment fluid will tend to flow through the flow passage above the gravel pack, thereby rendering it impossible to accurately position the treatment fluid at a desired location in the open-hole section. For example, if a portion of the open-hole section needs to be shut-off in order to reduce or prevent formation water from flowing into the wellbore, a treatment fluid is preferably used that reduces or eliminates the permeability of the gravel pack at the location where the water flows into the wellbore. However it has been experienced that the injected treatment fluid tends to flow through the flow passage above the gravel pack thereby spreading in the open-hole section and potentially affecting the permeability of the entire gravel instead of at the desired location only. 
     U.S. Pat. No. 4,995,456 discloses a wellbore completion assembly whereby a horizontal wellbore section is provided with a fluid-permeable liner provided with a cross-over sub and vanes for imparting a spiralling flow to a gravel pack slurry which is pumped into the horizontal wellbore section. The spiralling flow is intended to enhance the distribution of gravel pack particulate material in the horizontal wellbore section. 
     However there remains a need for an improved wellbore system and completion method, which overcomes the problems of the prior art. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a wellbore system comprising a borehole formed in an earth formation, the borehole having a borehole section containing a volume of gravel pack particles and at least one body of a swellable material, each body of swellable material being adapted to expand from an unexpanded state to an expanded state upon contact of the swellable material with a selected fluid, wherein a flow passage is present in said borehole section allowing fluid to bypass the volume of gravel pack particles when the body of swellable material is in the unexpanded state, and wherein the body of swellable material is arranged to substantially close the flow passage upon expansion of the body of swellable material to the expanded state. 
     Thus, by swelling of the swellable material, the flow passage becomes closed or vanishes, so that fluid no longer can flow unhindered in longitudinal direction through the borehole section. Also, locally produced sand is thereby prevented from spreading along the entire gravel pack, but instead remains in the wellbore location where it was produced. Furthermore, treatment fluid that is injected into the wellbore is confined to the injection location rather than spreading along the gravel pack. 
     In an advantageous embodiment, the body of swellable material is arranged to push the volume of gravel pack particles into the flow passage upon swelling of the swellable material, so that the flow passage gets blocked. Also, the body of swellable material, after expansion, can be arranged to completely fill the cross-section of the borehole section and thereby block the flow passage. 
     Suitably, the body of swellable material includes a sleeve arranged around a tubular element extending into said borehole section. The tubular element is, for example, a production liner provided with slots, openings or screens for the inflow of hydrocarbon fluid from the formation. 
     Movement of the volume of gravel pack particles into the flow passage is optimal if the sleeve is at least partly covered by the volume of gravel pack particles. 
     Preferably the tubular element is provided with a plurality of said sleeves mutually spaced along the tubular element. In this manner it is ensured that the annular space between the tubular element and the wellbore wall is formed into compartments which prevent fluid or formation sand from bypassing the gravel pack. In such arrangement the tubular element is suitably provided with fluid inlet means arranged at a portion of the tubular element located between a pair of adjacent sleeves. 
     In an alternative application, said at least one body of swellable material includes a plurality of particles of swellable material. Such application has the advantage that the particles of swellable material can be pumped into the wellbore section, and are allowed to flow into irregular wellbore portions. Preferably the particles of swellable material are intermixed with the gravel pack particles. To achieve adequate intermixing, the particles of swellable material and the gravel pack particles suitably have about equal density. This can be achieved, for example, by providing the particles of swellable material with a weighting material so as to increase their density. A suitable weighting material is Iron powder or a similar material. Since the function of the weighting material is to adapt the density of the swellable particles to the density of the gravel pack particles, a weighting material may be applied that lowers the density of the swellable particles in case the density of the swellable particles, absent the weighting material, exceeds the density of the gravel pack particles. 
     The wellbore system of the invention is most advantageous for application in wellbore sections that extend inclined or substantially horizontally. This is because it is generally difficult, if not impossible, to fill the entire cross-section of such inclined or substantially horizontal wellbore section with gravel particles. In most such applications an undesired flow passage remains above the volume of gravel pack particles. 
     Furthermore, the selected fluid can be fluid from the earth formation flowing into the wellbore section, such as water or oil, or fluid that is pumped from surface into the wellbore section. 
