You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
       [0001]    A variety of systems and methodologies are used to inject fluid into subterranean regions. For example, an injection string can be deployed downhole into a wellbore to deliver a desired injection fluid into the surrounding reservoir of an oil or gas well. Injection fluid is delivered downhole through the injection string and routed radially outward through openings in the injection string at the desired injection location. 
         [0002]    The injection string may comprise a base pipe with injection devices mounted on the base pipe at specific locations along the base pipe. The injection fluid is directed radially outward through the base pipe and into the injection devices which, in turn, control the fluid flow to the surrounding reservoir. Existing injection devices are attached to the base pipe with elastomer seals and/or welding, but such attachment methods can have detrimental effects with respect to system temperature sensitivity, potential for corrosion, loss of strength and reduced system life. Existing injection devices also tend have relatively large cross sections and tend to be constructed with expensive, exotic materials to resist the corrosive downhole environment. As a result, the functionality of the system is reduced while the cost of the system is increased. 
       SUMMARY 
       [0003]    In general, the present invention provides a system and method to facilitate injection of a fluid into a subterranean region. An injection string is provided with a plurality of injection devices positioned to facilitate fluid injection when the system is deployed in a wellbore. Each injection device comprises a base pipe and a surrounding structure mounted over the base pipe. The surrounding structure is designed to create a desired injection fluid flow path from a base pipe radial flow opening to an outlet of the surrounding structure. Various features are incorporated into or used in cooperation with the surrounding structure to facilitate system functionality and to increase the life of the system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0005]      FIG. 1  is a schematic front elevation view of a well system having an injection string deployed in a wellbore, according to an embodiment of the present invention; 
           [0006]      FIG. 2  is a sectional view of a portion of a surrounding injection structure mounted on a base pipe, according to an embodiment of the present invention; 
           [0007]      FIG. 3  is a partial, cross-sectional view taken generally along line  3 - 3  of  FIG. 2 , according to an embodiment of the present invention; 
           [0008]      FIG. 4  is a sectional view of a portion of another surrounding injection structure mounted on a base pipe, according to an alternate embodiment of the present invention; 
           [0009]      FIG. 5  is a partial, cross-sectional view taken generally along line  5 - 5  of  FIG. 4 , according to an alternate embodiment of the present invention; 
           [0010]      FIG. 6  is a sectional view of a portion of another surrounding injection structure mounted on a base pipe, according to an alternate embodiment of the present invention; 
           [0011]      FIG. 7  is a partial, cross-sectional view taken generally along line  7 - 7  of  FIG. 6 , according to an alternate embodiment of the present invention; and 
           [0012]      FIG. 8  is a partial, sectional view of an embodiment of a tool that can be used to move seal rings into sealing engagement between the base pipe and the surrounding structure, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    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. 
         [0014]    The present invention relates to a system and method used in facilitating and controlling the injection of fluid into a subterranean region. For example, the system and method can be used to control the injection of fluids into the subterranean reservoir of an oil well or a gas well. The system comprises an injection string having a plurality of injection devices positioned at desired locations along the injection string to facilitate injection of fluid into a wellbore and into the surrounding subterranean region, e.g. reservoir, along a length of the injection string. The design of the injection devices provides the overall injection string with a relatively slim cross-section characterized by a low ratio of maximum outside diameter to base pipe outside diameter. The injection string construction also enables use of lower-cost materials while avoiding expensive, exotic materials even in high-temperature and high injection pressure well applications. 
         [0015]    In one embodiment, the injection devices are formed with an all metal construction that eliminates the needs for elastomer seals and other expensive materials and components. For example, seals can be formed along a base pipe with non-elastomer rings, such as metal rings that may be in the form of metal, wedge-shaped, compressed seal rings. The lack of the elastomer seals also avoids limitations with respect to elevated temperatures and with respect to the lifespan of the injection string. In this same embodiment, the injection devices can be constructed without requiring welding of components to the base pipe and without requiring turning or milling of the base pipe. In some embodiments, the injection devices are provided with a removable cover that provides easy access to injection nozzles on a low pressure side of the injection nozzles. 
