Wellhead assembly

Abstract

A wellhead assembly (10) includes a casing head (16) configured to be mounted on a surface casing (12); a casing spool (18) mounted on the casing head and including a plurality of axially offset first through bores, each of which includes an annular first seat; a plurality of first casing hangers (36), each of which is supported on a corresponding first seat and configured to suspend a respective production casing through the surface casing; a tubing head (20) mounted on the casing spool and including a plurality of axially offset second through bores, each of which is aligned with a corresponding first through bore and each of which includes an annular second seat; and a plurality of tubing hangers, each of which includes an axial tubing hanger bore and each of which is supported on a corresponding second seat and is configured to suspend a respective production tubing through a corresponding production casing.

Description

[0001] WELLHEAD ASSEMBLY

[0002] The present disclosure is directed to wellhead assembly for use in drilling and completing multiple individual wells through a single casing spool, casing head and surface casing. More specifically, the disclosure is directed to a wellhead assembly in which multiple production casings are suspended from the casing spool and a separate well is completed through each production casing. BACKGROUND OF THE DISCLOSURE

[0003] Prior art oil and gas wellhead assemblies are commonly designed for drilling and completing a single well. Such wellhead assemblies usually include a casing head mounted to a surface casing, a casing spool mounted on the casing head and a tubing head mounted on the casing spool. The casing spool typically includes a single axially aligned through bore having an annular seat on which a production casing is landed. The tubing head likewise usually includes a single axially aligned through bore having a seat on which a tubing hanger is landed. Once the production casing is installed and cemented, the tubing head is mounted to the casing spool, the wellbore is drilled to depth through the tubing head and the production casing, and the well is completed by suspending the production tubing through the production casing.

[0004] In certain situations, a wellsite may be designed to have multiple wells. This typically requires that a wellhead assembly of the type described above be provided for each well. However, this arrangement requires a relatively large footprint, which is undesirable from the perspective of certain environmental protection laws. Although some prior art wellhead assemblies are designed to suspend multiple production tubings within a single production casing, these wellhead assemblies are used to access two or more production zones within a single wellbore. As a result, such wellhead assemblies are not useful for completing separate wells.

[0005] Therefore, a need exists for a wellhead assembly which can enable the drilling and completion of multiple wells through a single casing spool.

[0006] SUMMARY OF THE DISCLOSURE

[0007] In accordance with the present disclosure a wellhead assembly is provided which comprises a casing head configured to be mounted on a surface casing; a casing spool mounted on the casing head, the casing spool comprising a plurality of axially offset first through bores, each of which includes an annular first seat; a plurality of first casing hangers, each first casing hanger being supported on a corresponding first seat and being configured to suspend a respective production casing through the surface casing; a tubing head mounted on the casing spool, the tubing head comprising a plurality of axially offset second through bores, each of which is aligned with a corresponding first through bore and each of which includes an annular second seat; and a plurality of tubing hangers, each of which comprises an axial tubing hanger bore, each tubing hanger being supported on a corresponding second seat and being configured to suspend a respective production tubing through a corresponding production casing.

[0008] In accordance with one embodiment, the casing head may comprise an axial third through bore having an annular third seat on which a third casing hanger is supported. In this embodiment, the third casing hanger is configured to suspend an intermediate casing through the surface casing and the production casings are suspended through the intermediate casing.

[0009] In accordance with another embodiment, the casing spool may comprise an axially aligned fourth through bore having an annular fourth seat on which a separator hanger may be supported. In this embodiment, the separator hanger is configured to suspend a separator string between the production casing strings. In some embodiments the separator hanger may comprise an elongated tubular body having an upper end to which a load nut is connected and a lower end to which the separator string is connected. In this embodiment, the load nut is configured to engage the fourth seat to thereby suspend the separator hanger from the casing spool. In some embodiments the load nut may axially movably connected to the upper end of the body. In some embodiments the body may comprise an axial bore which extends completely therethrough and communicates with an interior of the separator string.

[0010] In certain embodiments the separator hanger may comprise a downward facing rim which is configured to land on a temporary support member during installation of the separator string. In some embodiments the separator hanger may comprise an upward facing shoulder on which the casing spool is landed during installation of the casing spool.

[0011] In accordance with yet another embodiment, the wellhead assembly may comprise a Christmas tree mounted on the tubing spool and comprising a plurality of production bores, each of which is aligned with a corresponding second through bore and each of which is connected to a corresponding tubing hanger bore. In some embodiments the Christmas tree may comprise a single valve block through which the production bores extend. In certain embodiments the Christmas tree may comprise a plurality of first valve members, each of which is mounted in the vale block in a position to control flow through a respective production bore.

