CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present document is based on and claims priority to U.S. Provisional Application Ser. No. 61/274,246, filed Aug. 14, 2009. 
     
    
     BACKGROUND 
       [0002]    During and after many downhole wellbore operations, such as hydraulic fracturing, a controlled release of chemicals often is required. A variety of chemical treatments, e.g. acid etching, have been used to enhance hydrocarbon production in such wellbore operations. It has been a challenging job to deliver chemicals in real-time and in a controlled manner. A number of solutions have been proposed, such as the use of a fluid plug, a canister, and encapsulation. Additionally, the use of micro-coil chemical delivery has been attempted, but the turbulent flow can be problematic for the micro-coil tubing. 
         [0003]    For example, when using micro-coil, one of the major technical challenges is coil survivability. During hydraulic fracturing operations, the micro-coil is subjected to significant fluid drag force due to the large annular flow rate between the micro-coil and the production tube or casing. This external drag force, combined with the weight of the micro-coil, can lead to premature failure/breakage of the micro-coil. This survivability issue severely limits the maximum flow rate that can be pumped during the fracturing job and also limits the maximum depth that can be reached for the chemical delivery. 
         [0004]    Existing micro-coil chemical delivery systems have focused on isotropic metal tubes, such as stainless steel and Inconel tubes. This is a viable solution as long as the target flow rate and/or the treatment depth are relatively small. However, the micro-coil is not able to withstand large flow rates and/or placement at substantial well depths. 
       SUMMARY 
       [0005]    In general, a system and method is described as enabling use of micro-coil at substantial depths and/or with substantial flow rates. A composite micro-coil is formed as a tubing with a multi-layered tubing wall. The composite tubing wall provides substantial strength and longevity which enables deployment of the micro-coil in a much wider variety of well treatment applications, such as applications having substantial flow rates and/or applications at substantial well depths. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Certain embodiments will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0007]      FIG. 1  is a schematic front elevation view of one embodiment of a well system having a micro-coil tubing deployed in a wellbore for a well treatment application; 
           [0008]      FIG. 2  is a cross-sectional view of one embodiment of a micro-coil; 
           [0009]      FIG. 3  is a cross-sectional view similar to that of  FIG. 2  but showing an alternate embodiment of the micro-coil; and 
           [0010]      FIG. 4  is a graph comparing performance of embodiments of composite micro-coil relative to conventional types of tubing. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
         [0012]    The embodiments described herein generally relate to a system and method of using composite micro-coils to deliver chemicals downhole in a wellbore. The micro-coil may be employed for real-time chemical delivery, and the unique design substantially enhances survivability. During a variety of well treatment applications, such as hydraulic fracturing operations, the composite micro-coil is able to better withstand the significant fluid drag forces due to large annular flow rates between the micro-coil and the production tubing or casing. The improved strength of the composite micro-coil enables the micro-coil to withstand the external drag force in combination with the weight of the micro-coil to avoid premature failure/breakage of the micro-coil which tended to occur in conventional systems. 
         [0013]    According to one embodiment, a production well stimulation application utilizes the composite micro-coil to provide real-time delivery of chemicals downhole for enhanced oil and/or gas production. In this particular example, the micro-coil is a multi-layer micro-coil incorporating a carbon-epoxy composite between metallic layers. However, other layers and materials may be employed, as discussed in greater detail below. 
         [0014]    The multi-layer composite design withstands the higher flow rates and is able to reach deeper locations than conventional tubings, such as conventional metal tubes. By confining chemicals inside, the composite micro-coil solution also minimizes detrimental impacts on the production well, downhole tools, and the natural environment. Also, it allows controlled chemical release at the target depth without premature reaction or loss. The controlled parameters may include the mixing depth, mixing time, flow/mixing rate, and total chemical quantities. 
         [0015]    Referring generally to  FIG. 1 , one embodiment of a well system  20  is illustrated as having a well treatment system  22  deployed in a wellbore  24 . The wellbore  24  extends down from a surface location and into or through a subterranean formation  26  below surface equipment  28 , such as a wellhead. By way of example, the wellbore  24  may be cased with a casing  30  having perforations  32  which allow injection of the treatment fluid into the surrounding subterranean formation  26 . 
         [0016]    The well treatment system  22  is illustrated schematically and may comprise a variety of configurations and components. By way of example, the well treatment system  22  comprises hydraulic fracturing equipment  34  deployed downhole into wellbore  24  by a suitable conveyance  36 , e.g. production tubing or coiled tubing. The well system  20  also comprises at least one micro-coil  38  which is deployed down through the wellbore within, or along the exterior of, conveyance  36  and well treatment system  22 . The micro-coil  38  is a composite micro-coil which may be formed in a tubular configuration having a multi-layered wall, as described in greater detail below. A variety of desired chemicals may be delivered downhole through an internal flow passage of the composite micro-coil  38  for delivery into the wellbore  24  and/or into the surrounding subterranean formation  26 . In an application, the micro-coil is conveyed downhole into the wellbore  24 , and a chemical treatment is delivered downhole through an interior flow passage of the micro-coil. 
         [0017]    Referring generally to  FIG. 2 , one embodiment of the composite micro-coil  38  is illustrated in cross-section. In this example, the composite micro-coil  38  is generally tubular having an internal flow passage  40  and a composite tubular wall  42  comprising a plurality of layers  44 . By way of specific example, the composite tubular wall  42  may be in the form of a triple layer sandwich construction, in which a non-metallic middle layer  46 , e.g. composite layer, is sandwiched between a metal internal layer  48  and a metal external layer  50 . 
         [0018]    The composite micro-coil  38  may be formed with a variety of geometries depending on the specific well treatment application for which it is designed. Examples of geometries are provided in the following Table 1 in conjunction with  FIG. 2 : 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 D1 
                 D2 
                 D3 
                 D4 
                 t1 
                 t2 
                 t3 
                 t 
               