         [0016]    Referring generally to  FIG. 1 , an example of a well system  20  is illustrated according to an embodiment of the present invention. The well system  20  comprises an injection string  22  that may be deployed downhole into a wellbore  24 . By way of example, wellbore  24  may comprise a deviated wellbore section  26 , e.g. a generally horizontal wellbore section; however the injection string  22  can be used in a many types of wellbores formed at a variety of orientations. In the embodiment illustrated, wellbore  24  and deviated wellbore section  26  extend through a subterranean region  28 , such as an oil reservoir or a gas reservoir. 
         [0017]    In the example illustrated, injection string  22  comprises a plurality of injection devices  30  that are used to control the injection of fluid from injection string  22  into the surrounding subterranean region  28 . The injection devices  30  are spaced from each other and positioned at desired locations along the injection string  22 . The wellbore  24  may be divided into isolated sections via isolation devices  32 , such as packers, to facilitate the controlled flow of injection fluid from the injection devices  30  into the surrounding subterranean region  28 . 
         [0018]    Referring generally to  FIG. 2 , an embodiment of one of the injection devices  30  is illustrated in partial cross-section. It should be noted that the injection device  30  is generally tubular in shape with an axis  34  extending along the center of the tubular structure. In the embodiment illustrated, injection device  30  comprises a base pipe  36  and a surrounding structure  38  mounted over the base pipe  36 . One or more flow openings  40 , such as radial flow openings, extend through base pipe  36  to enable flow of injection fluid from an interior of base pipe  36  into the surrounding structure  38 . 
         [0019]    The surrounding structure  38  may be constructed with a variety of components arranged in different configurations depending on the specific injection application. In the embodiment illustrated, surrounding structure  38  is constructed to create an upstream chamber  42 , with respect to injection fluid flow, and a downstream chamber  44  separated by a nozzle section  46 . Nozzle section  46  may comprise a nozzle  48  mounted in a flow passage  50 . By way of example, nozzle  48  may be an interchangeable nozzle that is removably mounted in flow passage  50  via a suitable fastening mechanism  52 , such as a threaded region. 
         [0020]    In the example illustrated, flow passage  50  is a formed in a generally axial direction through a nozzle retainer ring  54  which encircles base pipe  36 , as further illustrated in  FIG. 3 . It should be noted that a plurality of axial, flow passages  50  can be formed through nozzle retainer ring  54  to receive corresponding nozzles  48 . As illustrated, nozzle retainer ring  54  is located on one axial side of the one or more flow openings  40 , while an end ring  56  is located on an opposite side of flow openings  40 . A plate section  58  extends from nozzle retainer ring  54  to end ring  56  to create upstream chamber  42  for receiving injection fluid through openings  40 . The plate section  58  can be attached to nozzle retainer ring  54  and end ring  56  by a variety of suitable fastening techniques, such as welding. 
         [0021]    A flow-through ring  60  also may be mounted around base pipe  36  at a location downstream of nozzle retainer ring  54 , as illustrated. Flow-through ring  60  may comprise one or more flow passages  62  that facilitate continued flow of injection fluid from downstream chamber  44  to an injection device outlet  64 . In the example illustrated, flow-through ring  60  is connected to nozzle retainer ring  54  by a plurality of connector rods  66 . A removable cover  68  is mounted to span from nozzle retainer ring  54  to flow-through ring  60  over downstream chamber  44 , thereby defining the downstream chamber. 
         [0022]    Removable cover  68  is attached to nozzle retainer ring  54  and/or flow-through ring  60  by fasteners  70  which may be in the form of bolts, screws, threads or other suitable fasteners. The fastener  70  can be released to remove cover  68  and to provide access to nozzles  48  on a low pressure side of nozzle section  36  and nozzles  48 . In some embodiments, removable cover  68  can be designed as a slidable cover that, for example, slidingly engages flow-through ring  60  to permit adjustments to the size of downstream chamber  44 . The length of cover  68  spanning downstream chamber  44  can be selected to avoid or limit the erosion of components due to the discharge of injection fluid from nozzles  48 . Adjustments to the size of downstream chamber  44  can be accommodated by slidably engaging connector rods  66  with nozzle retainer ring  54  or flow-through ring  60 . With respect to  FIG. 2 , it should be noted that a portion of removable cover  68  has been cut out to illustrate the position of connecting rods  66 . 