     In another aspect of the invention there is provided a method of completing a borehole formed in an earth formation, the method comprising: 
     inserting a volume of gravel pack particles into a borehole section of the borehole; 
     inserting at least one body of swellable material into the borehole section, each body of swellable material being adapted to expand from an unexpanded state to an expanded state upon contact of the swellable material with a selected fluid, wherein a flow passage is present in said borehole section allowing fluid to bypass the volume of gravel pack particles when the body of swellable material is in the unexpanded state, and wherein the body of swellable material is arranged to substantially close the flow passage upon expansion of the body of swellable material to the expanded state; and 
     allowing the body of swellable material to expand due to contact of the swellable material with the selected fluid, thereby substantially closing the flow passage. 
     Preferably the body of swellable material pushes the volume of gravel pack particles into the flow passage upon swelling of the swellable material. 
     To allow accurate placement of a treatment fluid in the borehole section, the method suitably further comprises injecting a treatment fluid into the volume of gravel pack material after the volume of gravel pack material is pushed into the flow passage. For example, if the purpose of the treatment fluid is to shut-off a selected portion of the wellbore, the treatment fluid suitably is adapted to locally reduce or eliminate the permeability of the gravel pack material in such portion. 
     The swellable material may be an elastomer adapted to swell when in contact with water and/or oil. Examples of materials that swell upon contact with hydrocarbon fluid are natural rubber, nitrile rubber, hydrogenated nitrile rubber, acrylate butadiene rubber, poly acrylate rubber, butyl rubber, brominated butyl rubber, chlorinated butyl rubber, chlorinated polyethylene, neoprene rubber, styrene butadiene copolymer rubber, sulphonated polyethylene, ethylene acrylate rubber, epichlorohydrin ethylene oxide copolymer, ethylene-propylene-copolymer (peroxide crosslinked), ethylene-propylene-copolymer (sulphur crosslinked), ethylene-propylene-diene terpolymer rubber, ethylene vinyl acetate copolymer, fluoro rubbers, fluoro silicone rubber, and silicone rubbers. Preferred materials are EP(D)M rubber (ethylene-propylene-copolymer, either peroxide or sulphur crosslinked), EPT rubber (ethylene-propylene-diene terpolymer rubber), butyl rubber, brominated butyl rubber, chlorinated butyl rubber, or chlorinated polyethylene. 
     Instead of, or in addition to, the swellable material being adapted to swell upon contact with hydrocarbon fluid, the swellable material may be adapted to swell upon contact with water. Suitably, such water-swellable material may be selected from rubbers based on acrylonitrile butadiene (NBR), hydrogenated nitrile butadiene (HNBR), acrylonitrile butadiene carboxy monomer (XNBR), fluorinated hydrocarbon (FKM), perfluoroelastomers (FFKM), tetrafluoroethylene/propylene (TFE/P), or ethylene propylene diene monomer (EPDM). In order to enhance the swelling capacity of the water-swellable material, even for saline water conditions, said material suitably is a matrix material wherein a compound soluble in water is incorporated in the matrix material in a manner that the matrix material substantially prevents or restricts migration of the compound out of the swellable seal and allows migration of water into the swellable seal by osmosis so as to induce swelling of the swellable seal upon migration of said water into the swellable seal. Said compound suitably comprises a salt, for example at least 20 weight % salt based on the combined weight of the matrix material and the salt, preferably at least 35 weight % salt based on the combined weight of the matrix material and the salt. In order to prevent, or reduce, leaching of the compound out of the matrix material, it is preferred that the matrix material is substantially impermeable to said compound or to ions of said compound. The compound can be present in the matrix material, for example, in the form of a plurality of compound particles dispersed in the matrix material. If the matrix material is an elastomer, the compound can be mixed into the matrix material prior to vulcanisation thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described hereinafter in more detail and by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  schematically shows a borehole extending into an earth formation, provided with an embodiment of the wellbore system of the invention; 
