Abstract
A hydrocarbon-forming gas compression method comprising: a hydrate formation step in which water and hydrate-forming gas are mixed at a first pressure and a first temperature, resulting in the formation of hydrate; a decomposition step in which the hydrate is warmed, and the hydrate is decomposed to re-generate hydrate-forming gas at a second pressure higher than the first pressure.
Claims
1. A hydrocarbon-forming gas compression method comprising: a hydrate formation step in which water and hydrate-forming gas are mixed at a first pressure and a first temperature, resulting in the formation of hydrate; a decomposition step in which the hydrate is warmed, and the hydrate is decomposed to re-generate hydrate-forming gas at a second pressure higher than the first pressure.
2. The method according to claim 1 in which, in the hydrate formation step, water obtained from a natural water supply, is used to cool the mixture and to provide said water for the hydrate formation step.
3. The method according to claim 1, in which the hydrate-forming gas is natural gas exiting from a reservoir.
4. The method according to claim 3 in which during or following the hydrate formation step, water containing salt impurities is removed from the hydrate.
5. The method according to claim 1 in which the hydrate-forming gas comprises at least 50% of at least one of nitrogen, argon or carbon dioxide.
6. The method according to claim 1 in which the hydrate formation step the mixture is cooled using water from a first water supply, and during the decomposition step the mixture is warmed using water from a second water supply, the second water supply having a higher temperature than the first water supply.
7. The method according to claim 6, wherein the water from the second water supply is heating using excess heat of a hydrocarbon production facility.
8. The method according to claim 1 in which the mixture is located within at least two chambers, the hydrate formation step being performed in a first one of the two chambers at a time when the decomposition step is being performed in a second of the chambers.
9. The method according to claim 1 comprising storing the hydrate-forming gas resulting from the decomposition step.
10. The method according to claim 1 in which the water of the mixture contains an anti-agglomeration (AA) reagent.
11. A hydrocarbon-forming gas compression system comprising: a mixing device comprising an inlet for hydrocarbon-forming gas, an inlet for water, an outlet for water and an outlet for hydrates; a tank comprising an inlet for hydrates communicatively coupled to said outlet for hydrates from the mixing device, an outlet for compressed gas, an outlet for water; and a heat exchanging system for heating the tank.
12. The system according to claim 11, wherein the heat exchanging system comprises a circuit, the circuit comprising a water transport pipe to a heat source and a return pipe for transporting water or steam with a higher temperature than the water in the water transport pipe to the tank.
13. The system according to claim 11, wherein the tank and the mixing device are attached to a leg of a production platform and wherein the inlet for water is communicatively coupled to the surrounding seawater.
14. The system according to claim 11 further including a discharge mechanism for discharging fluid in the tank.
15. The system according to claim 11, wherein the mixing device comprises a screw expander.
16. The system according to claim 11, wherein the tank comprises a plurality of pressure control valves.
17. The system according to claim 11, wherein the tank comprises a sluice valve.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] Embodiments of the invention will now be described for the sake of example only with reference to the following figures, in which:
[0027] FIG. 1 is a known pressure-temperature phase diagram indicating the range of temperature and pressure at which stable hydrates are formed;
[0028] FIG. 2 is a known diagram illustration the energy content of a mixture of water and a hydrate-forming gas during a reversible phase transition;
[0029] FIG. 3 illustrates schematically and in cross-section a system which can be used in an embodiment of the invention;
[0030] FIG. 4 illustrates schematically a first embodiment of a mixing device as shown in FIG. 3;
[0031] FIG. 5 illustrates schematically an implementation of the systems of FIGS. 3 and 4;
[0032] FIG. 6 illustrates schematically a further embodiment of the invention;
[0033] FIG. 7 illustrates schematically an embodiment of a tank;
[0034] FIG. 8 illustrates schematically another embodiment of a tank; and
[0035] FIG. 9 is a flow diagram.
