Carbon injection Christmas tree

Abstract

A christmas tree 100 for use in injecting carbon dioxide into a wellbore includes an injection line 104, 108 which is connectable to an external injection flowline 58 and a valve member 156 which is p

Description

The present disclosure is directed to a Christmas tree for use in injecting carbon dioxide into a subterranean formation. More particularly, the disclosure is directed to a Christmas tree having an injection line connected to an external injection flowline and a valve member positioned in the injection line. The valve member is operated by pressure in the injection flowline. During the injection operation, if the pressure in the injection flowline drops below a predetermined threshold pressure, the valve member will close automatically to seal off the injection line and thereby isolate the wellbore. BACKGROUND OF THE DISCLOSURE Geological carbon sequestration involves storing carbon dioxide in subterranean geologic formations. In this process, the geologic formation is accessed through a wellbore, and carbon dioxide from various sources is injected through the wellbore and into the geologic formation for long term storage. In order to control the injection of fluid into and contain the fluid within the wellbore, the wellbore is often completed in a manner similar to an oil or gas well. Thus, the wellbore is topped with a wellhead, drilled to the depth of the geologic formation, and cased with a metal pipe. A Christmas tree is then mounted to the wellhead to control fluid flow into and out of the wellbore. The carbon dioxide is normally injected through a tubing string suspended from a tubing hanger, which, depending on the configuration of the Christmas tree, is landed in either the wellhead or the tree. Christmas trees used for carbon injection are similar in many respects to oil and gas well Christmas trees, especially subsea oil and gas well Christmas trees. Thus, carbon injection Christmas trees can include a vertically extending injection bore, which is fluidly connected through the tubing hanger to the tubing string, and a laterally extending inlet bore, which extends between the injection bore and a flowline connector. The flowline connector is connectable to an injection flowline extending from a processing facility that prepares the carbon dioxide for injection. During the injection operation, carbon dioxide from the processing facility is injected through the injection flowline, into the inlet bore, through the injection bore and the tubing string, and into the geologic formation. Due to the similarities between carbon injection Christmas trees and oil and gas well Christmas trees, the terms used to label certain features of oil and gas well Christmas trees are often used to identify similar features of carbon injection Christmas trees. Thus, the vertically extending injection bore may be referred to as the production bore, and the laterally extending inlet bore may be referred to as the production outlet. In the following, these terms and others commonly used to label certain features of oil and gas well Christmas trees will be used to identify the similar features of the carbon injection Christmas trees described herein. Similar to oil and gas well Christmas trees, carbon injection Christmas trees include a number of valve members for controlling fluid flow into and out of the wellbore. For example, a typical carbon injection Christmas tree may include a production master valve (“PMV”) in the production bore below the production outlet, a production swab valve (“PSV”) in the production bore above the production outlet, and a production wing valve (“PWV”) in the production outlet. In addition, in the event the carbon injection Christmas tree includes an annulus bore connected to the annulus surrounding the tubing string and an annulus outlet connected to the annulus bore, the Christmas tree may include an annulus master valve (“AMV”) in the annulus bore below the annulus outlet, an annulus swab valve (“ASV”) in the annulus bore above the annulus outlet, and an annulus wing valve (“AWV”) in the annulus outlet. Furthermore, if the carbon injection Christmas tree includes a crossover line extending between the production outlet and the annulus outlet, the Christmas tree may include a crossover valve (“XOV”) in the crossover line. Each of these valves is operated by a respective valve actuator. In the case of a subsea carbon injection Christmas tree, the PMV, PWV, AWV, and XOV can include remotely controlled electrically or hydraulically operated actuators, and the AMV, PSV, and ASV may comprise manual actuators configured for operation by, e.g., a remotely operated vehicle (“ROV”). In carbon injection Christmas trees, the production bore and production outlet define an injection line extending from the production flowline connector to the tubing hanger. During the injection mode of operation, control of flow through the injection line is provided by the PMV and PWV. In the event of a system failure or sudden loss of pressure in the injection flowline, the PMV and PWV must be able to close relatively quickly in order to close off the injection line and thereby isolate the wellbore from the environment. Accordingly, the PMV and PVW can be fail close valves which, in the event of a system failure, are capable of closing within a relatively short time frame (e.g., on the order of 60s-120s) without requiring an external command to do so. The fail close functionality provided by the PMV and PVW actuators can include one or more large springs designed to generate a force sufficient to rapidly close the valve upon loss of electrical or hydraulic power to the actuator. The actuators can also include a mechanism to hold the valve in the open position against this closing force. Consequently, current PMV actuators and PVW actuators are typically large, complex devices. Such actuators normally require their corresponding valves to be linearly actuated (i.e., gate valves) and sufficiently robust to handle the large closing force generated by