[0012] In accordance with a further embodiment, the wellhead assembly may comprise a valve assembly which is removably mountable to the valve block over any one of the production bores. In this embodiment, the valve assembly may comprise a number of second valve members for controlling flow through the production bore.

[0013] The present disclosure is also directed to a method for drilling and completing a plurality of wells. In accordance with one embodiment, the method comprises drilling a wellbore to a first depth; installing a surface casing in the wellbore; mounting a casing head on the surface casing; mounting a casing spool on the casing head, the casing spool comprising a plurality of axially offset first through bores, each of which includes an annular first seat; connecting each of a plurality of production casings to a respective first casing hanger; lowering each production casing through a respective first through bore until the casing hanger lands on the first seat; mounting a tubing head on the casing spool, the tubing head comprising a plurality of axially offset second through bores, each of which is aligned with a corresponding first through bore and each of which includes an annular second seat; connecting each of a plurality of production tubings to a respective tubing hanger, each of which comprises an axial tubing hanger bore; lowering each production tubing through a respective second through bore and production casing until the tubing hanger lands on the second seat; and securing each tubing hanger to the tubing head.

[0014] In accordance with another embodiment, the method may further comprise, prior to the step of mounting the casing spool to the casing head, drilling the wellbore to a second depth; and connecting an intermediate casing to a third casing hanger and lowering the intermediate casing through the casing head until the third casing hanger lands on an annular third seat in the casing head.

[0015] In certain embodiments the casing spool may comprise an axially aligned fourth through bore having an annular fourth seat. In this embodiment, the method may further comprise, prior to the step of mounting the tubing head on the casing spool, connecting a separator string to a separator hanger and lowering the separator string through the fourth through bore until the separator hanger lands on the fourth seat.

[0016] In some embodiments the method may also comprise, prior to the step of mounting the tubing head on the casing spool but after the step of landing the separator hanger on the fourth seat, cementing the production casings by pumping cement into the wellbore through the separator hanger and the separator string.

[0017] In another aspect of the disclosure, the method may also comprise, prior to the steps of connecting each of a plurality of production tubings to a respective tubing hanger and lowering each production tubing through a respective second through bore and production casing until the tubing hanger lands on the second seat, installing a respective tubing hanger, without the production tubing connected thereto, in each second through bore except a first one of the second through bores; providing a drilling adapter comprising a single axially offset drilling bore and a number of axially offset sockets; mounting the drilling adapter to the tubing head so that the drilling bore is aligned with said first one of the second through bores and the sockets are aligned with the remaining second through bores; connecting a blowout preventer (“BOP”) to the drilling adapter over the drilling bore; and drilling a first wellbore to a third depth through the BOP, the drilling bore and said first one the second through bores.

[0018] In some embodiments the step of connecting each of a plurality of production tubings to a respective tubing hanger and lowering each production tubing through a respective second through bore and production casing until the tubing hanger lands on the second seat may comprise connecting a first one of the production tubings to a respective tubing hanger and lowering said first one of the production tubings through the BOP, the drilling bore and said first one of the second through bores until the tubing hanger lands on the second seat.

[0019] In certain embodiments the method may further comprise disconnecting the drilling adapter from the tubing head; mounting the drilling adapter to the tubing head so that the drilling bore is aligned with a second one of the second through bores and the sockets are aligned with the remaining second through bores; and drilling a second wellbore to a third depth through the BOP, the drilling bore and said second one of the second through bores. In this embodiment, the step of connecting each of a plurality of production tubings to a respective tubing hanger and lowering each production tubing through a respective second through bore and production casing until the tubing hanger lands on the second seat may comprise connecting a second one of the production tubings to a respective tubing hanger and lowering said second one of the production tubings through the BOP, the drilling bore and said second one of the second through bores until the tubing hanger lands on the second seat.

[0020] In accordance with yet another embodiment, the method may comprise, after the step of securing each tubing hanger to the tubing head, mounting a Christmas tree to the tubing head, the Christmas tree comprising a plurality of production bores, each of which is aligned with a corresponding second through bore and each of which is connected to a corresponding tubing hanger bore.

[0021] The wellhead assembly of the present disclosure thus enables the drilling and completion of a plurality of separate wells through a single casing spool, casing head and surface casing. As a result, the need for a plurality of separate wellhead assemblies, one for each well, is eliminated. For wellsites supporting multiple wells, use of the single wellhead assembly of the present disclosure will therefore result in a significant reduction in equipment expenditures, operating costs and footprint compared to multiple independent wellhead assemblies.

[0022] These and other objects and advantages of the present disclosure will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers may be used to denote similar components in the various embodiments.

