Abstract:
The invention is related in general to equipment and methods for preparing curved fibers in a batch or continuous process. Fiber strands comprising a thermoplastic material are placed on a cylindrical surface and heated such that they soften and become malleable. The fiber strands assume the curvature of the cylindrical surface and, upon cooling below their softening temperature, retain the curvature. The curved-fiber strands are then cut to a desired length.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The current application is a divisional of U.S. application Ser. No. 14/056,949, filed on Oct. 18, 2013 and published on Feb. 13, 2014 as US2014/0045731, which is a divisional of U.S. application Ser. No. 12/890,969, filed on Sep. 27, 2010, published on Apr. 7, 2011 as US2011/0082058 and granted on Dec. 31, 2013 as U.S. Pat. No. 8,617,444, the entire content of which are hereby incorporated by reference into the current application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    The invention is related in general to equipment and methods for preparing curved fibers. 
         [0004]    The inclusion of fibers in fluids for servicing subterranean wells has become a well-known practice. For example, fibers are added to drilling fluids to help minimize lost circulation. Similarly, fibers may be added to cement slurries to help minimize lost circulation; fibers can also improve flexural strength of set cement. In hydraulic fracturing operations, fibers may be added to proppant to help prevent proppant flowback as the well produces. Fibers are also used to improve the solids-carrying capacity of fluids as they are pumped downhole for various purposes. 
         [0005]    A thorough presentation of the use of fibers in well-service fluids may be found in the following publications. 
         [0006]    Armstrong K. et al.: “Advanced Fracturing Fluids Improve Well Economics,”  Oilfield Review  7, no. 3 (Autumn 1995): 34-51. 
         [0007]    Abbas R. et al.: “A Safety Net for Controlling Lost Circulation,”  Oilfield Review  15, no. 4 (Winter 2003/2004): 20-27. 
         [0008]    Bivins C. et al.: “New Fibers for Hydraulic Fracturing,”  Oilfield Review  17, no. 2 (Summer 2005): 34-43. 
         [0009]    Daccord G. et al.: “Cement-Formation Interactions,” in Nelson E. B. and Guillot D. (eds.):  Well Cementing -2 nd  Edition, Houston: Schlumberger (2006): 191-232. 
         [0010]    More recently, it has been discovered that fluids containing fibers (also known as fiber laden fluids) may be used as plugs that prevent the commingling of two other well-service fluids in a tubular body. For example, a fiber-laden fluid may be used to prevent contact between drilling fluid and a cement slurry—fluids that are often incompatible and would cause operational problems should they commingle. 
         [0011]    Until now, fibers employed in well-service fluids have been linear; that is, the length of the fiber extends along a straight or nearly straight line. However, it has been discovered that curved fibers have utility in the domain of well-service fluids. This type of fibers is also documented in copending patent application no EP09290761.7 to Services Petroliers Schlumberger. 
         [0012]    In civil engineering, so-called “curved fibers”, available, for example, from CNBM International Corporation, 5/F, Longbo Building, 3 Nanlishi Rd., Xicheng District, Beijing City, China, under the product name “Curved Polymer Fibre” are used for the reinforcement of concrete. However, such curved fibers are crimped or twisted in a helical pattern along their length. The bulk fiber shape is still linear, because the helix extends in a straight line. 
         [0013]    It therefore remains desirable to have equipment and methods by which curved fibers may be prepared. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention solves the problems mentioned herein. 
         [0015]    In a first aspect, the invention pertains to an apparatus for preparing curved fibers in batches. Views of a preferred embodiment of the apparatus are presented in  FIGS. 1-3 . 
         [0016]    In a second aspect, the invention relates to an apparatus for preparing curved fibers continuously. A view of a preferred embodiment of the apparatus is presented in  FIG. 4 . 
         [0017]    The invention also aims at a batch method for preparing curved fibers. The first aspect of the invention is employed to perform the method. 
         [0018]    Also disclosed is a continuous method for preparing curved fibers. The second aspect of the invention is employed to perform the method. A cylindrical screw feeder is selected whose diameter between the threads provides a desired curvature. One or more continuous strands of fiber are also selected. Before operating the apparatus, it is first necessary to thread the fiber strands throughout the entire mechanism. In the rolling zone, the fiber strand passes through the opposing wheels and continues between the threads of the screw feeder until it reaches the unrolling zone, whereupon the fiber strands are guided off the screw feeder through opposing wheels and then fed into the cutting means. 
         [0019]    The fibers suitable to be used in the present invention have a workability-temperature range, defined herein as a temperature higher than the softening temperature (often the glass-transition temperature) of the fiber composition, and lower than the melting point of the fiber composition. For fiber compositions that do not melt, the upper limit is the decomposition temperature of the fiber composition. When heated to a temperature within the workability-temperature range, the fiber becomes malleable and assumes the curvature of the cylindrical surface. When cooled to a temperature below the softening temperature, the fiber is no longer malleable and therefore maintains the curvature. 