the springs. As a result, these requirements add significantly to the cost and complexity of carbon injection Christmas trees. SUMMARY OF THE DISCLOSURE In accordance with the present disclosure, a subsea carbon injection Christmas tree is provided for injecting carbon dioxide into a wellbore through which a tubing string extends, the tubing string being suspended from a tubing hanger having a tubing hanger production bore connected to the tubing string. In some embodiments, the Christmas tree comprises a production bore connectable to the tubing hanger production bore; a production outlet connected to the production bore, the production outlet and the production bore defining an injection line through the tree to which an external injection flowline is connectable; and a first valve member positioned in the injection line, the first valve member comprising a valve closure member which is maintained in the open position by pressure in the injection flowline. The first valve member is configured such that the valve closure member automatically moves from the open position to a closed position when the pressure in the injection flowline drops below a predetermined threshold pressure. In some embodiments, the Christmas tree may further comprise a rotary actuated second valve member positioned in the injection line. In some embodiments, the second valve member may be positioned in the production bore and the Christmas tree may further comprise a rotary actuated third valve member positioned in the production outlet. In some embodiments, at least one of the second and third valve members may comprise a rotary electric actuator. The rotary electric actuator may comprise an on-board battery for powering the actuator, and the rotary electric actuator may be operated by a power and communication router (“PCR”). In some embodiments, the rotary electric actuator may be programmed to close its corresponding valve upon loss of communication with the PCR. In accordance with some embodiments of the disclosure, the first valve member may comprise a check valve. For example, the first valve member may comprise an actuator which is operated by the pressure in the injection flowline. In some embodiments, the actuator may comprise a cylinder within which a piston is slidably supported, the piston being connected to the valve closure member and being sealed to the cylinder to thereby form a pressure chamber within the cylinder; a conduit which fluidly connects the pressure chamber with the injection flowline; and a biasing member which biases the piston toward the pressure chamber. In this example, pressure in the injection flowline is communicated through the conduit to the pressure chamber to thereby maintain the valve closure member in the open position. The present disclosure is also directed to a method of operating a Christmas tree for use in injecting carbon dioxide into a wellbore, the Christmas tree comprising a production bore connectable to the wellbore and a production outlet connected to the production bore, the production outlet and the production bore defining an injection line through the Christmas tree to which an external injection flowline is connectable. In some embodiments, the method may comprise, in the event of a system failure, automatically closing a check valve in the injection line to provide a first isolation barrier against pressure in the wellbore; and then actuating a first power-operated valve in the injection line to provide a second isolation barrier against pressure in the wellbore. In some embodiments, the method may further comprise,after the check valve is closed to provide the first isolation barrier against pressure in the wellbore, actuating a second power-operated valve in the injection line to provide a third isolation barrier against pressure in the wellbore. In some embodiments, the check valve may be configured to automatically close when pressure in the injection flowline drops below a predetermined threshold pressure. Thus, it may be seen that, compared to the prior art Christmas trees discussed above, the Christmas tree of the present disclosure is designed such that the PMV and PWV are not required to be safety critical valves. Instead, the Christmas tree is provided with a rapid, automatic fail close first valve member which is capable of closing off the injection line and isolating the wellbore until the PMV and PWV can be closed. As a result, the PMV and PWV can be low-cost rotary actuated valves (e.g., ball valves) with simple and compact rotary electric actuators. 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. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of an example of a prior art subsea carbon injection Christmas tree; Figure 2 is a schematic representation of the valve control system of the Christmas tree shown in Figure 1; Figure 3 is a schematic representation of an illustrative embodiment of the subsea carbon injection Christmas tree of the present disclosure; Figure 4 is a schematic representation of an exemplary valve control system of the Christmas tree shown in Figure 3; Figure 5 is a cross sectional representation of one embodiment of a valve closure member which may be used in the Christmas tree of Figure 3; and Figure 6 is a cross sectional representation of another embodiment of a valve closure member which may be used in the Christmas tree of Figure 3. DETAILED DESCRIPTION In reference to Figures 1 and 2, an exemplary carbon injection Christmas tree is described. The Christmas tree of this example, which is indicated generally by reference number 10, is configured as a vertical-type Christmas tree, although other carbon injection Christmas trees are known which are configured as horizontal-type Christmas trees. As depicted in Figure 1, the Christmas tree 10 includes a body 12, which is shown secured by a suitable connector 14 to a wellhead 16 positioned at the upper end of a wellbore (not shown). The wellhead 16 supports a tubing hanger 18, which is connected to the top of a tubing string 20 that extends through the wellbore to