[0023] BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a longitudinal cross sectional view of an illustrative embodiment of the wellhead assembly of the present disclosure;

[0025] Figure 2 is a longitudinal cross sectional view of the casing spool and casing head components of the wellhead assembly of Figure 1 shown with the casing hanger and separator hanger components installed therein;

[0026] Figure 3 is a longitudinal cross sectional view of the casing spool component of the wellhead assembly of Figure 1 shown without the casing hanger and separator hanger components installed therein; Figure 4 is a transverse cross sectional view of the tubing head component of the wellhead assembly of Figure 1 taken along line A - A;

[0027] Figure 5 is a longitudinal cross sectional view of the separator hanger component of the wellhead assembly of Figure 1 ;

[0028] Figure 6 is a longitudinal cross sectional view of the tubing head component of the wellhead assembly of Figure 1 shown with the tubing hanger components installed therein;

[0029] Figure 7 is a longitudinal cross sectional view of the tubing head component of the wellhead assembly of Figure 1 shown without the tubing hanger components installed therein;

[0030] Figure 8 is a transverse cross sectional view of the casing spool component of the wellhead assembly of Figure 1 taken along line B - B;

[0031] Figure 9 is a transverse cross sectional view of the casings and tubings associated with the wellhead assembly of Figure 1 taken along line C - C;

[0032] Figure 10 is a longitudinal cross sectional view of the tubing head and tree components of the wellhead assembly of Figure 1 ; and

[0033] Figures 11-36 are longitudinal cross sectional views of an illustrative procedure for installing the wellhead assembly of Figure 1 , with Figure 28 depicting an example of a drilling adapter which may be used during the procedure and Figure 30 depicting in enlarged detail the installation of a tubing hanger during the procedure.

[0034] DETAILED DESCRIPTION

[0035] The present disclosure is directed to a wellhead assembly for use in the completion of a plurality of wells through a single surface casing. The wellhead assembly generally includes a casing head which is configured to be mounted on the surface casing, a multi-bore casing spool which is mounted on the casing head, and a multi-bore tubing head which is mounted on the casing spool. More specifically, the casing spool includes a plurality of axially offset first through bores, each of which includes an annular first seat. Each of a plurality of first casing hangers is supported on a corresponding first seat and is configured to suspend a respective production casing through the surface casing. The tubing head includes a plurality of axially offset second through bores, each of which is aligned with a corresponding first through bore and each of which comprises an annular second seat. Each of a plurality of tubing hangers is supported on a corresponding second seat and is configured to suspend a respective production tubing through a corresponding production casing.

[0036] In certain applications the wellbore may require an intermediate casing between the surface casing and the production casings. In such applications, the casing head may comprise an axial third through bore having an annular third seat on which a third casing hanger is supported. In this embodiment the third seat is configured to suspend an intermediate casing through the surface casing, in which event the production casings would extend through the intermediate casing.

[0037] In certain embodiments the casing spool may comprise an axial fourth through bore having an annular fourth seat on which a separator hanger is supported. The separator hanger is configured to suspend a separator string through the intermediate casing to thereby maintain a desired spacing between the production casings.

[0038] In certain embodiments the wellhead assembly may also include a Christmas tree mounted on the tubing spool. The Christmas tree includes a plurality of production bores, each of which is connected to a corresponding tubing hanger bore that extends axially through a respective tubing hanger to a corresponding production tubing. In this manner, fluids may be independently extracted from or injected into each of the plurality of wells through a respective production bore.

[0039] The wellhead assembly of the present disclosure thus facilitates the drilling and completion of a plurality of separate wells through a single casing spool, casing head and surface casing. As a result, the need for a plurality of separate wellhead assemblies, one for each well, is eliminated. For wellsites supporting multiple wells, use of the single wellhead assembly of the present disclosure will therefore result in a significant reduction in equipment expenditures, operating costs and footprint compared to multiple independent wellhead assemblies.

[0040] An illustrative embodiment of the wellhead assembly of the present disclosure will be described with reference to Figures 1-11. Referring to Figure 1 , the wellhead assembly of this embodiment, which is indicated generally by reference number 10, is shown mounted to a surface casing 12 installed in a wellbore which in this example has been drilled through a conductor pipe 14.

[0041] The wellhead assembly 10 includes a casing head 16 mounted on the surface casing 12, a casing spool 18 mounted on the casing head, and a tubing head 20 mounted on the casing spool.

[0042] Referring also to Figure 2, the casing head 16 includes an axial through bore 22 having an enlarged diameter lower portion 24 within which the upper end of the surface casing 12 is received. The top of the enlarged diameter lower portion 24 defines an annular shoulder 26 which rests on the top of the surface casing 12 to thereby support the wellhead assembly 10 at the top of the wellbore. The casing head 16 may be secured to the surface casing 12 by any suitable means, such as a conventional slip connector 28, and may be sealed to the surface casing with, e.g., a number of conventional sealing members 30.

