A SYSTEM FOR CO2 STORAGE

Abstract

The present invention relates to a system for CO.sub.2 storage, comprising a subsea CO.sub.2 storage unit having at least one inlet for receiving liquid CO.sub.2 and at least one outlet connected to at least one injection well in a subterranean reservoir, the subsea storage unit comprising at least one pipeline, the subsea storage unit being configured to contain and store CO.sub.2 and to transfer heat from the surrounding seawater to the contained CO.sub.2. The invention also relates to a method for CO.sub.2 storage.

Claims

1. A system for CO.sub.2 storage, comprising a subsea CO.sub.2 storage unit having at least one inlet for receiving liquid CO.sub.2 and at least one outlet connected to at least one injection well in a subterranean reservoir, the subsea storage unit comprising at least one pipeline, the subsea storage unit being configured to contain and store CO.sub.2 and to transfer heat from the surrounding seawater to the contained CO.sub.2.

2. The system according to claim 1, wherein the subsea CO.sub.2 storage unit comprises a pipe rack comprising a manifold connected to a plurality of pipeline branches.

3-4. (canceled)

5. The system according to claim 2, wherein the subsea CO.sub.2 storage unit comprises spacers along the pipe rack.

6-7. (canceled)

8. The system according to claim 1, wherein the subsea CO.sub.2 storage unit comprises a bundled pipeline comprising at least one internal pipeline within, and fluidically connected to, an external pipeline.

9. (canceled)

10. The system according to claim 1, wherein the subsea CO.sub.2 storage unit comprises a single pipeline.

11. (canceled)

12. The system according to claim 1, wherein the subsea storage unit lies at the bottom of the sea.

13. The system according to claim 10, wherein the subsea CO.sub.2 storage unit is not horizontally disposed.

14. The system according to claim 1, wherein the subsea CO.sub.2 storage unit comprises multiple inlets and/or multiple outlets.

15-16. (canceled)

17. The system according to claim 1, wherein the subsea CO.sub.2 storage unit contains CO.sub.2 both in a liquid phase and in a vapor phase.

18. (canceled)

19. The system according to claim 1, comprising at least one CO.sub.2 transfer line with one end connected to an inlet of the subsea CO.sub.2 storage unit and the other end connected to a CO.sub.2 source.

20. The system according to claim 1, comprising a liquid CO.sub.2 injection line with one end connected to a liquid CO.sub.2 outlet of the subsea CO.sub.2 storage unit and the other end connected to the at least one injection well.

21. The system according to claim 1, comprising a vapor CO.sub.2 injection line with one end connected to a vapor CO.sub.2 outlet of the subsea CO.sub.2 storage unit and the other end connected to the at least one injection well.

22. (canceled)

23. The system according to claim 1, comprising an expansion unit along the liquid CO.sub.2 injection line configured to convert liquid CO.sub.2 to vapor CO.sub.2.

24. The system according to claim 1, comprising a pipeline end module connected at an inlet to the CO.sub.2 subsea storage unit and/or at an outlet to the CO.sub.2 subsea storage unit.

25. (canceled)

26. A method for CO.sub.2 storage, the method comprising steps of: feeding liquid CO.sub.2 to at least one inlet of a subsea CO.sub.2 storage unit in a system for CO.sub.2 storage, the subsea CO.sub.2 storage unit comprising at least one pipeline; containing and storing CO.sub.2 in the subsea CO.sub.2 storage unit; transferring heat from the surrounding seawater to the contained CO.sub.2; injecting CO.sub.2 from at least one outlet of the subsea CO.sub.2 storage unit into at least one injection well of a subterranean reservoir.

27-29. (canceled)

30. The method according to claim 26, wherein a portion of the liquid CO.sub.2 fed to the subsea CO.sub.2 storage unit transitions to a vapor phase within the subsea storage unit.

31. The method according to claim 26, wherein the liquid CO.sub.2 is fed to the at least one inlet of the subsea CO.sub.2 storage unit from a CO.sub.2 source, the CO.sub.2 source being in the form of a pipeline connected to an onshore storage unit, the pressure of the liquid CO.sub.2 fed to the subsea CO.sub.2 storage unit lying from 4 MPa to 20 MPa upon entry into the subsea storage unit.