         [0023]    A screen  72 , such as a sand screen, may be positioned over outlet  64 . In the example illustrated, screen  72  is connected with flow-through ring  60  by a connector plate  74  that may be fastened to screen  72  and flow-through ring  60  by a suitable fastener mechanism, such as welding. Screen  72  is designed and positioned to prevent the back flow of sand and other particulates into injection device  30 . 
         [0024]    In the embodiment illustrated, surrounding structure  38  is sealed with respect to base pipe  36  via seal rings  76  formed from a non-elastomer material. For example, seal rings  76  may be constructed as metal seal rings that are movable to enable insertion between base pipe  36  and the components of surrounding structure  38 . In one example, seal rings  76  are inserted between base pipe  36  and surrounding structure  38  on opposite sides of flow openings  40 . In the embodiment illustrated, seal rings  76  are sealably engaged between base pipe  36  and nozzle retainer ring  54  and between base pipe  36  and end ring  56 . The seal rings  76  may be formed of a ductile, metal material having a leading wedge  78  to facilitate insertion of the seal rings  76  between base pipe  36  and surrounding structure  38 . The leading wedge  78  can be oriented to engage a corresponding chamfer region  79  of the cooperating component of surrounding structure  38 . In some applications, the leading wedge sections  78  of separate seal rings  76  can be oriented in opposite directions to facilitate the maintenance of surrounding structure  38  at a desired axial position with respect to base pipe  36 . Additionally, seal rings  76  can be inserted to form the desired seals at various stages of construction during formation of surrounding structure  38 . 
         [0025]    In operation, injection fluid is pumped down through injection string  22  along the interior of the base pipe  36 . The lower end of base pipe  36  is closed which forces the injection fluid to flow radially outward through the flow openings  40  of each injection device  30 . From flow openings  40 , the injection fluid enters upstream chamber  42  and is directed axially along passages  50 , through nozzles  48 , and into downstream chamber  40 . The injection fluid flows through downstream chamber  40  and along flow passages  62  until exiting the injection device  30  via outlet  64 . In the example illustrated, the injection fluid flows outwardly through screen  72 , into the corresponding isolated wellbore section, and on into the surrounding subterranean region. 
         [0026]    Another embodiment of injection device  30  is illustrated in  FIG. 4 . In this embodiment, upstream chamber  42  is formed in nozzle retainer ring  54 , and downstream chamber  44  is formed between end ring  56  and nozzle retainer ring  54 . Injection fluid flows from the interior of base pipe  36 , through flow opening  40 , and into upstream chamber  42 . The injection fluid then flows through nozzle  48  in a first direction, e.g. a first axial direction, and into downstream chamber  44  between nozzle retainer ring  54  and end ring  56 , as illustrated by fluid flow arrows  80 . The flow of injection fluid is then reversed such that the fluid flows in another direction, e.g. an opposite axial direction, through one or more passages  82  formed through nozzle retainer ring  54 , as further illustrated in  FIG. 5 . The injection fluid then flows past flow-through ring  60  and out of injection device  30  through outlet  64 . 
         [0027]    In the embodiment illustrated in  FIGS. 4 and 5 , removable cover  68  is mounted between nozzle retainer ring  54  and end ring  56  generally at an end of the injection device  30 . Cover  68  may again be removably mounted to nozzle retainer ring  54  by suitable fasteners  70 . Additionally, slidable connector rods  66  may be connected between nozzle retainer ring  54  and end ring  56  to enable adjustment of the size of downstream chamber  44 . Nozzle  48  and removable cover  68  are positioned such that removal of cover  68  provides easy access to the one or more nozzles  48  on a low-pressure side of the nozzles. If interchangeable nozzles are employed, removable cover  68  provides easy access for interchanging the nozzles. Plate  58  can be used to connect nozzle retainer ring  54  and flow-through ring  60  and may be attached to the rings  54 ,  60  by a suitable fastening mechanism, such as welding. 
         [0028]    In this embodiment, the use of elastomer seals and welds to the base pipe  36  also can be avoided. Non-elastomer seal rings  76 , such as metal seal rings, are used to seal between base pipe  36  and surrounding structure  38 . In the example illustrated, seal rings  76  are used on opposite sides of base pipe flow openings  40  to form seals between nozzle retainer ring  54  and the base pipe  36 . The wedge portions  78  of seal rings  76  can be oriented in opposed directions, e.g. facing each other, to form a secure seal between the base pipe and the surrounding structure. 