[0043] In certain applications, the length and / or quality of the wellbore may require the use of a so-called intermediate casing 32. In such applications, the casing head 16 may be provided with an annular seat 34 on which a casing hanger 36 is supported. The casing hanger 36 is configured to engage and suspend the intermediate casing 32 through the surface casing 12. In the example shown in Figure 2, the casing hanger 36 is a slip-type casing hanger which is positioned on the seat 34 prior to engaging the intermediate casing 32. However, it should be understood that any suitable casing hanger may be used instead. A conventional sealing assembly 38 may be positioned above the casing hanger 36 in order to seal off the annulus 40 between the casing head 16 and the intermediate casing 32. The annulus 40 may be accessed through a number of lateral ports 42, each of which may be closed by a removable plug 44.

[0044] The casing spool 18 is landed on the top of the casing head 16 and is secured thereto using conventional means, such as a plurality of bolts 46. The casing head 18 may also be sealed to the top of the casing head 16 using a conventional gasket 48. Referring also to Figures 3 and 4, the casing spool 18 includes a plurality of axially offset through bores 50 which extend from a top of the casing spool to a single enlarged diameter socket 52. The socket 52 is configured to receive an upper end of the intermediate casing 32 (if present), which may be sealed to the socket using one or more conventional sealing members 54.

[0045] In the illustrative embodiment of the wellhead assembly 10 shown in the drawings, the casing spool 18 has four axially offset through bores 50 (Fig. 4). However, the casing spool 18 may comprise as few as two through bores 50 or as many through bores as may practicably be accommodated given the size and cost constraints imposed on the wellhead assembly 10 by a particular application. Also, it should be noted that in the context of the present disclosure, the term “axially offset” should be taken to mean that the through bores 50 are offset from, but generally parallel to, the axial centerline of the casing spool 18.

[0046] Each through bore 50 includes an annular seat 56 on which a respective casing hanger 58 is supported. Each casing hanger 58 is configured to engage and suspend a respective production casing 60 through the surface casing 12 (or through the intermediate casing 32 if one is present). In the example shown in Figure 2, each casing hanger 58 comprises a mandrel-type casing hanger which is threaded or otherwise secured to the top of its respective production casing 60 prior to being landed on the seat 56. However, it should be understood that the casing hanger 58 may comprise any suitable casing hanger. Each casing hanger 58 may be sealed to its corresponding through bore 50 using a number of appropriate sealing members 62 in order to seal off the annulus 64 between the production casings 60 and the intermediate casing 32. The annulus 64 may be accessed through a number (e.g., two) of ports 66, each of which may be closed by, e.g., a blind flange 68 (as shown on the left-hand side of Fig. 2) or selectively opened and closed by, e.g., a suitable valve member 70 (as shown on the righthand side of Fig. 2).

[0047] In accordance with one embodiment of the present disclosure, the casing spool 18 may also include an axially aligned through bore 72 having an annular seat 74 on which a separator hanger 76 is supported. The separator hanger 76 is configured to engage and suspend a separator string 78 through the surface casing 12 (or the intermediate casing 32 if one is present). The separator string 78 maintains a desired separation between the production casings 60 by in effect defining individual spaces or “slots” through which the production casings can extend.

[0048] Referring also to Figure 5, the separator hanger 76 in accordance with one embodiment of the disclosure may comprise an elongated tubular body 80 having an upper end 82 to which a load nut 84 is connected, a lower end 86 to which the separator string 78 is connected, and an axial bore 88 which extends completely through the body 80 and communicates with the interior of the separator string. The load nut 84 is configured to be supported on the seat 74 to thereby suspend the separator hanger 76 in the through bore 72. In one embodiment, the load nut 84 may be threadedly connected to the upper end 82 to enable the load nut to be axially movable relative to the body 80 (for reasons which will be made apparent below). The separator hanger 75 may also comprise an annular upward facing shoulder 90 and a downward facing rim 92 located below the shoulder 90. The purposes of the shoulder 90 and the rim 92 will be made apparent below.

[0049] Referring to Figures 1 and 6-8, the tubing head 20 is landed on the top of the casing spool 18 and is secured thereto using conventional means, such as a plurality of bolts 94. The tubing head 20 may also be sealed to the top of the casing spool 18 by a conventional gasket 96. Similar to the casing spool 18, the tubing head 20 includes a plurality of axially offset through bores 98, each of which is aligned with a corresponding through bore 50 in the casing spool. In the present embodiment, in which the casing spool 18 includes four through bores 50, the tubing head 20 may ideally also include four through bores 98 (Fig. 8).