         FIG. 2  schematically shows detail A of  FIG. 1 ; 
         FIG. 3  schematically shows cross-section  3 - 3  of  FIG. 2 ; 
         FIG. 4  schematically shows detail A of  FIG. 1  after swelling of a body of swellable material; and 
         FIG. 5  schematically shows cross-section  5 - 5  of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , there is shown a borehole extending into an earth formation  2 , in the form of wellbore  1  having a vertical upper wellbore section  4  provided with a scheme of casings and an open-hole lower section  8  that extends substantially horizontally into a reservoir zone  10  containing hydrocarbon fluid. For ease of reference, the scheme of casings is referred to hereinafter as casing  6 . A tubular production liner  12  extends from a wellhead  14  at surface  16  through the upper wellbore section  4  and into open-hole lower section  8 , whereby a production packer  18  seals the production liner  12  to the lower end of the casing  6 . Production liner  12  has a lower part  20  provided with a plurality of sleeves  22   a,    22   b,    22   c,    22   d  of elastomeric material susceptible of swelling with a selected fluid, such as water and/or oil. In the present example, the elastomeric material is selected to swell upon contact with oil from reservoir zone  10 . Sleeves  22   a,    22   b,    22   c ,  22   d  are spaced from each other in longitudinal direction of the liner  12 , whereby liner portions  24  between sleeves  22   a,    22   b,    22   c,    22   d  are provided with small openings or slots  23  ( FIG. 2 ), which provide fluid communication between the interior and the exterior of the liner  12 . Liner portions  24  can be provided in the form of sandscreens, slotted pipes or other devices suitable for inflow of produced hydrocarbon fluid into liner  12 , or outflow of treatment fluid from liner  12  into wellbore  1 . The open-hole section  8  of wellbore  1  is furthermore provided with a gravel pack  26  containing particulate material such as gravel, sand and the like, as is well known in wellbore completions. For ease of reference the volume of gravel pack particles  26  is referred to hereinafter as “gravel pack  26 .” 
     Referring further to  FIGS. 2 and 3 , there is shown detail A of  FIG. 1 , including open-hole section  8  provided with gravel pack  26  and liner  12 . Only one elastomeric sleeve  22   b  is shown for ease of reference, the other elastomeric sleeves  22   a,    22   c,    22   d  being similar to sleeve  22   b.  Gravel pack  26  does not occupy the entire cross-sectional area of the open-hole section  8 , but instead leaves a flow passage  30  in the open-hole section  8  through which fluid can flow in axial direction of the open-hole section  8  and thereby bypass the gravel pack  26 . 
     In  FIGS. 4 and 5  is shown detail A of  FIG. 1  after swelling of the elastomer of sleeve  22   b  due to contact with water or oil from the earth formation, whereby sleeve  22   b  has increased in diameter and thereby has pushed gravel pack  26  into flow passage  30 . As a result, flow passage  30  is blocked, or in other words, the flow passage vanishes at the location opposite the sleeve  22   b  so that fluid no longer can bypass gravel pack  26 . 
     During normal operation, wellbore  1  is drilled from surface  16  using a drilling rig (not shown), and casings  6  are installed in vertical wellbore section  4 . Production liner  12  is then installed in the wellbore so that sleeves  22   a,    22   b,    22   c,    22   d  of swellable elastomer are located in the reservoir zone  10  of earth formation  2 . Thereafter, a slurry of gravel pack particles and a viscous fluid, such as crude oil or a polymer-type water-based fluid, is pumped into open-hole section  8  of wellbore  1 . For this purpose, end part  20  of production liner  12  is provided with a cross-over sub assembly (not shown) that packs off open-hole section  8  and allows the gravel pack slurry to be pumped via liner  12  into a portion of open-hole section  8  below the cross-over assembly. There the gravel pack particles settle out from the slurry in open-hole section  8  to form gravel pack  26 , while the viscous fluid is circulated back to surface via the cross-over sub assembly and the annulus formed between liner  12  and wellbore wall or casing  6 . The cross-over sub assembly will not be described in more detail since it does not form part of the invention, and since it is a well known tool for completing wellbores. Production packer  18  is installed between the  12  and the lower end of casing  6  after gravel pack  26  has been placed in wellbore  1 . 