               
                   
                 (inch) 
                 (inch) 
                 (inch) 
                 (inch) 
                 (inch) 
                 (inch) 
                 (inch) 
                 (inch) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Composite 
                 0.120 
                 0.170 
                 0.220 
                 0.250 
                 0.015 
                 0.025 
                 0.025 
                 0.065 
               
               
                 design 1 
               
               
                 Composite 
                 0.152 
                 0.172 
                 0.230 
                 0.250 
                 0.010 
                 0.029 
                 0.010 
                 0.049 
               
               
                 design 2 
               
               
                 Composite 
                 0.120 
                 0.140 
                 0.230 
                 0.250 
                 0.010 
                 0.045 
                 0.010 
                 0.065 
               
               
                 design 3 
               
               
                   
               
             
          
         
       
     
         [0019]    In this example, the inside diameter of the composite micro-coil  38  (the diameter of internal flow passage  40 ) is D 1  and the outside diameter of the composite micro-coil  38  is D 4 . In Table 1 above, t1 is the thickness of metal external layer  50  which is equal to (D 4 −D 3 )/2. The thickness of composite layer  46  is labeled t2 and is equal to (D 3 −D 2 )/2. The thickness of metal internal layer  48  is labeled t3 and is equal to (D 2 −D 1 )/2.An overall thickness of tubular wall  42  is labeled t which is equal to (D 4 −D 1 )/2 . 
         [0020]    Table 1 provides examples of the composite micro-coil  38  which have substantially greater strength and provide superior performance when compared to conventional designs. In this example, t1, t2, t3, and t, respectively, correspond to the thickness of the outside metal layer  50  (e.g. a layer of Inconel  825 ); the thickness of composite layer  46  (e.g. a layer of carbon-epoxy composite, 40% fiber volume); the thickness of inside metal layer  48  (e.g. a layer of stainless steel), and the total wall thickness. It should be noted with respect to Table 1, composite design 1 and composite design 3 have the same total wall thickness but different thickness distribution among the three layers  44  of materials. 
         [0021]    As further illustrated in  FIG. 3 , the composite micro-coil  38  also may incorporate additional or alternate layers. In the embodiment of  FIG. 3 , the composite micro-coil  38  incorporates one or more layers  52 , such as bonding layers. An example of a material for forming a bonding layer is a suitable epoxy. The bonding layer  52  may be positioned between the non-metallic layer  46  and one or both of the metal layers  48  and  50 . In the embodiment illustrated, the bonding layers  52  have negligible thickness but can add greater dependability and strength when employed in certain applications. Each layer  52  also may be formed from a thermoset, thermoplastic or thermoplastic elastomer material, e.g. nylon, disposed between the non-metallic layer  46  and one or both of the metal layers  48  and  50 . Similarly, a jacket layer  54  may be placed around the outer metal layer  50  to both facilitate manufacturing and to reduce friction along an exterior of the composite micro-coil  38 . 
         [0022]    Referring again to  FIGS. 2 and 3 , the layers  44  may be formed from a variety of materials. In one embodiment, the inside metal layer  48  is made of stainless steel; the outside metal layer  50  is made of a nickel-iron-chromium alloy, such as Inconel; and the middle composite layer is made of a carbon-epoxy material. A fiber volume of the composite layer may vary, e.g. from about 30% to about 70% or from about 20% to about 75%, and is illustrated by fibers  56  in  FIG. 3 . In this embodiment, the materials selected to form layers  48 ,  50  and  46  are listed in the following Table 2: 
         [0000]                                                  TABLE 2                       density (kg/m 3 )   tensile strength (kpsi)                                        stainless steel   8000   70           Inconel 825   8000   85           Carbon-epoxy   1800   420                        
The sandwich construction shown in  FIGS. 2 and 3  allows a variety of dimensional and material choices based on strength requirements, cost constraints, and operating environments.
 