the subterranean formation into which the carbon dioxide is injected. The tubing hanger 18 includes an axially extending tubing hanger production bore 22, which is connected to the interior of the tubing string 20, and an axially extending tubing hanger annulus bore 24, which is connected to the tubing annulus surrounding the tubing string. The Christmas tree 10 includes a production bore 26, which is connected to the tubing hanger production bore 22, and an annulus bore 28, which is connected to the tubing hanger annulus bore 24. The production bore 26 is connected to a production outlet 30 which in turn is connected to a production flowline connector 32, and the annulus bore 28 is connected to an annulus outlet 34 which in turn is connected to an annulus flowline connector 36. In addition, the production outlet 30 and the annulus outlet 34 are connected to each other by a crossover line 38. The Christmas tree 10 also includes a number of valve members for controlling fluid flow into and out of the wellbore. In the example shown in Figure 1, the Christmas tree 10 includes a production master valve (“PMV”) 40 located in the production bore 26 below the production outlet 30, a production swab valve (“PSV”) 42 located in the production bore above the production outlet, a production wing valve (“PWV”) 44 located in the production outlet between the crossover line 38 and the production flowline connector 32, an annulus master valve (“AMV”) 46 located in the annulus bore 28 below the annulus outlet 34, an annulus swab valve (“ASW”) 48 located in the annulus bore above the annulus outlet, an annulus wing valve (“AWV”) 50 located in the annulus outlet between the annulus bore and the crossover line, and a crossover valve (“XOV”) 52 located in the crossover line. In order to control the pressure of the fluid injected into the wellbore, the Christmas tree 10 also includes a choke valve 54 located in the production outlet between the PWV 44 and the production flowline connector 32. Also, although not part of the Christmas tree 10 per se, a surface controlled subsurface safety valve (“SCSSV”) 56 may be positioned in the tubing string 20 below the tubing hanger 18. During the injection mode of operation of the Christmas tree 10, the PMV 40 and PWV 44 are opened, and the PSV 42 and XOV 52 are closed. In this configuration, carbon dioxide from an injection flowline 58 connected to the production flowline connector 32 may be injected through the choke valve 54, the production outlet 30, the production bore 26, and the tubing string 20 and into the subterranean formation. During this operation, the AMV 46 and AWV 50 may be selectively opened to allow access to the tubing annulus through an annulus flowline 60 connected to the annulus flowline connector 36. When the PMV 40 and AMV 46 are open, the PSV 42 and ASV 48 provide a single pressure barrier between the wellbore and the environment. In order to provide a second pressure barrier between the wellbore and the environment during the injection operation, the Christmas tree 10 may include a tree cap 62 to seal the production and annulus bores 26, 28 above the PSV 42 and ASW 48, respectively. Once the injection operation is completed, the PMV 40 and PWV 44 (and also the AMV 46 and AWV 50, if previously opened) are closed to maintain pressure in the wellbore. The valve members just described may comprise manual actuators or power-operated actuators. The manual actuators may be operated by, e.g., a remotely operated vehicle (“ROV”). The power-operated actuators may comprise, e.g., electric, hydraulic or pneumatic actuators, and these actuators may be remotely-controlled. For purposes of the present disclosure, the term “manual valve” should be understood to mean a valve having a manual actuator. Similarly, the term “power-operated valve” should be understood to mean a valve having a power-operated actuator. In the tree 10 depicted in Figure 1, the PMV 40, PWV 44, AWV 50 and XOV 52 are linearly actuated valves (e.g., gate valves) which are operated by respective remotely controlled electric actuators 64, 66, 68, and 70. As illustrated in Figure 2, each actuator 64, 66, 68, 70 comprises a rotary electric actuator 64a, 66a, 68a, 70a which is connected to the valve through a rotary-to-linear converter 64b, 66b, 68b, 70b. The choke valve 54 is a rotary actuated valve operated by a remotely controlled rotary electric actuator 72. The actuator 72 may comprise the G2iTM rotary electric actuator manufactured by TechnipFMC PLC of Houston, Texas. The G2i™ actuator is a self-contained device including a rotary actuator and an on-board battery unit for powering the actuator, eliminating the need for an external power source. In addition, the G2i™ actuator is ROV installable and independently retrievable using a remotely operated vehicle (“ROV”). In the example Christmas tree 10, the PSV 42, AMV 46, and ASV 48 are rotary or linearly actuated valves having respective manual actuators 74, 76, 78 configured for operation by, e.g., an ROV. In addition, the SCSSV 56 includes a conventional remotely controlled electric actuator 80. In reference to Figure 2, the Christmas tree 10 can include a power and communication router (“PCR”) 82 for controlling the operation of the PMV 40, PWV 44, AWV 50, XOV 52, and choke valve 54. The PCR 82 is connected (e.g., wirelessly or via an umbilical) to a control module located, e.g., on a topside facility (not shown). The PCR 82 controls the supply of electrical power and / or communication signals to the PMV actuator 64, PWV actuator 66, AWV actuator 68, XOV actuator 70, and choke valve actuator 72. In the present example, the PMV actuator 64, PWV actuator 66, AWV actuator 68, and XOV actuator 70 are powered by respective batteries contained in a central battery module (“CBM”) 84 connected to the PCR 82, and the PCR controls the supply of power from each battery to its respective actuator in response to control signals received from the topside control module. One example of a prior art PCR which may be used with the Christmas tree 10 is disclosed