32. The method according to claim 26, wherein the liquid CO.sub.2 is fed to the at least one inlet of the subsea CO.sub.2 storage unit from a CO.sub.2 source, the CO.sub.2 source being in the form of a CO.sub.2 storage unit of a shuttle ship, the pressure of the liquid CO.sub.2 fed to the subsea CO.sub.2 storage unit upon entry into the subsea storage unit lying from 0.5 MPa to 3 MPa and the temperature of the liquid CO.sub.2 fed to the subsea CO.sub.2 storage unit lies from 20 C. to 30 C. upon entry into the subsea CO.sub.2 storage unit.

33. The method according to claim 26, wherein CO.sub.2 is injected as a stream of liquid CO.sub.2, a stream of vapor CO.sub.2 or a combination of a stream of liquid CO.sub.2 and a stream of vapor CO.sub.2, from the subsea CO.sub.2 storage unit to the at least one injection well.

34. The method according to claim 26 wherein liquid CO.sub.2 is collected from the subsea CO.sub.2 storage unit and converted to vapor CO.sub.2 before being injected into the at least one injection well.

35. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Non-limiting examples will now be described in reference to the accompanying drawings, where:

[0054] FIG. 1 shows an illustration of the system according to an embodiment.

[0055] FIG. 2 shows an illustration of the system according to an embodiment.

[0056] FIG. 3 shows an illustration of the system according to an embodiment.

[0057] FIG. 4 shows an illustration of phase transition of CO.sub.2 in the subsea CO.sub.2 storage unit according to an embodiment.

[0058] FIG. 5a shows an illustration of the system according to an embodiment, with multiple subsea CO.sub.2 storage units comprising pipe racks.

[0059] FIG. 5b shows an illustration of the system according to an embodiment, with multiple subsea CO.sub.2 storage units comprising bundled pipelines.

[0060] FIG. 5c shows an illustration of the system according to an embodiment, with multiple subsea CO.sub.2 storage units comprising single pipelines.

[0061] FIG. 6 shows a schematic illustration of a countercurrent in a subsea CO.sub.2 storage unit according to an embodiment.

[0062] FIG. 7 shows a schematic illustration of the system according to an embodiment.

DETAILED DESCRIPTION

[0063] The invention will now be described in detail without limitation in the following description.

[0064] The system of the present invention comprises a subsea CO.sub.2 storage unit comprising at least one pipeline, and having at least one inlet for receiving liquid CO.sub.2 and at least one outlet connected to at least one injection well in a subterranean reservoir. A subsea CO.sub.2 storage unit refers to a CO.sub.2 storage unit that is submerged underwater, or more specifically, that is submerged in the waters of a sea or ocean. In other words, the entire CO.sub.2 storage unit is below the water surface. A CO.sub.2 storage unit refers to a device that is used for containing and/or storing CO.sub.2 for a period of time. Such a period of time may be equal to or greater than that over which the CO.sub.2 needs to be heated to a desired temperature by the surrounding seawater. A subsea CO.sub.2 storage unit configured to contain CO.sub.2 refers to the holding of CO.sub.2 inside the subsea CO.sub.2 storage unit. A subsea CO.sub.2 storage unit configured to store CO.sub.2 refers to the holding of CO.sub.2 inside the subsea CO2 storage unit for a future use. A subsea CO.sub.2 storage unit configured to transfer heat from the surrounding seawater to the contained CO.sub.2 refers to the heat transfer from the surrounding seawater, through one or more walls of the subsea CO.sub.2 storage unit and to the contained CO.sub.2 by means of convection and conduction. The storage of the CO.sub.2 in the CO.sub.2 storage unit is transient, as the CO.sub.2 is ultimately injected into the subterranean reservoir.

[0065] A subterranean reservoir refers to a hydrocarbon reservoir within a subterranean formation. The reservoir may be positioned offshore and found at a depth below sea level that is, for example, greater than 1 km such as from 2 km to 4 km or of such order. This hydrocarbon reservoir may be partly, substantially or fully depletedi.e. the hydrocarbons in the reservoir may have been previously produced at the time the system of the invention is implemented. A reservoir is an underground portion wherein a fluid such as CO.sub.2 or hydrocarbons can be contained without substantially diffusing to neighboring portions. In this respect, the reservoir can be considered as a geological enclosure within a subterranean formation. For example, the neighboring portions may be made of rock material having a lower porosity than the rock material of the reservoir itself. In some variations, a layer of clay may be present above the reservoir. In some variations, a water-containing layer may be present below the reservoir. In some variations, the reservoir may be partly delimited by a crack creating a porosity discontinuity through which a fluid may not easily flow. The reservoir may be of an elongated shape, with for example, a height of from 20 to 300 m and/or a lateral dimension of from 2 km to 15 km, for example from 3 to 10 km.

