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]    To meet the demand for natural resources, companies invest significant amounts of time and money in searching for and extracting oil, natural gas, and other subterranean resources from the earth. Systems are often employed to access and extract the desired resource. These systems may be located onshore or offshore, depending on the location of the resource, and generally include a wellhead assembly through which the resource is extracted. These wellhead assemblies may include a wide variety of components, such as various casings, hangers, valves, fluid conduits, and the like, that control system operations. Sometimes it is difficult, as well as expensive, to get direct access to a well through the wellhead assembly while maintaining pressure-containing barriers to protect against release to the surrounding environment. 
         [0002]    Wellhead assemblies may include a tree, i.e., an assembly of pipes, valves and fittings coupled to a wellhead housing or hub to control the flow of oil and gas produced from the well and/or to control the flow of fluids injected into the well, a spool, or other completion member. Completion members are manufactured for surface or subsea applications, and can be vertical, horizontal, or a variation or hybrid thereof in configuration. 
         [0003]    Vertical completion members generally include one or more production passages containing valves, where each production passage is in-line with the production tubing. Vertical completion members generally may be removed while leaving the completion (e.g., the production tubing hanger and production tubing) in place; however, if it is necessary to pull the completion, a vertical completion member may be removed and replaced with a blowout preventer (BOP), a lengthy operation that may leave the well in a vulnerable condition during plugging and/or killing operations and/or exchange of the completion member and BOP pressure-control devices. 
         [0004]    Horizontal completion members may be arranged with production control valves offset from the production tubing and with the tubing hanger locked and sealed in the member passage (instead of the wellhead) after the completion member is installed. With a horizontal configuration, the completion (e.g., the production tubing hanger and production tubing) may be removed without having to remove the completion member from the wellhead housing. However, if the member needs to be removed, the entire completion typically also is removed. 
         [0005]    To manage expected maintenance costs, which are especially high for an offshore well, the well operator may select equipment best suited for the expected type of maintenance predicted to be required over the life of the well. For example, a well operator may predict whether there will be a greater need in the future to pull the completion member from the well for repair, or pull the completion, either for repair or for additional work in the well. Depending on the predicted maintenance events, an operator will decide whether the horizontal or vertical configuration, or a variation or hybrid thereof, each with its own advantages and disadvantages, is best suited for the expected conditions. For instance, with a vertical configuration, it is more efficient to pull the completion member and leave the completion in place. However, if the completion is pulled, the completion member is pulled as well, increasing the time and expense of pulling the completion. Conversely, with a horizontal configuration, it is more efficient to pull the completion, leaving the completion member in place. However, if the completion member is pulled, the completion is pulled as well, increasing the time and expense of pulling the member. 
         [0006]    Another factor an operator may weigh in completion member selection is the relative bore size available for access. With the production valves offset from the production tubing, a horizontal configuration generally has a relatively larger bore. This allows the tubing and tubing hanger to be removed, for instance, or other downhole operations to be performed, without having to remove the completion member from the wellhead or disturb any external connectors to flowlines, service lines, or the like—thereby saving risk, time, and cost. Moreover, due to its large bore configuration, the horizontal configuration can accommodate larger equipment such as electrical submersible pump (ESP) completions. 
         [0007]    An additional factor an operator may weigh in completion member selection relates to the operational impact of the so-called dual barrier requirement. Regulations in certain jurisdictions and other industry practices require a subsea well access system to provide at least two full-bore pressure-containing safety barriers between the well and open water environment at all times. For a vertical configuration, these barriers may be provided by valves such as master valves and swab valves, for example, which may be actuated to open at any time while a safety package is in place. 
         [0008]    For a horizontal configuration, pressure-containing barriers may be provided by crown plugs sealed in the vertical passage of the tubing hanger above the production outlet and in the vertical passage of an internal tree cap landed in the completion member above the tubing hanger, where a so-called tree cap may be used with a tree, spool, or any other completion member. However, the well can be accessed only after the crown plugs have been physically removed. Removal and installation of crown plugs in a horizontal configuration each require a separate trip by wireline, slickline, braided line, or coiled tubing, and such subsea well intervention operations are generally very expensive, often based on hourly or daily rig charges. Moreover, in some cases the plug removal can be made more difficult due to the presence of corrosion, encrustation, debris, differential pressure across the plug, etc., thereby further adding to the cost of intervention. 