in U.S. Patent Application Publication No. US 2020 / 0182003 A1, which is hereby incorporated herein by reference. In carbon injection Christmas trees, the production bore 26 and production outlet 30 define an injection line extending from the production flowline connector 32 to the tubing hanger production bore 22. The injection line 26 / 30 is the main conduit between the injection flowline 58 and the tubing string 20, and control of flow through the injection line is critical. During the injection mode of operation of the Christmas tree 10, control of flow through the injection line 26 / 30 is provided by the PMV 40 and PWV 44. In the event of a system failure or sudden loss of pressure in the injection flowline 58, the PMV 40 and PWV 44 must be able to close relatively quickly in order to close off the injection line 26 / 30 and thereby isolate the wellbore. Thus, in the illustrative Christmas tree 10 shown in Figure 1, the PMV 40 and PWV 44 (and also the AMV 46 and AWV 50, in this example) are considered safety critical valves. As such, they must be configured as fail close valves which, in the event of a system failure, are capable of closing within a relatively short time frame (e.g., on the order of 60s-120s) without requiring an external command to do so. The fail close functionality is provided by the PMV actuator 64 and PWV actuator 66 (and also the AWV actuator 68, in this example). Such electrically operated fail close actuators typically include one or more large springs (e.g., compression springs) designed to generate a force sufficient to rapidly close the valve upon loss of electrical power to the actuator. These actuators normally must also include a mechanism to hold the valve in the open position against this closing force. Consequently, the PMV actuator 64 and PWV actuator 66 (and also the AWV actuator 68, in this example) are usually large, complex devices. In addition, such actuators require their corresponding valves to be linearly actuated (i.e., gate valves) and sufficiently robust to handle the large closing force generated by the springs. These requirements add significantly to the cost and complexity of the Christmas tree 10. In accordance with the present disclosure, a carbon injection Christmas tree is provided that can eliminate the need for such large, complex, and expensive valves and actuators. An illustrative embodiment of such a carbon injection Christmas tree will be described with reference to Figures 3 and 4. The carbon injection Christmas tree of this embodiment is indicated generally by reference number 100. The Christmas tree 100 includes a body 102, which in this example is secured by a suitable connector 14 to a wellhead 16 positioned at the upper end of a wellbore (not shown). The wellhead 16 supports a tubing hanger 18 which is connected to the top of a tubing string 20 that extends through the wellbore to the subterranean formation into which the carbon dioxide is injected. The tubing hanger 18 includes an axially extending tubing hanger production bore 22, which is connected to the interior of the tubing string 20, and an axially extending tubing hanger annulus bore 24, which is connected to the tubing annulus surrounding the tubing string. The Christmas tree 100 includes a production bore 104 connected to the tubing hanger production bore 22 and an annulus bore 106 connected to the tubing hanger annulus bore 24. The production bore 104 is connected to a production outlet 108, which in turn is connected to a production flowline connector 110, and the annulus bore 104 is connected to an annulus outlet 112 which in turn is connected to an annulus flowline connector 114. The production bore 104, and the production outlet 108 together define an injection line extending through the Christmas tree 100 between the production flowline connector 110 and the tubing hanger production bore 22. As with the Christmas 9 tree 10, the production outlet 108 and the annulus outlet 112 may be connected to each other by a crossover line 116. Although the Christmas tree 100 is configured as a vertical type Christmas tree, it could have any practical configuration. For example, the Christmas tree 100 could be configured as a horizontal type Christmas tree. In this case, the tubing hanger 18 would be landed in the production bore 104, and the production outlet 108 would be connected to a side port in the tubing hanger 18, which in turn is connected to the tubing hanger production bore 22. Other configurations for the Christmas tree are also possible. As will be made apparent below, the configuration of the Christmas tree and its structural and functional relationships with the wellhead 16 and tubing hanger 18 are not important to achieving the advantages afforded by the present disclosure. Similar to the Christmas tree 10, the Christmas tree 100 may include a number of valve members for controlling fluid flow into and out of the wellbore. For example, the Christmas tree 100 may include a PMV 118 located in the production bore 104 below the production outlet 108, a PSV 120 located in the production bore above the production outlet, a PWV 122 located in the production outlet between the crossover line 116 and the production flowline connector 110, an AMV 124 located in the annulus bore 106 below the annulus outlet 112, an ASW126 located in the annulus bore above the annulus outlet, an AWV 128 located in the annulus outlet between the annulus bore and the crossover line, and an XOV 130 located in the crossover line. In some embodiments, the Christmas tree 100 may also include a choke valve 132 located in the production outlet 108 between the PWV 122 and the production flowline connector 110, and a tree cap 134 to seal the production and annulus bores 104, 106 above the PSV 120 and ASW 126 during the injection mode of operation of the tree. Also, although not part of the Christmas tree 100 per se, an SCSSV 136 may be positioned in the tubing string 20 below the tubing hanger 18. Each of the valves includes a respective actuator. For example, the PMV 118, PWV 122, AWV 