DETAILED DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a phase diagram illustrating phase transitions which occur in a mixture of water with a hydrate-forming gas. The area of the diagram above the graph corresponds to stable hydrates, while the area below the graph corresponds to separate gas and water phases. The specific hydrate-forming gas which was used to generate FIG. 1 has the composition: [0037] N.sub.2: 2% [0038] CO.sub.2: 2% [0039] Methane (C1) 63.6% [0040] Ethane (C2): 10.9% [0041] Proplene (C3): 9.8% [0042] i-Butane (i-C4): 1.3% [0043] n-Butane (n-C4): 3.9% [0044] i-Pentane (i-C5): 1.1% [0045] n-Pentane (n-C5): 1.7%
which is a typical natural gas composition. Although the exact form of FIG. 1 varies depending on the gas composition, the general shape of the graph remains the same. In particular it will be observed that for a relatively small temperature range (10° C. to 25° C.) the pressure below which hydrates are stable increases remarkably, from under 20 bars to over 200 bars.
[0046] Dotted line 101 indicates a 50 bara pressure as an exemplary pressure of hydrocarbons when emerging from a well. Arrow 102 indicates a possible temperature of 5° C. near the seabed or deep below the water surface. Arrow 103 shows the path of heating up the stable hydrates at a 50 bara pressure from a temperature of 5° C. to 18° C., and at those conditions (50 bara and 18° C.) a phase transition to gas and water will occur. If the hydrates are placed in a confined space, the pressure will rise when more hydrates are melting, and the state moves along the curve upwards until all hydrates are melted, at an exemplary pressure of around 1000 bara.
[0047] A 50 bara pressure is mentioned as an exemplary pressure of hydrocarbons emerging from a well. In a practical implementation of the concept disclosed herein, a conventional choke may be omitted in order to make use of the well pressure. A choke is a conventional valve used to regulate or reduce pressure of hydrocarbons emerging from a well.
[0048] FIG. 2 shows schematically the four transitions which occur during a reversible process of hydrate formation and decomposition employed in the embodiments of the invention described below. A first horizontal axis of the diagram represents temperature. The vertical axis represents the energy which is contained in a mixture of water and hydrate-forming gas during the process. The second horizontal axis illustrates schematically the state of the mixture, i.e. the phase change between solid state (i.e. hydrates have been formed) and melted state (i.e. the hydrates have decomposed).
[0049] Consider for example, the state marked A as a starting state. In this state, the water and hydrate-forming gas are present together in a chamber (typically with the gas in a layer above the water), and the temperature and pressure of the system are slightly below a phase transition temperature. Accordingly the state A is unstable, and a transition occurs (a process marked as 1), in which hydrate crystals are formed. Significant energy is expelled in this process, and this energy must be removed from the system for process 1 to be completed, resulting in hydrate crystals in state B. In process 2, the hydrate crystals are very slightly heated to a temperature above the phase transition temperature (a process marked as 2), where again the hydrates (now in state C) are unstable. In process 3, the hydrate crystals melt, regenerating the hydrate-forming gas and separately the water. Significant energy must be input to the system during process 3, resulting in decomposed gas and water (state D). Finally, slight cooling of the system returns the mixture to state A (process 4).
[0050] Note that the small amounts of energy respectively absorbed and released in processes 2 and 4 cancel each other, as do the much larger amounts of energy respectively released and absorbed in processes 1 and 3. Processes 1 and 3 typically require a heat pump, and/or external warm and cool media which the gas-water mixture can exchange heat with. The heat exchange with an external supply of energy amounts to energy consumed to the system to achieve the gas compression.
[0051] Referring to FIG. 3 a system is illustrated schematically which can be used for compressing gas based on a hydrate cycle under the influence of temperature differences occurring in existing offshore production platforms. Starting from the right-hand side of the schematic drawing, seawater 301 and hydrate-forming gas 302 are mixed together in a mixing device 303. The seawater is taken in from the surrounding sea (which may also be an ocean, lake or other volume of water) and the gas 302 may be taken from a hydrocarbon producing well. Only part of the seawater 301 forms hydrates and the remaining part of the seawater 304 is released again into the surrounding sea. The temperature of the released seawater is higher than the seawater taken into the mixing device because the hydrate formation process releases energy, as described in connection with FIG. 2 when moving from state A to state B. The salt content of the released seawater is also higher because the hydrate formation process uses only water molecules. Hydrates 305 are produced and transported to compressing device 307. The mixing device 303 will be described in more detail below with reference to FIG. 4.