[0066] An injection well refers to a well that is used to inject CO.sub.2 into a subterranean reservoir. This well may be a dry well, or in other words a well placed on a platform. Alternatively, this well may be a wet well, or in other words a well placed on a subsea template.

[0067] Different embodiments of the present invention are described below. Some embodiments disclose the system with the subsea CO.sub.2 storage unit comprising a pipe rack.

[0068] Other embodiments disclose the system with the subsea CO.sub.2 storage unit comprising a bundled pipeline.

[0069] Other embodiments disclose the system with the subsea CO.sub.2 storage unit comprising a single pipeline.

General Principle

[0070] FIG. 1 displays a general illustration of the system for CO.sub.2 storage 100 according to an embodiment wherein the system for CO.sub.2 storage 100 comprises a subsea CO.sub.2 storage unit 102 comprising at least one pipeline 104. The subsea CO.sub.2 storage unit 102 may be fed liquid via an inlet 106. The inlet 106 may be connected to one end of a transfer line 108 that may supply the liquid CO.sub.2 from a CO.sub.2 source 110 such as, for example, a CO.sub.2 storage unit of a shuttle ship or offshore platform to which the other end of the transfer line 108 is connected. The transfer line 108 may be connected to the storage unit of the offshore platform via a connection unit, such as, for example, a loading buoy. The transfer line 108 may for example be in the form of a flexible riser. Additionally or alternatively, the CO.sub.2 transfer line 108 may be connected to another CO.sub.2 source 110, such as, for example, an onshore storage unit, from which it may feed the liquid CO.sub.2 to the inlet 106. The transfer line 108 may be connected to the storage unit of the onshore storage unit via a connection unit, such as, for example, an onshore terminal.

[0071] As illustrated in FIG. 2, a pipeline 129 connected to the onshore storage unit may be connected directly at one end to the transfer line 108. Alternatively, the pipeline 129 connected to an onshore storage unit may be tapped at a point along its length for connection to the transfer line 108, the end of the pipeline 129 bypassing the subsea CO.sub.2 storage unit and continuing to provide CO.sub.2 to at least one injection well 126 of a subterranean reservoir 124a, 124b. According to some embodiments, liquid CO.sub.2 may be supplied from a shuttle ship via an offshore platform positioned on the at least one injection well 126, or via a dedicated riser. The shuttle ship may feed liquid CO.sub.2 via the transfer line 108 (such as for example a cryogenic flexible line) to the offshore platform or riser with the assistance of a tower loading unit. In other words, the offshore platform or the riser may be used as an intermediary point between the shuttle ship and the subsea storage unit 102. Some or all of the supplied CO.sub.2 may then be diverted through the offshore platform via the transfer line 108 to the subsea storage unit 102. After the CO.sub.2 is diverted towards the riser with the assistance of a tower loading unit, CO.sub.2 may be pumped through the transfer line 108 to the subsea storage unit 102.

[0072] Alternatively, multiple transfer lines 108 connected to the same or different CO.sub.2 sources 110 may be simultaneously connected to multiple inlets 106 of the subsea CO.sub.2 storage unit 102 to feed different streams of CO.sub.2 to the subsea CO.sub.2 storage unit 102. For example, a supply of liquid CO.sub.2 may be fed from the pipeline 129 connected to an onshore storage unit to an inlet 106 while liquid CO.sub.2 is simultaneously being fed from a shuttle ship via another transfer line 108 connected to an inlet 106.

[0073] Referring again to FIG. 1, the subsea CO.sub.2 storage unit 102 may contain and store the CO.sub.2 in the subsea CO.sub.2 storage unit 102. The subsea CO.sub.2 storage unit 102 is preferably at a fixed position relative to the bottom of the sea. The subsea CO.sub.2 storage unit 102 may be horizontally disposed. According to some embodiments, the subsea CO.sub.2 storage unit 102 may lie at the bottom of the sea. Alternatively, the subsea CO.sub.2 storage unit may have sea water surrounding the entire subsea CO.sub.2 storage unit 102, the subsea CO.sub.2 storage unit 102 floating at a certain depth from the water surface.