128, XOV 130, and choke valve 132 may include respective remotely controlled electric actuators 138, 140, 142, 144, 146. Also, the PSV 120, AMV 124, and ASV 126 may include respective manual actuators 148, 150, and 152 configured for operation by, e.g., an ROV. Finally, the SCSSV 136 may comprise a remotely controlled electric actuator 154. Further details of the PMV 10 actuator 138, PVW actuator 140, A VW actuator 142, XOV actuator 144, and choke valve actuator 146 will be provided below. As discussed above, during the injection mode of operation, the ability to control flow through the injection line is critical. In the event of a system failure or a sudden loss of pressure in the injection flowline 58, the Christmas tree 100 must be able to close off the injection line relatively quickly in order to isolate the wellbore from the environment. In the Christmas tree 10, this ability is provided by configuring the PMV 40 and PVW 44 as safety critical valves, that is, fail close valves capable of closing relatively quickly without an external command to do so. However, this requires that the valves 40, 44 be linearly actuated and robust and that their respective actuators 64, 66 be large and complex. In accordance with the present disclosure, the Christmas tree 100 is designed such that the PMV 118 and PVW 122 are not required to be safety critical valves. Instead, the Christmas tree 100 is provided with a rapid, automatic fail close valve member which is capable of closing off the injection line and isolating the wellbore until the PMV 118 and PVW 122 can be closed. As a result, the PMV 118 and PVW 122 can be low-cost rotary actuated valves (e.g., ball valves) with simple and compact rotary electric actuators. As depicted in Figure 3, an automatic rapid close valve member 156 is shown positioned in the production outlet 108 between the choke valve 132 and the production flowline connector 110. It should be noted, however, that the valve member 156 can be positioned anywhere in the injection line or the Christmas tree 100. A feature of the valve member 156 is that it is operated by pressure in the injection flowline 58. That is, it is maintained in the open position by pressure in the injection flowline but will automatically close once the pressure in the injection flowline drops below a predetermined threshold value. In some embodiments, the valve member 156 is configured as a check valve. An example of a check valve suitable for use in the Christmas tree 100 is shown in Figure 5. The check valve of this embodiment, which is indicated generally by reference number 156a, includes a body 158 having a flowbore 160 within which a valve closure member 162 is movably supported. In some embodiments, the body 158 may comprise part of the body 102 of the tree 100, and the flowbore 160 may form part of or be continuous with the injection line (i.e., the production outlet 108 and production bore 104) in the tree. Although the check valve 156a is depicted in Figure 5 as being located in the production outlet 108 proximate to the production flowline connector 110, it can be located anywhere in the injection line between the injection flowline 58 and the tubing hanger production bore 22. In the embodiment shown in Figure 5, the valve closure member 162 is slidably supported on an axially extending shaft 164 having a base 166 which is connected to the body 158 by a number of radially extending, circumferentially spaced struts 168. The valve closure member 162 includes an annular sealing surface 170 configured to sealingly engage a corresponding seat 172 formed on the body 158 concentrically with the flowbore 160. A biasing member (e.g., a compression spring) 174 is operatively engaged between the valve closure member 162 and the base 166 and functions to bias the valve closure member towards the seat 170. The biasing member 174 may be located at any suitable location, such as in a recess 176 formed at an end of the valve closure member 162 opposite the sealing surface 170. In operation, a predetermined threshold pressure in the injection flowline 58 will force the valve closure member 162 into the open position against the force of the biasing member 174. In this position, which is shown in Figure 5, carbon dioxide will flow from the injection flowline 58 through the flowbore 160 and into the injection line in the Christmas tree 100. Thus, provided the pressure in the injection flowline 58 is greater than the predetermined threshold pressure, the pressure in the injection flowline will maintain the valve closure member 162 in the open position. However, if the pressure in the injection flowline 58 drops below the predetermined threshold pressure, the biasing member 174 will force the valve closure member 162 into the closed position against the seat 172. In this position, the check valve 156a will close off the injection line and thereby isolate the wellbore. In this particular example, the predetermined threshold pressure is the pressure that generates a resultant opening force (i.e., leftward force, as viewed in Figure 5) on the valve closure member 162 that is greater than the force generated by the biasing member 174. This pressure can be selected by designing the valve closure member 162 and the biasing member 174 appropriately. The check valve 156a of this embodiment is a fast-acting and reliable closure member. Once the pressure in the injection flowline 58 drops below the predetermined threshold pressure, the biasing member 174 will instantaneously close the check valve to seal off the injection line. In addition, since the check valve 156a comprises only one moving part (i.e., the valve closure member 162), the probability of failure is low. Another example of an automatic rapid close valve member 156 which is suitable for use in the Christmas tree 100 of the present disclosure is shown in Figure 6. The valve member of this embodiment, which is indicated generally by reference number 156b, is designed as a linearly actuated valve having an actuator 178 which is operated by pressure in the injection