         [0029]    Another embodiment of injection device  30  is illustrated in  FIG. 6 . In this embodiment, the components of surrounding structure  38  are arranged similarly to those of the embodiment illustrated in  FIG. 4 . However, a variety of changes can be incorporated, including the use of additional passages  82  further illustrated in the cross-sectional view of  FIG. 7 . Furthermore, the length of removable cover  68  can be shortened by incorporating an erosion resistant surface or plate  84  into end ring  56 . The erosion resistant plate  84  is positioned so that injection fluid exiting nozzle  48  is directed against the erosion resistant plate before being redirected through passages  82 . 
         [0030]    In the embodiment of  FIG. 6 , additional flow openings  40  are provided through base pipe  36  to provide a greater flow area for injection fluids flowing from an interior of base pipe  36  to upstream chamber  42 . Additionally, the fastener  70  used to removably attach cover  68  between nozzle retainer ring  54  and end ring  56  may comprise a threaded region  86  by which removable cover  68  is threadably engaged with nozzle retainer ring  54  and/or end ring  56 . Similar to the embodiments described above, the configuration of surrounding structure  38  in the embodiment of  FIG. 6  also enables a low profile injection device  30  that requires no elastomers. Although the surrounding structure  38  can be attached and sealed to base pipe  36  by welds  88 , seal rings  76  also can be employed, as described above. 
         [0031]    In any of the embodiments described above, one or more of the seal rings  76  can be moved in an axial direction and forced between base pipe  36  and the surrounding structure  38  in a manner that ensures sealing even if the base pipe  36  has non circular regions or if other abnormalities exist between base pipe  36  and the surrounding structure  38 . Each seal ring  76  can be moved in an axial direction such that portions of the movable seal ring move farther in the axial direction than other portions of the seal ring to ensure formation of a seal even if irregularities exist between the base pipe  36  and the surrounding structure  38 . 
         [0032]    As illustrated in  FIG. 8 , a tool  90  can be used to move selected seal rings  76  in an axial direction such that portions of the seal ring  76  move farther axially then other portions. In other words, an end face  92  of the seal ring  76  does not remain in the same plane, thus allowing the seal ring  76  to compensate for a non circular base pipe or other irregularities between the base pipe  36  and surrounding structure  38 . Although a variety of tools  90  can be employed, one example comprises a base structure  94 , such as a common cylinder ring, to which a plurality of individual pistons  96  are individually coupled for movement against the face  92  of seal ring  76 . Pistons  96  may be actuated hydraulically, electrically, or by other suitable actuation mechanisms. By clamping or otherwise securing base structure  94  around the base pipe  36  (at least temporarily), actuation of individual pistons  96  forces seal ring  76  to move axially until portions of the seal ring move a sufficient axial distance to secure a seal along the entire seal region between base pipe  36  and surrounding structure  38 . By way of example, the wedge portion  78  of seal ring  76  should move farther into the mating chamfer region  79  where a non-circular base pipe has a smaller diameter. 
         [0033]    The overall well system  20  can be constructed in a variety of configurations for use in many environments and applications. For example, various types of base pipe structures and injection device structures can be employed. Additionally, the number of injection devices, the length of the injection region, and the orientation of the wellbore can vary from one application to another. The injection devices also may incorporate a variety of components in different arrangements depending on the injection fluid and operational or environmental factors. The injection fluid may comprise a variety of liquids, including water, or gases depending on the environment and injection parameters. Furthermore, the base pipe and the surrounding structure can be formed with a variety of materials. In some embodiments, the base pipe and all components of the surrounding structure are formed from metals. The metal seal rings and the removable covers can be formed in a variety of sizes and configurations. 
         [0034]    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.

Summary:
A technique involves injection of a fluid into a subterranean region. An injection string is provided with a plurality of injection devices positioned to enable fluid injection into the subterranean region when the system is deployed in a wellbore. Each injection device comprises a base pipe and a surrounding structure mounted over the base pipe. The surrounding structure is designed to create a desired injection fluid flow path from a base pipe radial flow opening to an outlet of the surrounding structure. Various features are incorporated into or used in cooperation with the surrounding structure to facilitate system functionality and to increase the life of the system.