         [0009]    An actuatable valve also may provide a pressure-containing barrier in one or more location. However, regardless of whether a crown plug or an actuatable valve is provided as a pressure-containing barrier in the tubing hanger bore, the location is problematic as tubing hangers may already have complex elements such as contingency plug profiles, for example. Moreover, providing a valve such as a gate valve in an internal tree cap as a final pressure-containing barrier may not provide complete control against leakage to the environment. 
     
    
     
       DRAWINGS 
         [0010]    Embodiments of a well access system are described with reference to the following figures. The same numbers are used throughout the figures to reference like features and components. The features depicted in the figures are not necessarily shown to scale. Certain features of the embodiments may be shown exaggerated in scale or in somewhat schematic form, and some details of elements may not be shown in the interest of clarity and conciseness. 
           [0011]      FIG. 1  shows a cross-sectional view of a well access system in accordance with one or more aspects of the present disclosure; 
           [0012]      FIG. 2  shows a cross-sectional view of a well access system in accordance with one or more aspects of additional embodiments of the present disclosure; 
           [0013]      FIG. 3  shows a cross-sectional view of a well access system in accordance with one or more aspects of additional embodiments of the present disclosure; 
           [0014]      FIG. 4  shows a cross-sectional view of a well access system in accordance with one or more aspects of additional embodiments of the present disclosure; and 
           [0015]      FIG. 5  shows a cross-sectional view of a well access system in accordance with one or more aspects of additional embodiments of the present disclosure. 
           [0016]      FIG. 6  shows a cross-sectional view of an upper portion of the well access system in accordance with one or more aspects of additional embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The present disclosure relates generally to systems and methods for providing access to a well without the operational need to replace pressure-containing barriers between the well and the environment. 
         [0018]    One or more specific embodiments of the present disclosure will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
         [0019]    When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, any use of any form of the terms “connect,” “engage,” “couple,” “attach,” or any other term describing an interaction between elements is intended to mean either an indirect or a direct interaction between the elements described. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. The use of “top,” “bottom,” “above,” “below,” and variations of these terms is made for convenience, but does not require any particular orientation of the components. 
         [0020]    Certain terms are used throughout the description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. 
         [0021]    Accordingly, disclosed herein is a system and method for well access that may include and/or be used with a tree, spool, or other completion member. The tree may be a subsea tree or a surface tree, a vertical tree or a horizontal tree, a mono bore or a multi bore tree, a production tree or an injection tree, or any combination or further variation. The spool may be an adapter spool, a tubing spool, a flow spool, or any completion spool member. Completion members may be used in combination; for example, a tree may be installable on other components of the well access system, such as installable on or within a wellhead and/or a tubing spool, for instance. 
         [0022]      FIG. 1  is a schematic diagram that illustrates a well access system  10 . The illustrated well access system  10  can be configured to extract various minerals and natural resources, including hydrocarbons (e.g., oil and/or natural gas), or configured to inject substances into the earth. In some embodiments, the well access system  10  is land-based (e.g., a surface system), and in other embodiments, the well access system  10  is subsea (e.g., a subsea system). As illustrated in  FIG. 1  in accordance with one or more embodiments of the present disclosure, well access system  10  is a subsea system that includes a horizontal tree  12  directly or indirectly connected to wellhead hub  18 , which can be a high pressure wellhead housing or tubing spool or another wellhead member or tree member. The wellhead hub  18  generally is disposed at the upper termination of a well bore and provides for the connection to the well. 
         [0023]    In  FIG. 1 , tree  12  having a bore  13  is shown as mounted on tree connector  16 . Subsea trees generally have at least two bores, one of which communicates with the production tubing (the production bore)  20  via production line  22 , and the other of which communicates with the production tubing annulus (the annulus bore)  24  via annulus flow line  26 . A tubing hanger  44  with a production bore is landed in the tree  12  and supports the production tubing  20  extending into the well. 