[0050] Each through bore 98 includes an annular seat 100 on which a corresponding tubing hanger 102 is supported. Each tubing hanger 102 is configured to engage and suspend a respective production tubing 104 through a corresponding production casing 60 (Fig. 1 ). In addition, each tubing hanger 102 includes an axial tubing hanger bore 106 which is connected to the interior of its respective production tubing 104. The tubing hangers 102 may be secured in their respective through bores 98 by means to be described below.

[0051] In the example embodiment shown in the Figures, each tubing hanger 102 comprises a mandrel-type tubing hanger which is threaded or otherwise secured to the top of its respective production tubing 104 prior to being landed on the seat 100. Each tubing hanger 102 may also be sealed to its corresponding through bore 98 using a number of appropriate sealing members 108 in order to seal off the annulus 110 between the production tubing 104 and the through bore. Each annulus 110 may be accessed through a respective port 112 which may be closed by a blind flange (not shown) or selectively opened and closed by, e.g., a suitable valve member 114. In some embodiments, an upper end of each casing hanger 58 may extend into a corresponding socket 116 in the lower end of the through bore 98 and be sealed thereto using a number of conventional sealing members 116.

[0052] As shown in Figure 6, one or more of the tubing hangers 102 may be configured to suspend an electrical submersible pump (“ESP”). In this example, the tubing hanger 102 (i.e. , the left-hand tubing hanger shown in Fig. 6) is provided with an axial feedthrough bore 120 through which power and control cables may be routed to the ESP (not shown).

[0053] A particular advantage of the wellhead assembly 10 of the present disclosure is the ability to drill and complete multiple separate wells through a reasonably sized surface casing 12. With reference to Figure 9, in one example the casing spool 18 may be configured to enable the installation of four 6 5 / 8” production casings 60 through a 20” intermediate casing 32 which in turn is installed through a 26” surface casing 12. In this example, the casing spool 18 can also accommodate a separator hanger 76 which is sufficiently sized to suspend a 2 7 / 8” separator string 78. With the addition of the tubing head 20, the wellhead assembly 10 can thus facilitate the drilling and completion of four separate wells through the surface casing 12.

[0054] Referring now to Figures 1 and 10, in some embodiments of the disclosure the wellhead assembly 10 may also include a Christmas tree 122 mounted to the top of the tubing head 20 and secured thereto with conventional means, such as a plurality of bolts 124. The Christmas tree 122 includes a plurality of production bores 126, each of which is aligned with a corresponding through bore 98 in the tubing head 20. In embodiments of the disclosure in which the tubing head 20 has four through bores 98, the Christmas tree 122 likewise may include four production bores 126. Each production bore 126 is connected to the tubing hanger bore 106 of a respective tubing hanger 102. In certain embodiments each tubing hanger 102 may comprise an axially extending neck portion 128 which is received in its corresponding production bore 126 and sealed thereto with a number of suitable sealing members 128a.

[0055] In certain embodiments the Christmas tree 122 may comprise a single valve block 130 through which the production bores 126 extend. In addition, the valve block 130 may include a plurality of lower master valves 132, each of which is positioned to control flow through a corresponding production bore 126. Prior to placing one of the wells into production, a valve assembly comprising one or more of an upper master valve 134, a tee fitting 136, a production wing valve 138, a swab valve 140, a tree adapter 142 and a tree cap 144 may be mounted to the valve block 130 over the corresponding production bore 126 for that well. The remaining production bores 126 may be terminated with a tree adapter 142 and a tree cap 144. If required, a pressure gauge 146 may be mounted in one or more of the tree caps 144. Once production is completed on that well, the valve assembly may be disconnected from the valve block 130 and re-mounted over a different production bore 126 in order to commence production through that production bore. In an alternative embodiment, separate valve assemblies may be provided for each production bore 126.

[0056] In embodiments in which an ESP (not shown) is deployed with the wellhead assembly 10, power and control cables 148 for the ESP may be routed from, e.g., a junction box 150 mounted on or near the wellhead assembly 10 and through a feedthrough bore 152 in the Christmas tree 122 which is aligned with the feedthrough bore 120 in the tubing hanger 102 from which the ESP is suspended.

[0057] One example of the procedure for drilling and completing a plurality of wells using the wellhead assembly 10 of the present disclosure will now be described with reference to Figures 11-36. To start the process, the conductor pipe 14 is installed in the ground in the usual manner, a first section of wellbore 154 is drilled through the conductor pipe to a desired depth, and the surface casing 12 is run into the wellbore and cemented (Fig. 11 ). The casing head 16 is then lowered onto and secured to the surface casing 12, after which a conventional diverter 156 is connected to the top of the casing head (Fig. 12). Following this, a test plug 158 may be installed in the casing head 16 to enable the diverter 156 to be pressure tested (Fig. 13). After the pressure test, the test plug 158 is removed and a bowl protector 160 is installed in the through bore 22 of the casing head 16 using a conventional installation tool 162 (Fig. 14). The wellbore 154 is then drilled to a desired depth, and the intermediate casing 32 is run into the wellbore and cemented in place (Fig. 15).