[0074] The transfer line 108 feeds the liquid CO.sub.2 into the subsea CO.sub.2 storage unit 102 at a temperature depending on the pressure within the CO.sub.2 source 110. By way of example, this temperature may lie between 20 C. and 30 C. upon entry into the subsea CO.sub.2 storage unit 102, preferably between 24 C. and 28 C., by way of illustration at approximately 26 C.

[0075] The surrounding seawater may be of a temperature lying from 0 C. to 30 C., or for example from 5 C. to 20 C. and may transfer heat to the liquid CO.sub.2 contained within the subsea CO.sub.2 storage unit 102 so that it reaches approximately the same temperature.

[0076] The subsea CO.sub.2 storage unit 102 comprises one or more pipelines 104. Each pipeline 104 may have an internal surface area of, for example, 5 m.sup.2 or less, preferably from 2 m.sup.2 to 5 m.sup.2 per cubic meter of internal volume. If the pipelines 104 have a cylindrical shape with a circular cross-section, the internal diameter of the pipelines 104 may for example range from 50 cm to 2 m. The internal volume of the subsea CO.sub.2 storage unit 102 available for CO.sub.2 storage may lie from 1,000 m.sup.3 to 100,000 m.sup.3, preferably from 2,500 m.sup.3 to 75,000 m.sup.3, more preferably from 5,000 m.sup.3 to 50,000 m.sup.3. The pipelines 104 may comprise or be made of steel, such as, for example, carbon steel, or alloy steel.

[0077] Preferably, the subsea CO.sub.2 storage unit 102, once in use, contains substantially only CO.sub.2 and no other substance (such as in particular seawater).

[0078] According to some embodiments, CO.sub.2 in the CO.sub.2 storage unit of a shuttle ship, and upon entry into the subsea CO.sub.2 storage unit 102, may be at a pressure lying from 0.5 MPa to 3 MPa, preferably lying from 1.5 MPa to 2.5 MPa, more preferably at approximately 2 MPa, when the transfer line 108 feeds the liquid CO2 into the subsea CO2 storage unit 102 at a temperature lying from 20 C. to 30 C. upon entry into the subsea CO2 storage unit 102, preferably lying from 24 C. to 28 C. According to other embodiments, CO.sub.2 in the pipeline 129 connected to an onshore storage unit, and upon entry into the subsea CO.sub.2 storage unit 102, may be at a pressure lying from 4 MPa to 20 MPa, preferably lying from 4 MPa to 15 MPa. In some variations, a pressure reduction valve may be provided at or upstream of the inlet of the CO.sub.2 storage unit 102. The CO.sub.2 in the subsea CO.sub.2 storage unit 102 may experience a pressure increase owing to the static pressure within the subsea CO.sub.2 storage unit 102.

[0079] As a result, the CO.sub.2 not only experiences a heat transfer conditioning step of temperature increase, but may also experience a pressure conditioning step in preparation for injection into the subterranean reservoir 124.

[0080] The subsea CO.sub.2 storage unit 102 may contain CO.sub.2 both in a liquid phase 114 and in a vapor phase 112 as a portion of the liquid CO.sub.2 may transition to a vapor phase within the subsea CO.sub.2 storage unit 102 as a result of the heat transfer and pressure conditions of the subsea CO.sub.2 storage unit 102.

[0081] The pressure in the subsea CO.sub.2 storage unit 102 (especially after a period of storage time) may lie, for example, from 3 MPa to 7 MPa, preferably from 4 MPa to 5 MPa, more preferably at approximately 4 MPa. After a period of storage time, liquid CO.sub.2 may be at equilibrium with vapor CO.sub.2 within the subsea CO.sub.2 storage unit 102; in that case, the pressure within the subsea CO.sub.2 storage unit 102 depends on the temperature in the unit, which is itself preferably approximately equal to the surrounding sea temperature.

[0082] The vapor CO.sub.2 112 may be in an upper portion of the subsea CO.sub.2 storage unit 102, (i.e. a portion which is closer to the water surface than the rest of the unit). According to some embodiments, the subsea CO.sub.2 storage may not be horizontally disposed, especially if it lies on a portion of the sea bottom which is sloped. In this instance, vapor CO.sub.2 may occupy an upper portion of the subsea CO.sub.2 storage unit 102 that is elevated relative to the rest of the subsea CO.sub.2 storage unit 102.