     Although it is desired that gravel pack  26  occupies the entire annular space between liner part  20  and the wall of open-hole section  8 , it has proved difficult, or even impossible, to fill the entire annular space with gravel pack particles. The problem is more pronounced in horizontal, or inclined, wellbore sections where the particles have a tendency due to gravity to fall to the lower side of the wellbore section. Thus, in the present instance of substantially horizontal open-hole section  8 , it is almost inevitable that the flow passage  30  remains between the volume of gravel pack particles  26  and the wellbore wall. 
     When oil starts flowing from reservoir zone  10  into open-hole section  8 , such oil contacts sleeves  22   a,    22   b,    22   c,    22   d,  thereby inducing the elastomer of the sleeves to swell. As a result sleeves  22   a,    22   b,    22   c,    22   d  expand in diameter and thereby push gravel pack  26  into flow passage  30  which, as a result, gradually vanishes at the location of sleeves  22   a ,  22   b,    22   c,    22   d.  After sleeves  22   a,    22   b,    22   c,    22   d  have expanded, gravel pack  26  completely fills the annular space between each sleeve  22   a,    22   b,    22   c,    22   d  and the wellbore wall in open-hole section  8 . In this manner, gravel pack  26  divides open-hole section  8  into compartments that prevent free flow of fluid and rock particles through the open-hole section  8  in longitudinal direction thereof. Thus, sand particles from the rock formation can only locally flow into gravel pack  26  rather than flowing along the whole length thereof as in the prior art. It is thereby achieved that any negative effect on the permeability of gravel pack  26  as a result of such inflow of sand particles is confined to local spots of the gravel pack. Oil from reservoir zone  10  flows through gravel pack  26  into openings or slots  23  and from there through liner  12  to surface. 
     The method of the invention also enables better placement of treatment fluid in open-hole section  8  of the wellbore. For example, if such fluid is pumped via liner  12  and openings  23  into open-hole section  8 , the fluid can no longer freely flow in longitudinal direction through the open-hole section  8  by virtue of the compartments formed in gravel pack  26 . This allows the treatment fluid to be placed more accurately in open-hole section  8 . In an exemplary application, it may be desired to shut-off a selected portion of open-hole section  8  if after some time of continued oil production, formation water starts flowing into such portion of open-hole section  8 . A treatment fluid that substantially reduces, or eliminates, the permeability of gravel pack  26  is then pumped via production liner  12  and openings  23  into gravel pack  26  at the selected location. Due to the compartments formed in gravel pack  26 , the treatment cannot freely flow in longitudinal direction through open-hole section  8 , so that the treatment fluid can be accurately placed at the desired location of gravel pack  26 . As a result, only the desired portion of open-hole section  8  is shut-off, while other portions of open-hole section  8  remain unaffected by the treatment fluid. 
     In an alternative embodiment of the wellbore system of the invention, particles of swellable material susceptible of swelling upon contact with water and/or oil are intermixed with the particulate material of the gravel pack. Suitably such particles of swellable material are made of one or more of the swellable elastomers described hereinbefore. The elastomer particles can be mixed into the gravel pack slurry at surface and pumped with the slurry into the wellbore section. Also the gravel pack slurry can be pumped first into the wellbore, whereafter the elastomer particles are pumped into the gravel pack. Upon flow of oil or water from the earth formation into the wellbore section, the elastomer particles start swelling. As a result the volume of the combined gravel pack particles and elastomer particles increases so that the volume is pushed into the flow passage which thereby gradually becomes blocked and eventually completely vanishes. In this manner it is achieved that injected fluid, such as treatment fluid, and sand particles from the formation can no longer bypass the gravel pack. 
     In the above detailed description it is indicated that the body of swellable material or the swellable particles swell by contact with oil or water from the earth formation. However it is envisaged that swelling of the swellable body or the swellable particles also can be triggered by inducing the selected fluid to flow from surface into the borehole, for example by pumping oil or water into the borehole to contact the body of swellable material or the swellable particles. 
     Furthermore, it is to be understood that the procedure described hereinbefore, whereby a slurry of gravel pack particles and a viscous fluid is pumped into the wellbore, includes applications whereby the gravel particles not only are pumped into the open-hole section of the wellbore, but also into fractures of the earth formation which are in communication with the wellbore. Such applications are sometimes referred to as “frac and pack.”