         [0023]    However, the configuration of layers  44  and the selection of materials for forming layers  44  may vary according to the parameters of a given well treatment application and/or environment. For example, the non-metallic layer  46  may be a composite layer formed from a variety of materials to provide a major contribution to the superior performance of the composite micro-coil  38 . The composite material layer  46  may include a matrix  58  with embedded fibers  56 , as illustrated in  FIG. 3 . In alternative embodiments, the matrix material  58  may include any appropriate material such as epoxy, Peek, Pek, or any thermoset or any thermoplastic materials, among others; and the fiber may include any appropriate fiber such as carbon, Kevlar, glass, aluminum, ceramic, and steel, among others. The metal both on the metal internal layer  48  and the metal external layer  50  of the composite micro-coil  38  may be any appropriate metal, such as carbon steel, stainless steel, or Inconel, among others. The metal on the inside and outside of the micro-coil can be made of the same material or of different materials, as discussed above with respect to various embodiments. 
         [0024]    There are typically two primary external loads acting on the micro-coil  38  during fluid pumping, namely, the fluid drag force due to fracturing fluid flow and the gravity/buoyancy effects. The micro-coil  38  has to survive the combination of these two external loads by virtue of its strength. This leads to the load-resistance governing equation, which provides the safe operation envelopes for the micro-coils in turbulent fracturing flow. A safe operation envelope provides the boundary of maximum flow rate and depth for a specific micro-coil. Thus it can be used as a convenient tool to evaluate the performance of various micro-coil designs. 
         [0025]    A plot of the safe operation envelopes for a variety of micro-coils is shown below in  FIG. 4 . For comparison purposes, conventional tubing structures, e.g. control lines, distributed temperature sensing (DTS) tubes, and chemical injection tubes are illustrated as conventional tubing structure group  60 . The group of conventional tubing structures  60  is contrasted with the performance of three different embodiments of the composite micro-coil  38  with respect to maximum flow rate and maximum depth. The three embodiments of the present, composite micro-coil  38  are labeled group  62 . As illustrated, the newly designed composite micro-coils  38  have substantially superior performance compared to existing tubing structures, such as existing metal tubes. The composite micro-coils  38  can reach higher flow rates at greater depths. Composite design 3, for example, can sustain a maximum flow rate of about 50 BPM at a target depth of about 10,000 ft. 
         [0026]    The structure of composite micro-coil  38  also enables use of the micro-coil in performing other functions, such as the transmission of signals, e.g. data and/or power signals. For example, in embodiments where the composite layer  46  is formed from a good insulation material, the inner metal layer  48  can be used as a conductor. Similarly, when the composite layer  46  is a good insulator, one or more of the layers  52  can be formed as a bonded jacket. By way of example, the bonded jacket layer  52  can be extruded over the outside of the metal internal layer  48  or on the outside of the composite layer  46  to create a conductive path. These embodiments allow use of the composite micro-coil  38  for powering of an electric tool downhole without requiring a dedicated power cable. 
         [0027]    Well system  20 , well treatment system  22 , and composite micro-coil  38  may be utilized in a variety of well applications and environments. The well treatment system  22  may be designed for fracturing applications or for a variety of other well treatment applications. Similarly, composite micro-coil  38  may be designed to deliver a variety of chemicals downhole to facilitate fracturing or a variety of other well service/well stimulation applications. In some applications, the composite micro-oil  38  may even be used to pump slurries, fibers, and other well stimulation materials downhole. The composite micro-coil  38  also may be routed downhole within or along many types of well equipment. 
         [0028]    Depending on the specific chemicals, treatment applications, and environments, the composite micro-coil may be constructed with several types of materials. In many applications, a non-metallic layer is sandwiched between metallic layers, but the composition of the non-metallic layer may vary according to the environment and strength requirements. Many types of composite materials incorporate fibers or other additives to strengthen the overall layer. As discussed above, the composite layer may be formed from a carbon-epoxy composite material with high specific strength and low specific density. However, many other types of materials may be employed to construct both the composite layer and the layers on either side of the composite layer. 
         [0029]    Accordingly, although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Such modifications are intended to be included within the scope of this invention as defined in the claims.
A technique utilizes micro-coil which is formed as a composite to enable use at substantial depths and/or with substantial flow rates. The micro-coil is formed as a tubing with a multi-layered tubing wall. The composite tubing wall provides substantial strength and longevity which allows deployment of the micro-coil in a much wider variety of well treatment applications, such as applications having substantial flow rates and/or applications at substantial well depths.