[0083] The subsea CO.sub.2 storage unit 102 may comprise at least one outlet. The subsea CO.sub.2 storage unit 102 may indeed comprise multiple outlets. The multiple outlets may be connected to one injection well. Additionally or alternatively, the multiple outlets may be connected to multiple injection wells. The subsea CO.sub.2 storage unit 102 may inject liquid CO.sub.2 from at least one liquid outlet 116 of the subsea CO.sub.2 storage unit 102 (preferably positioned in a lower portion of the subsea CO.sub.2 storage unit 102) into the at least one injection well 126 of the subterranean reservoir 124 via a CO.sub.2 liquid injection line 120 with one end connected to a liquid CO.sub.2 outlet 116 of the subsea CO.sub.2 storage unit 102 and the other end connected to the at least one injection well 126. A pump 103 may be positioned along the liquid injection line 120 to facilitate injection into the subterranean reservoir 124. An expansion unit (not shown) may be positioned along the liquid CO.sub.2 injection line 120 and, upon collection of liquid CO.sub.2 by the liquid CO.sub.2 injection line 120, may convert part or all of the liquid CO.sub.2 to vapor CO.sub.2 before being injected into the at least one injection well 126. To facilitate connection between the transfer line 108 and/or injection lines 120, 122, the system for CO.sub.2 storage 100 may comprise a pipeline end module (not shown) connected at an inlet to the CO.sub.2 subsea storage unit and/or at an outlet to the CO.sub.2 subsea storage unit.

[0084] The subsea CO.sub.2 storage unit 102 may inject vapor CO.sub.2 from at least one vapor outlet 118 of the subsea CO.sub.2 storage unit 102 (preferably positioned in an upper portion of the subsea CO.sub.2 storage unit 102) into at least one injection well 126 of a subterranean reservoir 124 via a CO.sub.2 vapor injection line 122 with one end connected to a vapor CO.sub.2 outlet 118 of the subsea CO.sub.2 storage unit 102 and the other end connected to the at least one injection well 126. In an advantageous embodiment, no compressor is provided on the CO.sub.2 vapor injection line 122, as the pressure within the CO.sub.2 storage unit 102 is sufficient to effect the injection of vapor CO.sub.2.

[0085] As demonstrated in FIG. 2, the CO.sub.2 liquid injection line 120 and/or the CO.sub.2 vapor injection line 122 may be entirely underwater and may be directly connected to the subsea wellhead 121 of a subterranean reservoir 124a. Additionally or alternatively, the CO.sub.2 liquid injection line 120 and/or the CO.sub.2 vapor injection line 122 may be connected to an injection well 126 via a rigid riser 128 of an offshore platform above the water surface, and injection to a subterranean reservoir 124b may be executed via the rigid riser 128.

[0086] Each CO.sub.2 injection line, and especially the CO.sub.2 liquid injection line 120 and/or the CO.sub.2 vapor injection line 122 if present, may be provided with at least one closable valve 127, in order to control whether CO.sub.2 is injected or not, and optionally in order to control the flow rate of the CO.sub.2 in the respective injection line.

[0087] According to some embodiments, steps of feeding liquid CO.sub.2 to the system for CO.sub.2 storage 100 and injecting CO.sub.2 from the subsea CO.sub.2 storage unit 102 into the subterranean reservoir 124a, 124b may be executed simultaneously. In other words, while liquid CO.sub.2 is being fed into the subsea CO.sub.2 storage unit 102, liquid CO.sub.2 and/or vapor CO.sub.2 may be also injected from one or multiple outlets 116, 118 of the subsea CO.sub.2 storage unit 102. During simultaneous feeding and injecting, average residence time of CO.sub.2 within the subsea CO.sub.2 storage unit 102 may by of example range from 3 to 12 hours.

[0088] The subsea CO.sub.2 storage unit 102 may experience pressure fluctuations over the course of CO.sub.2 feeding and CO.sub.2 injection, for example lying from 100 kPa to 200 kPa or of such order.