         [0024]    Typical designs of horizontal tree  12  have a fixed lateral connection or side outlet  14  (a production wing branch) to the production bore  20  closed by at least one production valve  30  for removal of production fluids from the production bore  20 . The annulus bore  24  also may have an annulus wing branch  15  with one or more respective annulus valve(s)  32 ,  34 . 
         [0025]    In  FIG. 1 , the top of the production bore and the top of the annulus bore are capped by an internal tree cap  40 , which seals off the various bores in the tree  12  and provides channels for operation of the various valves in the tree  12  by means of intervention equipment or remotely from an offshore installation. Internal tree cap  40  may be installed by equipment run through a riser connected to vessel, platform, or other pipe location on or closer to the sea surface, for example, or may be installed in open water by remotely operated vehicle (ROV) assistance. 
         [0026]    The internal tree cap  40  is sealed to the tubing hanger body  44  with one or more seal(s)  45 . The internal tree cap  40  is also sealed to the tree bore with one or more seal(s)  48 . The internal tree cap  40  includes a central production bore capable of fluid communication with production flow line  22  and/or production bore  20  and an annulus bore  28  capable of fluid communication with annulus flow line  26  and/or annulus bore  24 . 
         [0027]    In the embodiment of  FIG. 1 , a valve  60  is built into the internal tree cap  40 , which has relatively more volume for such valve than tubing hanger  44 . As shown in  FIG. 1 , valve  60  is a spherical gate valve; however, valve  60  built into internal tree cap  40  also may be a ball valve or any other type of valve for sealing a flow passageway in the desired environment. Valve  60  is hydraulically, mechanically, and/or electrically actuatable between an open position and a closed position using one or more of control lines  66 ,  68 . Control lines  66 ,  68  used to open and close valve  60  may be ported to a surface or subsurface control system or to ROV hot stabs, for example. When in the closed position, valve  60  may provide a production and/or annulus pressure-containing barrier to the environment. 
         [0028]    An open water tree cap  80  is shown installed within the bore, i.e. inside an internal profile, of internal tree cap  40 . Open water tree cap  80  may be installed in the inner bore of internal tree cap  40 . Open water tree cap  80  may be installed by equipment run through a riser connected to vessel, platform, or other pipe location on or closer to the sea surface, for example, or may be installed in open water by ROV. The open water tree cap  80  is sealed to the internal tree cap  40  with one or more seal(s)  85 . Open water tree cap  80 , when installed inside internal tree cap  40  and when valve  60  is in the closed position, may provide a production and/or annulus pressure-containing barrier to the environment. 
         [0029]    Referring to  FIG. 2 , another embodiment of the present disclosure, a plug  260  installed in the bore of the tubing hanger  44  or a plug (not shown) installed at location  280  in the bore of the internal tree cap  40  may provide a production and/or annulus pressure-containing barrier to the environment, where the open water tree cap  80  disclosed above may provide a production and/or annulus pressure-containing barrier to the environment. 
         [0030]    Referring to  FIG. 3 , another embodiment of the present disclosure, in addition to actuatable valve  60  installed in the bore of the internal tree cap  40 , an additional actuatable valve  300  is installed in the production bore  20  below the tubing hanger  44 . As shown in  FIG. 3 , valve  300  is a spherical gate valve; however, valve  300  installed below the tubing hanger also may be a ball valve or any other type of valve for sealing a flow passageway in the desired environment. Valve  300  is hydraulically, mechanically, and/or electrically actuatable between an open position and a closed position using one or more of control lines  301 ,  302 . Control lines  301 ,  302  used to open and close valve  300  may be ported to a surface or subsurface control system or to ROV hot stabs, for example. When in the closed position, valve  300  may provide a production and/or annulus pressure-containing barrier to the environment, with or without valve  60  also being in a closed position. The open water tree cap  80  disclosed above may provide a production and/or annulus pressure-containing barrier to the environment. 