         [0020]    Then, in the context of the present invention, fiber diameters up to about 300 micrometers may be employed; however, diameters between 100 to 300 micrometers are preferred and diameters between 200 and 280 micrometers are even more preferred. The curved-fiber length after cutting may be between about 1 mm and 50 mm, more preferably between 5 mm to 20 mm and most preferably between 10 mm to 20 mm. The Feret length of the fibers, defined as the straight-line distance between each end of the curved fiber, may be between 5 mm and 30 mm. 
         [0021]    In addition, the curvature radius of the fibers may be between about 5 mm and 50 mm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a cross-sectional view of an apparatus to prepare curved fibers in batches. 
           [0023]      FIG. 2  shows how the half cylinders and spacing rod are arranged in the apparatus to prepare curved fibers in batches. 
           [0024]      FIGS. 3A and 3B  show views of an armature bracket employed in the apparatus to prepare curved fibers in batches 
           [0025]      FIG. 4  is a diagram of an apparatus for preparing curved fibers continuously. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    As mentioned earlier, the present invention relates to apparatuses and methods for manufacturing curved fibers that are especially suitable for well application. An even more preferred application is the use of the present curved fibers in fluids employed during various operations performed during the construction and treatment of subterranean wells. Appropriate well-service fluids include (but are not limited to) drilling fluids, cement slurries, spacer fluids, chemical washes, completion fluids, acidizing fluids, fracturing fluids and gravel-pack fluids. 
         [0027]    The fibers envisioned for use in all aspects of the invention may comprise (but are not limited to) thermoplastic materials such as those in the following list: acrylonitrile butadiene styrene, cellulose acetate, cycloolefin copolymer, ethylene-vinyl acetate, ethylene-vinyl alcohol, acrylic/PVC alloy, polyacetal, polyacrylate, polyacrylonitrile, polyimide, polyimide-imide, polyaryletherketone, polybutadiene, polybutylene, polybutylene, terephthalate, polycaprolactone, polychlorotrifluoroethylene, polyethylene terephthalate, polycyclohexone dimethylene terepthlalate, polycarbonate, polyhydroxyalkanoate, polyketone, polyester, polyethylene, polyetheretherketoneketone, polyetherimide, polyethersulfone, polyethylenechlorinate, polyimide, polylactic acid, polymethylpentene, polyolefins, polyphenylene oxide, polyphenylene sulfide, polyphtalamide, polypropylene, polystyrene, polysulfone, polytrimethylene terephthalate, polyurethane, polyvinyl acetate, polyvinyl chloride and styrene-acrylonitrile. Of these, polyolefins, polyamides and polyesters are preferred. 
         [0028]    Fibers suitable for being use in the present invention have a “workability-temperature range”, defined herein as a temperature range higher than the softening temperature (often the glass-transition temperature) of the fiber composition, and lower than the melting point of the fiber composition. For fiber compositions that do not melt, the upper limit is the decomposition temperature of the fiber composition. When heated to a temperature within the workability-temperature range, the fiber becomes malleable and assumes the curvature of the cylindrical surface. When cooled to a temperature below the softening temperature, the fiber is no longer malleable and therefore maintains the curvature. 
         [0029]    After that, the present invention preferably involves fibers with diameters up to about 300 micrometers; however, diameters between 100 to 300 micrometers are preferred and diameters between 200 and 280 micrometers are even more preferred. The curved-fiber length after cutting may be between about 1 mm and 50 mm, more preferably between 5 mm to 20 mm and most preferably between 10 mm to 20 mm. The Feret length of the fibers, defined as the straight-line distance between each end of the curved fiber, may be between 5 mm and 30 mm. In addition, the curvature radius of the fibers may be between about 5 mm and 50 mm. 
         [0030]    The first aspect of the invention is an apparatus for preparing curved fibers in batches. Views of a preferred embodiment of the apparatus are presented in  FIGS. 1-3 . Curved bars  1  and  2  are arranged such that their concave surfaces face each other, but leave openings  7  between them.  FIGS. 1-3  show two half-cylinders; however, it will be appreciated that more than two curved bars, each forming a curvature less than 90 degrees, may be also be used. Such an arrangement would create more openings. A spacing rod  4  is placed between the curved bars to maintain the openings, and hanging means are used to secure the curved bars to the spacing rod  4 . In  FIGS. 1-3  the hanging means are two bolts  8  and  9 ; however, it will be appreciated that other devices such as pins, clips and screws may also be appropriate. A strand of fiber  3  is wrapped around the curved bars in a manner such that the entire fiber surface is in direct contact with the curved-bar surfaces. In this way, the entire fiber strand is exposed to the same curvature. After the curved bars are wrapped with fiber, a supporting means surrounds the apparatus and holds the wrapped fiber in place. The supporting means shown in  FIGS. 1-3  is an armature comprising two brackets  5  and  6 . An internal portion of each bracket is notched such that the bracket surface is approximately tangent to the wound fiber, thereby minimizing damage. Although the angle shown in  FIGS. 1-3  is 30°, it will be appreciated that angles between about 20° to about 50° would be appropriate. It will also be appreciated that the supporting means may comprise other devices such as tubes, triangular frames and the like. The effective external diameter of the half cylinders  1  and  2  is preferably between about 5 mm and 20 mm. The width of the spacing rod  4  is preferably between about 0.1 and 5 mm. The apparatus also comprises means for heating the fibers. 