[0089] Alternatively, steps of feeding liquid CO.sub.2 to the system for CO.sub.2 storage 100 and injecting CO.sub.2 from the subsea CO.sub.2 storage unit 102 into the subterranean reservoir 124a, 124b may be executed consecutively. In other words, the reservoir 124a, 124b may remain filled or partly filled during a certain period of time after feeding of liquid CO.sub.2 into the subsea CO.sub.2 storage unit 102, before injection into a subterranean reservoir 124a, 124b begins. Such a duration may be a short term pause, such as for example from 1 day to 7 days, or from 24 hours to 72 hours, or from 1 hour to 24 hours. Alternatively, such a duration may last for a number of weeks, such as for example less than two weeks, or from two weeks to one month. Alternatively, such a duration may last several months, such as from 1 to 6 months, or from 6 to 12 months, or from 12 to 18 months. Such a duration may indeed last for several years, such as less than five years, or more than five years, or less than 10 years, or more than 10 years. In this case, the subsea CO.sub.2 storage unit 102 acts as a buffer between feeding and injection.

[0090] These consecutive steps may for example be advantageous to ensure that the CO.sub.2 is properly conditioned to the desired temperature in the subsea CO.sub.2 storage unit 102.

[0091] Alternatively, steps of simultaneous feeding and injection may alternate with steps of only feeding or only injection. Additionally or alternatively, steps of simultaneous feeding and injection may include pauses without feeding and injection. By way of example, CO.sub.2 injection into the subterranean reservoir 124a, 124b may be carried out substantially continuously, while CO.sub.2 feeding to the subsea CO.sub.2 storage unit 102 may be carried out intermittently. In this case, the flow rate of CO.sub.2 injection is lower than the flow rate of CO.sub.2 feeding to the subsea CO.sub.2 storage unit 102. This may be advantageous especially when CO.sub.2 is fed to the subsea CO.sub.2 storage unit 102 by shuttle ships, and thus necessarily in a discontinuous manner.

[0092] The pressure within the subsea CO.sub.2 storage unit 102 may remain constant or relatively constant throughout the course of feeding into the subsea CO.sub.2 storage unit 102 and injecting the CO.sub.2 from the subsea CO.sub.2 storage unit 102 into the subterranean reservoir 124a, 124b. The subsea CO.sub.2 storage unit 102 may lie at a depth lying from 10 m to 3000 m, preferably from 30 m to 300 m.

[0093] According to some embodiments, CO.sub.2 may be injected, as a stream of liquid CO.sub.2, a stream of vapor CO.sub.2 or a combination of a stream of liquid CO.sub.2 and a stream of vapor CO.sub.2, from the subsea CO.sub.2 storage unit 102 to the at least one injection well 126. The CO.sub.2 injected into the subterranean reservoir 124a, 124b may therefore be a single-phase or two-phase mixture.

[0094] According to some embodiments, the subsea CO.sub.2 storage unit 102 may comprise a pipe rack. Alternatively, and according to some embodiments, the subsea CO.sub.2 storage unit 102 may comprise a bundled pipeline comprising at least one internal pipeline within, and fluidically connected to, an external pipeline. Alternatively, and according to some embodiments, the subsea CO.sub.2 storage unit 102 may comprise a single pipeline.

Pipe Rack

[0095] FIG. 3 to FIG. 5a relate to examples of subsea CO.sub.2 storage units comprising a pipe rack 130. Regarding FIG. 3, the pipe rack 130 comprises a manifold 132 connected to a plurality of pipeline branches 138. The pipeline branches 138 may extend substantially parallel to each other, and may form a two-dimensional, or, as illustrated, three-dimensional array. In this example, the manifold 132 comprises (e.g. 8) prongs 134 which are horizontally spaced from each other. Each prong 134 is connected to (e.g. 8) branches 138 which are vertically spaced from each other. The plurality of branches 138 may comprise a total of from 10 to 500 branches 138, preferably from 50 to 300 branches 138. Spacing between the branches 138 may be selected to be of certain dimensions, for example, from 0.1 m to 1 m, so as to optimize heat transfer across the pipe rack.

[0096] The pipe rack 130 may optionally be provided with an external and/or internal frame to hold the branches 138 (not shown). The subsea CO.sub.2 storage unit 102 may comprise spacers (not shown) along the pipe rack 130 to allow for thermal expansion of the branches 138. The spacers may be positioned perpendicularly to the branches 138.