[0058] After cementing, the casing hanger 36 is landed on the seat 34 and engaged with the intermediate casing, the top of the intermediate casing 32 is cut off and polished, and the sealing assembly 38 is installed to seal the annulus 40 between the intermediate casing and the surface casing 12 (Fig. 16). As shown in Figure 17, the separator hanger 76 is then connected to the top of the separator string 78, a suitable installation tool 164 is connected to the top of the separator hanger, and the separator string is lowered through the wellbore until the annular rim 92 on the bottom of the separator hanger lands on a temporary support member, such as a C-plate 166, which is positioned on the top of the casing head 16. Following this, the installation tool 164 is disconnected from the separator hanger 76 and the casing spool 18 is lowered onto the separator hanger until a downward facing ledge 168 formed at the intersection of the through bores 50 and the socket 52 (see Fig. 3) lands on the upward facing shoulder 90 of the separator hanger (Fig. 18). The load nut 84 is then connected to the upper end 82 of the separator hanger 76, after which the installation tool 164 is once again connected to the separator hanger 76 and lifted to enable the C-plate 166 to be removed (Fig. 19). Following removal of the C-plate 166, the installation tool 164 is lowered until the casing spool 18 rests on the casing head 16 and the separator hanger 76 (or more precisely the load nut 84) lands on the seat 74 in the through bore 72 of the casing spool (Fig. 20).

[0059] After the casing spool 18 is installed and the separator string 78 is landed, the production casings 60 may be installed. As shown in Figure 21 , a suitable installation tool 170 is connected to the casing hanger 58 and the production casing 60 is lowered through its corresponding through bore 50 until the casing hanger lands on the seat 56. After all of the production casings 60 have been installed (Fig. 22), the casing hanger sealing members 62 may be pressure tested in a conventional manner. Once the pressure testing is completed, a blind flange 172 is bolted to the top of the casing spool 18, the installation tool 164 is once again connected to the upper end 82 of the separator hanger 76, and the production casings 60 are cemented by pumping cement through the installation tool, the separator hanger and the separator string 78 (Fig. 23).

[0060] Following the cementing procedure, the axial bore 88 through the separator hanger 76 is sealed with a plug 76a (Fig. 24), and the tubing head 20 is lowered and bolted onto the casing spool 18 (Fig. 24). The sealing members 118 between the upper end of the casing hangers 58 and the corresponding through bores 98 into which the upper ends extend may then be pressure tested in a conventional manner. Using an appropriate tubing hanger running and retrieval tool 174, tubing hangers 102 are installed in all but one of the through bores 98 of the tubing head 20 (Fig. 26). Starting with the one open through bore 98, the plurality of wells may now be drilled in succession to final depth.

[0061] In preparation for drilling, a drilling adapter 176 is bolted to the top of the tubing head 20 and a blowout preventer (“BOP”) 178 is connected to the top of the drilling adapter (Fig. 27). As shown in Figure 28, the drilling adapter 176 may comprise a plurality of studs 180 for bolting to the BOP 178, a single axially offset drilling bore 182 and a number of axially offset sockets 184. The total number of drilling bore 182 and sockets 184 ideally corresponds to the number of through bores 98 in the tubing head 20. Thus, in embodiments in which the tubing head 20 comprises four through bores 98, the drilling adapter will ideally include one drilling bore 182 and three sockets 184. In addition, the drilling bore 182 and sockets 184 are configured to be aligned with the through bores 98 when the drilling adapter 176 is connected to the tubing head 20. Thus, when the drilling bore 182 is aligned with any one of the through bores 98, the sockets 184 will be aligned with the all of the remaining through bores 98. In this manner, the drilling adapter 176 will allow each through bore 98 to be accessed in turn through the drilling bore 182 while the remaining through bores 98 are closed by the sockets 184.

[0062] Each socket 184 may include a vent port 186 which his sealed by a removable plug 188. As shown in Figure 27, each socket 184 may also be configured to receive the neck portion 128 of the tubing hangers 102 and to sealingly engage the sealing members 128a mounted on the neck portion.

[0063] Once the drilling adapter 176 is secured to the tubing head 20 with the drilling bore 182 aligned with a first through bore 98, a BOP test plug (not shown) may be installed in the through bore to enable the pressure integrity of the BOP to be tested in a conventional manner, after which the BOP test plug may be removed and a bowl protector (not shown) installed. The first wellbore may then be drilled to final depth through the BOP 178, the drilling adapter 176 and the first through bore 98. After the first wellbore is drilled to final depth, a production tubing 104 is connected to a first tubing hanger 102 and, using the tubing hanger running and retrieval tool 174, the tubing hanger is lowered and landed on the seat 100 in the first through bore 98, as shown in Figure 29.