         [0031]    Referring to  FIG. 4 , when the open water tree cap  80  of this disclosure is removed, the configuration of this disclosure allows a riser  430  to be installed, through insertion of stab  410  into the bore of the internal tree cap  40  and use of riser connector  460 , to facilitate a full production bore well kill. The stab  410  is sealed to the internal tree cap body  40  with one or more seal(s)  420  configured around the outer surface of the stab  410 . Riser  430  includes at least one production bore  440  and annulus bore  450  and can be a light weight intervention (LWI) riser, lower marine riser package (LMRP), lower riser package/emergency disconnect package (LRP/EDP), high pressure riser system, intervention riser, workover riser, or any other type of riser or equipment used for fluid communication between the well access system  10  and a vessel, platform, or other pipe location on or closer to the sea surface. In operation, when the open water tree cap  80  is removed to install the riser  430 , the valve  60  in internal tree cap  40  will be tested and closed. In addition, the surface controlled subsurface safety valves (SCSSV) downhole will be closed, allowing two barriers for the short period before the riser connection  460  is established on the well access system  10 . 
         [0032]      FIG. 5  illustrates one or more embodiments of the present disclosure in which well access system  110  includes a vertical tree or an adapter spool  112  directly or indirectly connected to wellhead hub  158 , which can be a high pressure wellhead housing or tubing spool or another wellhead member or tree member. 
         [0033]    In  FIG. 5 , tree or adapter spool  112  is shown as mounted on tree connector  116  and as having at least one production bore  120 , with one or more production valves  170 ,  172  therein, and at least one annulus bore extending from the production tubing/casing annulus in the well (not shown) through annulus bore  124  in tubing hanger  155  and through annulus bore  126  with one or more annulus valves  132 ,  134  in the annulus wing branch  115  of tree or adapter spool  112 . A tubing hanger  155  with a production bore is landed in the tree or adapter spool  112  and supports the production tubing  120  extending into the well. A side outlet  174  is equipped with a valve  176 . 
         [0034]    In  FIG. 5 , the top of the production bore and the top of the annulus bore are capped by an internal tree cap  140 , which seals off the various bores in the tree or adapter spool  112  and provides channels for operation of the various valves in the tree or adapter spool  112  by means of intervention equipment or remotely from an offshore installation. Internal tree cap  140  may be installed by equipment run through a riser connected to vessel, platform, or other pipe location on or closer to the sea surface, for example, or may be installed in open water by ROV. The internal tree cap  140  is sealed to the tree or adapter spool body  112  with one or more seal(s)  145 ,  146 . The internal tree cap  140  includes a central production bore capable of fluid communication with production bore  120  and an annulus bore  128  capable of fluid communication with annulus flow line  126 . 
         [0035]    In  FIG. 5 , an open water tree cap  180  is installed inside internal tree cap  140 . Open water tree cap  180  may be installed by equipment run through a riser connected to vessel, platform, or other pipe location on or closer to the sea surface, for example, or may be installed in open water by ROV. The open water tree cap  180  is sealed to the internal tree cap  140  with one or more seal(s)  185 . Open water tree cap  180 , when installed inside internal tree cap  140  and when one or more of valves  170  and  172  are in the closed position, may provide a production and/or annulus pressure-containing barrier to the environment. 
         [0036]      FIG. 6  shows an open water tree cap  680  installed around an internal tree cap  40  and sealed to an external profile of internal tree cap  40  with one or more seal(s)  685 . Although  FIG. 6  shows the configuration including a valve in the internal tree cap of  FIGS. 1 and 3 , any open water tree cap of the present disclosure may be installed inside an internal profile, or above and/or around an external profile, of an internal tree cap. 
         [0037]    The seals shown in  FIGS. 1-6  may be made of metal, elastomer, thermal plastic compounds, other sealing material, or a variation or combination thereof. 
         [0038]    The present invention can be applied with different surface intervention systems as well as subsea intervention systems including but not limited to a tensioned riser system, a compliant riser system, a spoolable compliant guide system, a subsea lubricator system, a light weight intervention system, and any other intervention system which includes a subsea intervention package connected above the subsea tree or adapter spool. 
         [0039]    Although the preceding description has been described herein with reference to particular means, materials and embodiments, it is not intended to be limited to the particulars disclosed herein; rather, it extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.

Summary:
A system and method for accessing a well is disclosed. The system includes a first cap positionable within a bore of a completion member such as a tree or a spool. The first cap has a sealable interface to accept a second cap, and the second cap when sealed to the first cap is capable of providing a barrier against a release of well fluid external to the well.