flowline 58. As shown in Figure 6, the valve member 156b may be configured as a conventional gate valve. In this example, the valve member 156b comprises a body 180 having a flowbore 182 across which a valve closure member 184 in the form of a gate is positioned. The gate 184 is slidably supported between a pair of ring-shaped seats 186 mounted concentrically in the flow bore 182 and includes a through bore 188 which, when the gate is in the open position (shown in Figure 6) is aligned with the flowbore. As with the valve closure member 156a, the body 180 may comprise part of the body 102 of the tree 100, and the flowbore 182 may form part of or be continuous with the injection line. Also, although the gate valve 156b is depicted in Figure 6 as being located in the production outlet 108 next to the production flowline connector 110, it could be located anywhere in the injection line between the injection flowline 58 and the tubing hanger production bore 22. In the embodiment of the automatic rapid close valve member 156b shown in Figure 6, the actuator 178 includes a cylinder 190 within which a piston 192 is slidably supported. The piston 192 is connected to a valve stem 194 which in turn is connected to the gate 184. The piston 192 is sealed to the cylinder 190 to thereby form a pressure chamber 196 within the cylinder which is connected to the injection flowline 58 via a conduit 198. The piston 192 is biased toward the pressure chamber 196 by a biasing member 200 (e.g., a compression spring), which may be positioned, e.g., on a side of the piston opposite the pressure chamber. In operation, pressure in the injection flowline 58 is communicated to the pressure chamber 196 through the conduit 198. As a result, a predetermined threshold pressure in the injection flowline 58 will force the piston 192 downward (as viewed in Figure 6) against the force of the biasing member 200 and thereby move the gate 184 into the open position. In this position, which is shown in Figure 6, carbon dioxide will flow from the injection flowline 58 through the flowbore 182 and the through bore 188 and into the injection line in the Christmas tree 100. Thus, provided the pressure in the injection flowline 58 is greater than the predetermined threshold pressure, the pressure in the injection flowline will maintain the gate 184 in the open position. However, if the pressure in the injection flowline 58 drops below the predetermined threshold pressure, the biasing member 200 will force the piston 192 upward (as viewed in Figure 6) and thereby move the gate 184 into the closed position. In this position, the valve member 156b will close the injection line and thereby isolate the wellbore. In this example, the predetermined threshold pressure is the pressure which generates a force on the piston 192 which is greater than the force generated by the biasing member 200. This pressure can be selected by designing the piston 192 and the biasing member 200 appropriately. Similar to the valve member 156a, the gate valve 156b is a fast acting and reliable closure member. Once the pressure in the injection flowline 58 drops below the predetermined threshold pressure, the biasing member 200 will instantaneously close the gate valve to seal off the injection line. In addition, since the gate valve 156b comprises a minimum of moving parts, the probability of failure is low. During the injection mode of operation of the Christmas tree 100, the PMV 118 and PWV 122 are opened, and the PSV 120 and XOV 130 are closed. In this configuration, carbon dioxide from the injection flowline 58 is injected through the valve member 156, the injection line (i.e., the production outlet 106 and the production bore 104), the tubing hanger production bore 22, and the tubing string 20 and into the subterranean formation. During this operation, the pressure of the fluid in the injection flowline 58 will maintain the valve member 156 in the open position. Should pressure in the injection flowline 58 drop below the threshold pressure for any reason, the valve member 156 will automatically close to seal off the injection line. Upon completion of the injection operation, the PMV 118 and PWV 122 (and also the AMV 124 and AWV 128, if previously opened) may be closed to maintain pressure in the wellbore. In the event of a system failure leading to a loss of pressure in the injection flowline 58, the valve member 156 will act automatically to rapidly close the injection line and thereby isolate the wellbore. This initial, rapid isolation of the wellbore enables the Christmas tree 100 to be greatly simplified. For example, with the wellbore isolated by the valve member 156 (and the SCSSV 136, if present), the need for the PMV 118 and PWV 122 to be safety-critical valves is eliminated. As a result, the PMV and PWV actuators 138, 140 are not required to fail close in a relatively short time frame (i.e., on the order of 60s-120s). Instead, these actuators can be smaller, simpler, and less powerful than conventional fail close actuators. This also enables the PMV 118 and PWV 122 to be simpler and less robust than typical fail close valves. In accordance with the present disclosure, the PMV and PWV actuators 138, 140 may comprise relatively simple, relatively low-cost rotary electric actuators. In some embodiments, the rotary electric actuators may comprise G2i™ rotary electric actuators of the type described above. These actuators enable the PMV 118 and PWV 122 to be low cost rotary actuated valves, such as ball valves or rotary operated gate valves. The AWV 128 and XOV 130 may likewise comprise similar relatively low cost rotary actuated valves operated by rotary electric actuators, such as the G2iTM rotary electric actuator. Similar to the choke valve 54 described above, the choke valve 132 may comprise a rotary actuated valve operated by, eg., a G2i™ actuator 146. Referring again to Figure 4, the Christmas tree 100 may, in some embodiments, include a PCR 202 (which may be similar, e.g., to the PCR 82 described above) for controlling the operation of the PMV 