[0064] The procedure for locking the tubing hanger 102 in position is illustrated in Figure 30, which is a split view showing on the left-hand side the tubing hanger prior to landing on the seat 100 and on the right-hand side the tubing hanger landed and locked to the tubing head 20. In one exemplary embodiment, the running and retrieval tool 174 may include an energizing mandrel 190 rotatably connected to a tubular body 192 having an upper end which is threadedly connected to a landing string 194 and a lower end which is threadedly connected to the neck portion 128 of the tubing hanger 102. The bottom of the energizing mandrel 190 engages the top of a locking mandrel 196 which is configured to engage a lock ring 198 supported on the tubing hanger 102. Once the tubing hanger 102 is landed on the seat 100, the running and retrieval tool 174 is operated to move the locking mandrel 196 downward and thereby force the lock ring 198 radially outwardly into a locking profile 200 formed in the through bore 98 to thereby secure the tubing hanger to the tubing head 20.

[0065] After the first wellbore has been drilled to depth, the tubing hanger running and retrieval tool 174 is removed and a back pressure valve 202 is installed in the tubing hanger bore 106 (Fig. 31 ). The drilling adapter 176 is then disconnected from the tubing head 20, rotated to align the drilling bore182 with a second through bore 98, and then reconnected to the tubing head (Fig. 31 ). At this point, the tubing hanger 102 which has previously been installed in the second through bore 98 is retrieved. A BOP test plug (not shown) may then be installed in the second through bore 98 for pressure testing of the BOP 178, after which a bowl protector (not shown) may be installed in preparation for drilling the second wellbore through the second through bore. Once drilling of the second wellbore is finished, a production tubing 104 is connected to the tubing hanger 102 and, using the procedure described above, the tubing hanger is lowered, landed and secured in the second through bore 98 (Fig. 33).

[0066] After the tubing hanger 102 has been landed and locked in the second through bore 98, a back pressure valve 202 is installed in the tubing hanger (Fig. 34). The drilling adapter 176 is then disconnected from the tubing head 20, rotated to align the drilling bore 182 with a third through bore 98 (if the tubing head has more than two through bores), and then reconnected to the tubing head. The process described above is then repeated until the third wellbore and any additional wellbores are completed. At this point the Christmas tree 122 may be installed on the tubing head 102 (Fig. 35). Once the integrity of the Christmas tree 122 has been verified, the back pressure valves 202, or any other valves which may have been used during testing, may be removed from the tubing hangers 102 (Fig. 36), at which point the tree may be put into production.

[0067] It should be recognized that, while the present disclosure has been presented with reference to certain embodiments, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the disclosure. For example, the various elements shown in the different embodiments may be combined in a manner not described above. Therefore, the following claims are to be construed to cover all equivalents falling within the true scope and spirit of the disclosure.

Claims

What is Claimed is:1 . A wellhead assembly comprising: a casing head configured to be mounted on a surface casing; a casing spool mounted on the casing head, the casing spool comprising a plurality of axially offset first through bores, each of which includes an annular first seat; a plurality of first casing hangers, each first casing hanger being supported on a corresponding first seat and being configured to suspend a respective production casing through the surface casing; a tubing head mounted on the casing spool, the tubing head comprising a plurality of axially offset second through bores, each of which is aligned with a corresponding first through bore and each of which includes an annular second seat; and a plurality of tubing hangers, each of which comprises an axial tubing hanger bore, each tubing hanger being supported on a corresponding second seat and being configured to suspend a respective production tubing through a corresponding production casing.

2. The wellhead assembly of claim 1 , wherein the casing head comprises an axial third through bore having an annular third seat on which a third casing hanger is supported, wherein the third casing hanger is configured to suspend an intermediate casing through the surface casing, and wherein the production casings are suspended through the intermediate casing.

3. The wellhead assembly of claim 1 , wherein the casing spool comprises an axially aligned fourth through bore having an annular fourth seat on which a separator hanger is supported, the separator hanger being configured to suspend a separator string between the production casing strings.

4. The wellhead assembly of claim 3, wherein the separator hanger comprises an elongated tubular body having an upper end to which a load nut is connected and a lower end to which the separator string is connected, wherein the load nut is configured to engage the fourth seat to thereby suspend the separator hanger from the casing spool.

5. The wellhead assembly of claim 4, wherein the load nut is axially movably connected to the upper end of the body.

6. The wellhead assembly of claim 4, wherein the body comprises anaxial bore which extends completely therethrough and communicates with an interior of the separator string.