[0097] Each pipeline branch 138 may be closed at the end opposite the manifold 132. Alternatively, the pipe rack 130 may comprise a second manifold connected to the pipelines branches, at the extremity thereof opposite the manifold 132. In this case, the pipeline branches 138 extend between the two manifolds. The pipe rack 130 may have a length lying from 10 m to 250 m, preferably from 50 m to 200 m, a rack height lying from 1 m to 30 m, preferably from 10 m to 20 m, and/or a rack width lying from 10 m to 100 m, preferably from 30 m to 60 m. The length of the pipe rack 130 may correspond to the length of the branches 138 extending from the manifold 132 (the total pipe length within the pipe rack 130 may be much larger, since the pipe rack 130 may comprise a large number of pipeline branches, for example, the subsea CO.sub.2 storage unit may have a total pipe length lying from 10 km to 90 km, preferably from 30 km to 70 km, more preferably from 40 km to 60 km).

[0098] In view of the relatively short length of the pipe rack 130 (in comparison with the other embodiments described below), the pipe rack may often be considered as lying in a substantially horizontal manner on the sea bottom, as the sea bottom slope can be neglected.

[0099] The diameter of the pipeline branches may lie from 50 cm to 150 cm, preferably from 70 cm to 115 cm, more preferably from 75 cm to 110 cm. The pipe rack 130 may for example have a total volume lying from 1000 m.sup.3 to 10,000 m.sup.3, preferably from 2500 m.sup.3 to 75,000 m.sup.3, more preferably from 5000 m.sup.3 to 50,000 m.sup.3. The ratio of vapor CO.sub.2 to liquid CO.sub.2 in the subsea CO.sub.2 storage unit 102 may vary over time. The subsea CO.sub.2 storage unit may, for example, where steps of providing liquid CO.sub.2 to the system for CO.sub.2 storage and injecting CO.sub.2 from the subsea CO.sub.2 storage unit 102 into the subterranean reservoir 124 may be executed consecutively, have a liquid fraction lying from 00% to 100%, for example from 70% to 80%, or for example at approximately 75% by weight; at the beginning of the injection step. According to such an embodiment, during injection the liquid fraction in the subsea CO.sub.2 storage unit may decrease.

[0100] The pipe rack 130 may comprise upper branches 138 containing CO.sub.2 vapor and lower branches 138 containing liquid CO.sub.2. The boundary between the vapor CO.sub.2 and liquid CO.sub.2 may move over time as the vapor to liquid CO.sub.2 weight ratio in the pipe rack 130 changes. Vapor CO.sub.2 may be injected to the wellhead of a subterranean reservoir 124 via a vapor CO.sub.2 outlet in one or more of the upper branches 138. Likewise, liquid CO.sub.2 may be injected via a liquid CO.sub.2 outlet in one or more of the lower branches 138 to the wellhead of a subterranean reservoir 124. In FIG. 4, liquid and vapor CO.sub.2 are both injected to the wellhead via a rigid riser 128.

Bundled Pipeline

[0101] FIG. 4, FIG. 5b, FIG. 6 and FIG. 7 relate to examples of subsea storage units comprising at least one bundled pipeline 137. The bundled pipeline 137 may comprise at least one internal pipeline concentrically arranged within, and fluidically connected to, an external pipeline. This may make it possible to optimize heat transfer owing internal heat exchange of the CO.sub.2 against itself. In particular, the CO.sub.2 may flow in a countercurrent manner in the internal pipeline and external pipeline.

[0102] More than one bundled pipelines 137 may be connected together, in series or in parallel.

[0103] The subsea CO.sub.2 storage unit 102 may also comprise one or more (non-bundled) pipelines in addition to, and fluidically connected the bundled pipeline(s) 137.

[0104] As shown in FIG. 4, the subsea CO.sub.2 storage unit 102 may comprise an inlet 106 connected to a lower bundled pipeline 137, which is itself fluidically connected to a plurality of pipeline branches 138 extending substantially horizontally above the lower bundled pipeline and for example vertically spaced from each other. CO.sub.2 may flow first in the bundled pipeline 137 (for example, first the internal pipeline 139 and second in the external pipeline 136); and then in the pipeline branches 138. The pipeline branches 138 may comprise a lower portion containing liquid CO.sub.2 and an upper portion containing vapor CO.sub.2. The flow path may run from the bundled pipeline 137 to the lower portion and then to the upper portion. A liquid outlet 116 may be connected to the lower portion, and a vapor outlet 118 may be connected to the upper portion.