[0052] The produced hydrates 305 are transported to tank 306, which will also be described in more detail below. The step of transporting hydrates provides a technical advantage over transporting gas, which would need to be compressed at this stage. The distance between elements 303 and 306 may be short, for example 1m, or may be long, as long as 100 km. At tank 306, the hydrates are heated to regenerate the hydrate-forming gas and separately the water, described as process 3 in connection with FIG. 2 in which state C transforms to state D. Water 307 and high pressure gas 308 are extracted. The water 307 can be released into the sea, while the pressurised gas can, for example, be stored in containers to be transported to consumers.
[0053] The inventors have realised that excess energy of the existing hydrocarbon producing facility can be used for the energy required to cause the phase transition from hydrates to gas. Temperature differences exist within the sea between the temperature at the seabed and the temperature below the waves.
[0054] Well fluids have typically also a higher temperature than seawater, which provides another temperature differential which could be used to cause a phase transition. However, there are also other opportunities to re-use excess energy at a facility such as a production platform which includes a variety of heavy machinery. One specific example of excess heat is a chimney for releasing gases from a burning process. FIG. 3 illustrates chimney 311 and a circuit including incoming cold water through a line 309 and a return line 310 with outgoing hot water or steam. The circuit may be closed or open. The water or other fluid in the circuit is used for transporting heat to tank 306.
[0055] A realistic numerical example of a process such as illustrated in FIG. 3 is as follows: 43 m.sup.3/min of seawater 301 enters mixing device 303 together with 1000 m.sup.3/min of gas 302. At 304, 38.9 m.sup.3/min of sea water is released again, also releasing 39.7 MW of energy. At 305, 5.9 m.sup.3/min of hydrates are transported to tank 306. Around 500 kg/min of water is pumped around the circuit 309 and 310 to transport 40.1 MW of energy from the chimney towards tank 306. The amount of released water at 307 is 4.7 m.sup.3/min and 1000 m.sup.3/min of compressed gas, at a pressure of 1000 bara, is released at 308. This specific example is not intended as a limiting example, and a range of other numerical examples can be used while achieving the same technical effect of providing compressed gas.
[0056] FIG. 4 illustrates a specific example 401 of mixing device 303. The device has an input 402 for seawater and an output 403 for letting out hydrates (around 10% of the output) and remaining seawater 405 which is released back into the sea as described before. The device comprises a housing 406 to contain a corresponding set of two screws 407, whereby the housing and the screw define recesses for receiving pressured gas 408 through inputs 409. The volume within the expander for receiving gas and hydrates increases towards the top of the device while the two screws turn. The expander is known as such to the skilled person, and can also be run in reverse to act as a compressor for different purposes. Some of the energy of the injected gas may also be used for making the screws turn.
[0057] FIG. 5A illustrates a possible practical implementation of the devices disclosed herein, whereby a production platform 501 placed on a leg 502 is set in the sea (or ocean) 503. The parts corresponding to those discussed in connection with FIG. 3 are indicated with corresponding reference numbers: mixing device 303, tank 306, chimney 311, while the connecting conduits are illustrated but not numbered again. The mixing device 303 is set against leg 502 at sufficient depth for intake of cold seawater. Mixing device 303 is embodied by screw expander 401 shown again in FIG. 5B.