7. The wellhead assembly of claim 4, wherein the separator hanger comprises a downward facing rim which is configured to land on a temporary support member during installation of the separator string.

8. The wellhead assembly of claim 7, wherein the separator hanger comprises an upward facing shoulder on which the casing spool is landed during installation of the casing spool.

9. The wellhead assembly of claim 1 , further comprising a Christmas tree mounted on the tubing spool, the Christmas tree comprising a plurality of production bores, each of which is aligned with a corresponding second through bore and each of which is connected to a corresponding tubing hanger bore.

10. The wellhead assembly of claim 9, wherein the Christmas tree comprises a single valve block through which the production bores extend.11 . The wellhead assembly of claim 10, wherein the Christmas tree comprises a plurality of first valve members, each of which is mounted in the vale block in a position to control flow through a respective production bore.

12. The wellhead assembly of claim 10, further comprising a valve assembly which is removably mountable to the valve block over any one of the production bores, the valve assembly comprising a number of second valve members for controlling flow through the production bore.

13. A method for drilling and completing a plurality of wells, the method comprising: drilling a wellbore to a first depth; installing a surface casing in the wellbore; mounting a casing head on the surface casing; mounting a casing spool on the casing head, the casing spool comprising a plurality of axially offset first through bores, each of which includes an annular first seat; connecting each of a plurality of production casings to a respective first casing hanger; lowering each production casing through a respective first through bore until the casing hanger lands on the first seat; mounting a tubing head on the casing spool, the tubing headcomprising a plurality of axially offset second through bores, each of which is aligned with a corresponding first through bore and each of which includes an annular second seat; connecting each of a plurality of production tubings to a respective tubing hanger, each of which comprises an axial tubing hanger bore; lowering each production tubing through a respective second through bore and production casing until the tubing hanger lands on the second seat; and securing each tubing hanger to the tubing head.

14. The method of claim 13 further comprising: prior to the step of mounting the casing spool to the casing head, drilling the wellbore to a second depth; and connecting an intermediate casing to a third casing hanger and lowering the intermediate casing through the casing head until the third casing hanger lands on an annular third seat in the casing head.

15. The method of claim 13, wherein the casing spool comprises an axially aligned fourth through bore having an annular fourth seat, and wherein the method further comprises: prior to the step of mounting the tubing head on the casing spool, connecting a separator string to a separator hanger and lowering the separator string through the fourth through bore until the separator hanger lands on the fourth seat.

16. The method of claim 15, further comprising: prior to the step of mounting the tubing head on the casing spool but after the step of landing the separator hanger on the fourth seat, cementing the production casings by pumping cement into the wellbore through the separator hanger and the separator string.

17. The method of claim 13, further comprising: prior to the steps of connecting each of a plurality of production tubings to a respective tubing hanger and lowering each production tubing through a respective second through bore and production casing until the tubing hanger lands on the second seat, installing a respective tubing hanger, without the production tubing connected thereto, in each second through bore except a first one of the second through bores;providing a drilling adapter comprising a single axially offset drilling bore and a number of axially offset sockets; mounting the drilling adapter to the tubing head so that the drilling bore is aligned with said first one of the second through bores and the sockets are aligned with the remaining second through bores; connecting a blowout preventer (“BOP”) to the drilling adapter over the drilling bore; and drilling a first wellbore to a third depth through the BOP, the drilling bore and said first one the second through bores.

18. The method of claim 17, wherein the step of connecting each of a plurality of production tubings to a respective tubing hanger and lowering each production tubing through a respective second through bore and production casing until the tubing hanger lands on the second seat comprises connecting a first one of the production tubings to a respective tubing hanger and lowering said first one of the production tubings through the BOP, the drilling bore and said first one of the second through bores until the tubing hanger lands on the second seat.

19. The method of claim 18, further comprising: disconnecting the drilling adapter from the tubing head; mounting the drilling adapter to the tubing head so that the drilling bore is aligned with a second one of the second through bores and the sockets are aligned with the remaining second through bores; and drilling a second wellbore to a third depth through the BOP, the drilling bore and said second one of the second through bores; wherein the step of connecting each of a plurality of production tubings to a respective tubing hanger and lowering each production tubing through a respective second through bore and production casing until the tubing hanger lands on the second seat comprises connecting a second one of the production tubings to a respective tubing hanger and lowering said second one of the production tubings through the BOP, the drilling bore and said second one of the second through bores until the tubing hanger lands on the second seat.

20. The method of claim 13, further comprising: after the step of securing each tubing hanger to the tubing head, mounting a Christmas tree to the tubing head, the Christmas tree comprising aplurality of production bores, each of which is aligned with a corresponding second through bore and each of which is connected to a corresponding tubing hanger bore.

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