[0105] As CO.sub.2 travels within the subsea CO.sub.2 storage unit 102, the surrounding seawater (at a temperature of for example approximately 5 C.) heats the CO.sub.2 through the walls of the bundled pipeline 137 and pipeline branches 138. During feeding, the temperature of the CO.sub.2 within the subsea CO.sub.2 storage unit 102 may not be uniform. By way of example, the temperature of the CO.sub.2 may increase up to approximately 0 C. along the internal pipeline 139 of the bundled pipeline 137, and then up to approximately 2 to 4 C. along the external pipeline 136 of the bundled pipeline 137. The temperature of the CO.sub.2 may reach for example approximately 5 C. in one or more of the pipeline branches 138 downstream of the bundled pipeline 137. The temperature within the subsea CO.sub.2 storage unit 102 may be uniform when there is no step of feeding. Additionally (i.e. at another point in time) or alternatively, the temperature within the subsea CO.sub.2 storage unit 102 may not be uniform when there is no step of feeding, such as for example during a period immediately after which feeding has been completed.

[0106] In some variants, and as shown in FIG. 7, the external pipeline 136 of a bundled pipeline may be connected to at least two pipeline branches 138, one below the bundled pipeline 137, and the other above the bundled pipeline 137. A liquid outlet 116 may be connected to the pipeline branch 138 below the bundled pipeline 137, and a vapor outlet 118 may be connected to the pipeline branch above the bundled pipeline 137.

[0107] As shown in FIG. 6, the subsea CO.sub.2 storage unit 102 may comprise at least two bundled pipelines 137 which may be fluidically connected, and possibly disposed in parallel, such as vertically spaced. CO.sub.2 may be fed to the internal pipelines 139 of bundled pipelines 137 and the external pipelines 136 may be fluidically connected together, as shown.

[0108] According to some embodiments, the bundled pipeline(s) may have a total expanded pipe length (internal pipe and external pipe) preferably lying from 1 km to 15 km, preferably from 5 km to 10 km, more preferably from 5 km to 7 km. When one or more non-bundled pipelines are connected to the bundled pipeline(s), the total expanded pipe length of the subsea CO.sub.2 storage unit 102 may lie for example from 1 km to 50 km.

Single Pipeline

[0109] Referring to FIG. 5c, the subsea CO.sub.2 storage unit 102 may comprise a single pipeline 160. Like with the bundled pipeline, according to some embodiments, the subsea CO.sub.2 storage unit 102 may have a total length lying from 1 km to 30 km, preferably from 1 km to 25 km, more preferably from 5 km to 7 km. An outlet positioned at one end of the single pipeline 160 may inject a one-phase liquid CO.sub.2 injection only. The liquid may be expanded to a vapor CO.sub.2 upon leaving the outlet.

Multiple Pipelines

[0110] The system may comprise multiple subsea CO.sub.2 storage units 102, using any combination of said embodiments. Referring to FIG. 5a to FIG. 5c, the system for CO.sub.2 storage comprises multiple subsea storage units comprising pipe racks 130 (FIG. 5a) or multiple bundles pipelines 137 (FIG. 5b) or multiple single pipes 160 (FIG. 5c). Liquid CO.sub.2 may be delivered via a common transfer line 108 to individual transfer lines 108a, 108b, 108c dedicated to each to each of the multiple subsea storage units. Liquid CO.sub.2 may flow through an outlet of each pipe rack 130/bundled pipeline 137/single pipeline 160, each pipe rack 130/bundled pipeline 137/single pipeline 160 being connected to its own liquid CO.sub.2 injection line and/or own vapor CO.sub.2 injection line 140a, 140b, 140c. Each CO.sub.2 vapor injection line or CO.sub.2 liquid injection line 140a, 140b, 140c may be used independently. Alternatively, at least two of the injection lines may provide CO.sub.2 to one common injection line 120, before the CO.sub.2 is injected into the at least one injection well 126 of a subterranean reservoir 124.

[0111] In addition, and independently, a subsea CO.sub.2 storage unit 102 may comprise a pipe rack 130 and a bundled pipeline 137, or a pipe rack 130 and a single pipeline 160, or a bundled pipeline 137 and a single pipeline 160, which may be serially fluidically connected.

[0112] Furthermore, a pipe rack 130 may comprise one or more bundled pipelines 137 as part or all of the branches 138 described above.