[0058] FIG. 5C illustrates the screw expander connected to an inlet pipe 504 which takes in the cold seawater. The outlet of the expander contains a mixture of hydrate slush and water, and an outlet pipe is used in the illustrated example which has an S-bend 505. The S-bend works in a manner similar to an air-lock in a kitchen sink, whereby the lighter hydrate slush exits through outlet 506 before the S-bend, while the heavier water is driven through the S-bend and continues in outlet pipe 507. As mentioned before the water in the outlet has a higher temperature and as shown in FIG. 5A a relatively long outlet pipe 507 is used to transport the warmer outlet water away from the mixing device by way of a chimney effect within pipe 507. Other devices for separating lighter hydrates from heavier seawater may be used, such as a centrifugal separator.
[0059] FIG. 6 illustrates tank 306 in more detail when used in the implementation illustrated in FIG. 6A as discussed before. The tank 306 has an inlet 601 for receiving the hydrates produced as described before. The tank further has an outlet 602 for letting out the pressurised gas, whereby the outlet 602 is provided at a top or at least near a higher part of the tank to take advantage of the lighter gas rising to the top of the tank. A further outlet 603 is provided to let out the water. The water is also pressurised and can be used for a specific purpose such as injection into the well, or can simply be released into the sea. The rate of releasing gas into the sea needs to be controlled carefully such that the gas is absorbed, whereby the rate of absorption depends on the temperature. Channels 309 and 310 of the water heating circuit are illustrated as leading the water past chimney 311. The inlet water 309 may be around 1000 bara at room temperature, while the returning steam may be at the same pressure, but below or above a super-critical state at a temperature at or over 400° C. The inlet water may be taken from a lower part of the tank 306, while the steam is injected at a top part of the tank. As illustrated, a preferred embodiment is the channels 309 and 310 being in open connection to the tank 306, but in an alternative embodiment the channels may form a closed circuit which is in temperature communication with the tank to exchange heat without releasing or taking out water from the tank. The tank further includes valves for pressure control, as described in more detail below in connection with FIGS. 7 and 8.
[0060] An example of efficiency achieved with the illustrated setup is a temperature difference of 445° C. between the cold 5° C. and steam of 450° C., an energy delivery of 25 MW, an energy exhaust of 19 MW.
[0061] FIG. 7 illustrates one optional arrangement of valves for controlling the pressure within tank 306. The gas outlet is controlled with a control valve 702, while the water outlet is controlled with control valve 701. The inlet of hydrates is regulated with a screw pump 703. The method of operating this tank is as follows: first hydrates are fed into the tank by rotating the screw of the screw pump; then the tank is closed and heat circulation is started to melt the hydrates and consequently the pressure will increase. When the pressure reaches a threshold pressure at which the gas control valve opens, the gas will flow into a container which is attached to the outlet. When the gas is let out, the water level will increase and when the pressure of the water column reaches a threshold pressure of the water control valve 701, water is released. This process can run continuously during production.
[0062] FIG. 8 illustrates alternative embodiments for managing the pressure in tank 306 including a rotatable sluice. The sluice is rotatable around an axis and has one or more outward facing chambers which can be filled up with a liquid or gas when they face an opening, but retain the fluid or gas (as well as the fluid or gas pressure) when facing away from an opening during rotation of the sluice. One opening faces the tank while another opening faces an outlet, so the chamber alternatingly faces the tank and the outlet. In FIG. 8A, a rotating valve 801 is provided at a lower end to act as the sluice for removing and replacing water from the tank 306, without significantly changing the pressure in the tank. Rotating the sluice does not require a large amount of energy. In FIG. B, hydrates are provided at an outlet of the rotating valve, such that when the valves rotates, water is let out while hydrates are let in. The use of a sluice has the technical advantage of increasing the efficiency of the system because rotating the sluice valve does not consume much energy when compared to screw pump 703 of FIG. 7. A valve may be provided at the top of the tank to remove compressed gas to a container or pipe for transporting the compressed gas away from the system.
[0063] FIG. 9 is a flow diagram illustrating the two main steps of the method disclosed herein, comprising (S1) mixing water and hydrate-forming gas to form hydrates and (S2) warming the hydrates in a confined space to produce pressurised gas. Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.