         [0031]    The description of the present invention also encompasses a batch method for preparing curved fibers. The first aspect of the invention is preferably employed to perform the batch method. Basically, a spacing rod  4  and curved bars  1  and  2  are chosen such that the combination provides a desired curvature and openings  7  between the curved bars. The spacing rod  4  is fixed to the curved bars  1  and  2  by hanging means  8  and  9 . 
         [0032]    A fiber strand  3  is selected. The fiber strand is wrapped around the curved bars  1  and  2  such that the entire strand is in direct contact with the curved-bar surfaces. In this way, all of the fiber is exposed to the same curvature. After wrapping, the supporting means  4  and  5  are placed over the assembly to hold the fiber strand in place. 
         [0033]    Next, the apparatus is heated such that the fiber composition is heated to a temperature within its workability-temperature range. After heating, the apparatus is cooled such that the fiber temperature is restored to a temperature below the softening temperature. After cooling, the supporting means  4  and  5  are removed, and the wrapped fiber is cut along openings  7  between the curved bars. After cutting, the rest of the apparatus is disassembled, and the resultant curved fibers are collected. 
         [0034]    The second aspect of the invention is an apparatus for preparing curved fibers continuously. A view of a preferred embodiment of the apparatus is presented in  FIG. 4 . One or more strands of fiber are guided onto a rotating cylindrical screw feeder  10  in a rolling zone  11 . As shown in  FIG. 4 , the means for transporting and guiding the fiber strands to the screw feeder  10  comprises two opposing wheels or rollers  17  that rotate in opposite directions. Those skilled in the art will recognize that other means for transporting and guiding fiber strands may also be appropriate. 
         [0035]    Rotation of the screw feeder  10  carries the fiber strands through a heating zone  12  and a cooling zone  13 . An unrolling zone  14  at the end of the screw feeder  10  comprises means for guiding the fiber strands away. As pictured in  FIG. 4 , the means for transporting and guiding the fiber strands away from the screw feeder  10  comprises two opposing wheels or rollers  18  that rotate in opposite directions. Those skilled in the art will recognize that other means for transporting and guiding fiber strands may also be appropriate. 
         [0036]    Next, the fiber strands encounter a cutting means  15  at which the fiber strands are chopped into desired lengths. After cutting, the curved fibers  16  are collected (not shown). 
         [0037]    A continuous method for preparing curved fibers is also part of the present invention. The second aspect of the invention is employed to perform the continuous method. A cylindrical screw feeder  10  is selected whose diameter between the threads provides a desired curvature. One or more continuous strands of fiber  19  are also selected. For the purposes of this invention, a continuous strand is defined as having an extended length such as a spool containing many meters of continuous filament, thereby making the continuous method practical. Before operating the apparatus, it is first necessary to thread the fiber strands throughout the mechanism. In the rolling zone  11 , the fiber strand  19  passes through the opposing wheels  17  and continues between the threads of the screw feeder until it reaches the unrolling zone  14 , whereupon the fiber strands  19  are guided off the screw feeder through opposing wheels  18  and then fed into the cutting means  15 . 
         [0038]    Once the threading process is complete, rotation of the screw feeder  10  commences and a heat source is activated in the heating zone  12  such that the fiber composition reaches a temperature within its workability-temperature range. After becoming malleable and assuming the curvature of the screw feeder  10 , the fiber strands  19  leave the heating zone and proceed to the cooling zone, in which the fiber-composition temperature falls below the softening temperature, and the fiber strands  19  lose their malleability. Next, the fiber strand  19  reaches the unrolling zone  14  and is guided away from screw feeder by opposing wheels  18 , whereupon it reaches the cutting means  15  and is chopped into desired curved fiber lengths  16 . The cut curved fibers  16  are then collected in a container (not shown). 
         [0039]    The heating means in both the first and second aspects of the invention may be (but is not limited to) one or more members of the list comprising: an oven, a heating rod, a forced-air heater and an infrared heater. 
         [0040]    As mentioned above, the curved fibers may be used as an additive in fluids employed during the construction and treatment of subterranean wells. It will be appreciated that the choice of thermoplastic material in the fibers would be dependent upon the temperature the fluid would encounter during the subterranean-well operation. One would preferably choose a material whose workability-temperature range begins at a temperature higher than that in the subterranean well. 
         [0041]    The preceding description has been presented with reference to presently preferred embodiments of the invention. Persons skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structures and methods of operation can be practiced without meaningfully departing from the principle, and scope of this invention. Accordingly, the foregoing description should not be read as pertaining only to the precise structures described and shown in the accompanying drawings, but rather should be read as consistent with and as support for the following claims, which are to have their fullest and fairest scope.