118, PWV 122, AWV 128, XOV 130 and choke valve 132. As discussed above, the PCR 202 is connected (e.g., wirelessly or via an umbilical) to a control module located, e.g., on a topside facility (not shown) and controls the supply of power and / or communication signals to the PMV actuator 138, PWV actuator 140, AWV actuator 142, XOV actuator 144 and choke valve actuator 146. In an embodiment of the Christmas tree 100 in which the actuators 138, 140, 142, 144, and 146 include rotary electric actuators, such as the G2i™ rotary electric actuator (or any rotary electric actuator comprising an on-board battery for powering the actuator), a CBM is not required. Instead, the on-board battery 15 will provide all the power required to operate the actuator. Thus, the use of the G2iTM actuator (or any rotary electric actuator comprising an on-board battery) greatly simplifies the valve control system of the Christmas tree 100. The G2iTM rotary electric actuator is a low speed, high torque actuator. Thus, once the actuator receives a signal from the PCR 202 to close, the actuator may take several minutes to fully close its associated valve. An advantage of the G2i™ actuator is that it may be programmed to fail close. Thus, in the event of a system failure in which communication between the PCR 202 and the G2i™ actuators is interrupted, the G2i™ actuators will nevertheless automatically close their respective valves. Combined with the automatic rapid close valve member 156, the use of such fail close rotary electric actuators for at least the PMV and PWV actuators138, 140 enables the Christmas tree 100 to provide the wellbore isolation functionality normally required for carbon injection Christmas trees. In the event of a system failure resulting in a loss of pressure in the injection flowline 58, the valve member 156 will quickly close off the injection line to provide initial isolation of the wellbore. The PMV and PWV actuators 138, 140 will simultaneously initiate the closing of the PMV 118 and PWV 122. Although rotary electric actuators such as the G2i actuator are typically much slower than conventional fail close actuators, the initial isolation provided by the valve member 156 will provide the necessary wellbore isolation until the PMV 118 and PWV 122 are fully closed. 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 illustrated above. Therefore, the following claims are to be construed to cover all equivalents falling within the true scope and spirit of the disclosure. The following embodiments represent exemplary embodiments of concepts contemplated herein. Any one of the following embodiments may be combined in a multiple dependent manner to depend from one or more clauses. Further, any combination of dependent embodiments (e.g., clauses that explicitly depend from a previous clause) may be combined while staying within the scope 16 of aspects contemplated herein. The following clauses are exemplary in nature and are not limiting. Clause 1. A Christmas tree for use in injecting carbon dioxide into a wellbore through which a tubing string extends, the tubing string is suspended from a tubing hanger having a tubing hanger production bore connected to the tubing string, the Christmas tree comprising a production bore connectable to the tubing hanger production bore, a production outlet connected to the production bore, the production outlet and the production bore defining an injection line through the tree to which an external injection flowline is connectable, and a first valve member positioned in the injection line, the first valve member comprising a valve closure member which is maintained in the open position by pressure in the injection flowline, wherein the first valve member is configured such that the valve closure member automatically moves from the open position to a closed position when the pressure in the injection flowline drops below a predetermined threshold pressure. Thus, the illustrative embodiment provides technological improvements over conventional techniques by implementing a pressure-operated valve that performs fail close functionalities when pressure is reduced within the injection flowline. Clause 2. The Christmas tree of clause 1, further comprising a rotary actuated second valve member positioned in the injection line. Clause 3. The Christmas tree of clause 1 or 2, wherein the second valve member is positioned in the production bore and the Christmas tree further comprises a rotary actuated third valve member positioned in the production outlet. Clause 4. The Christmas tree of clause 1,2 or 3, wherein at least one of the second and third valve members comprises a rotary electric actuator. Clause 5. The Christmas tree of clause 1,2,3, or 4, wherein the rotary electric actuator comprises an on-board battery for powering the actuator. Clause 6. The Christmas tree of clause 1,2,3, 4, or 5, wherein the rotary electric actuator is operated by a power and communication router (“PCR”). Clause 7. The Christmas tree of clause 1,2,3, 4, 5, or 6, wherein the rotary electric actuator is programmed to close its corresponding valve upon loss of communication with the PCR. Clause 8. The Christmas tree of clause 1,2,3, 4, 5, 6, or 7, wherein the first valve member comprises a check valve. Clause 9. The Christmas tree of clause 1,2,3, 4, 5, 6, 7, or 8, wherein the first valve member comprises an actuator which is operated by the pressure in the injection flowline. 5 Clause 10. The Christmas tree of claim 1,2, 3, 4, 5, 6, 7, 8, or 9, wherein the actuator comprises: a cylinder within which a piston is slidably supported, the piston being connected to the valve closure member and being sealed to the cylinder to thereby form a pressure chamber within the cylinder; a conduit which fluidly connects the pressure chamber with the injection flowline; and a biasing 10 member which biases the piston toward the pressure chamber; wherein pressure in the injection flowline is communicated through the conduit to the pressure chamber to thereby maintain the valve closure member in the open position.

Claims

24What is Claimed is:

1. A Christmas tree for use in injecting carbon dioxide into a wellbore through which a tubing string extends, the tubing string being suspended from a tubing hanger having a tubing hanger production bore connected to the tubing5 string, the Christmas tree comprising:a production bore connectable to the tubing hanger production bore;a production outlet connected to the production bore, the production outlet and the production bore defining an injection line through the tree to which10 an external injection flowline is connectable; anda first valve member positioned in the injection line, the first valve member comprising a valve closure member which is maintained in the open position by pressure in the injection flowline;wherein the first valve member is configured such that the valve15 closure member automatically moves from the open position to a closed position when the pressure in the injection flowline drops below a predetermined threshold pressure.

2. The Christmas tree of claim 1, further comprising a rotary actuated second valve member positioned in the injection line.20 3. The Christmas tree of claim 2, wherein the second valve member ispositioned in the production bore and the Christmas tree further comprises a rotary actuated third valve member positioned in the production outlet.

4. The Christmas tree of claim 3, wherein at least one of the second and third valve members comprises a rotary electric actuator.25 5. The Christmas tree of claim 4, wherein the rotary electric actuatorcomprises an on-board battery for powering the actuator.

6. The Christmas tree of claim 5, wherein the rotary electric actuator is operated by a power and communication router (“PCR”).

7. The Christmas tree of claim 6, wherein the rotary electric actuator is 30 programmed to close its corresponding valve upon loss of communication with the PCR.

8. The Christmas tree of any preceding claim, wherein the first valve member comprises a check valve.

9. The Christmas tree of any preceding claim, wherein the first valvemember comprises an actuator which is operated by the pressure in the injection flowline.

10. The Christmas tree of claim 9, wherein the actuator comprises: a cylinder within which a piston is slidably supported, the piston being connected to the valve closure member and being sealed to the cylinder to thereby form a pressure chamber within the cylinder;a conduit which fluidly connects the pressure chamber with the injection flowline; anda biasing member which biases the piston toward the pressure chamber;wherein pressure in the injection flowline is communicated through the conduit to the pressure chamber to thereby maintain the valve closure member in the open position.

11. A method of operating a Christmas tree for use in injecting carbon dioxide into a wellbore, the Christmas tree comprising a production bore connectable to the wellbore and a production outlet connected to the production bore, the production outlet and the production bore defining an injection line through the Christmas tree to which an external injection flowline is connectable, the method comprising:in the event pressure in the injection flowline drops below a predetermined threshold pressure, automatically closing a check valve in the injection line to provide a first isolation barrier against pressure in the wellbore; and thenactuating a first power-operated valve in the injection line to provide a second isolation barrier against pressure in the wellbore.

12. The method of claim 11, further comprising:after the check valve is closed to provide the first isolation barrier against pressure in the wellbore, actuating a second power-operated valve in the injection line to provide a third isolation barrier against pressure in the wellbore.

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