NO20151449A1

NO20151449A1 - System and method for production of hydrocarbons in offshore salt caverns

Info

Publication number: NO20151449A1
Application number: NO20151449A
Authority: NO
Inventors: Bjørn Rasmussen
Original assignee: Kalkulo As
Priority date: 2015-10-26
Filing date: 2015-10-26
Publication date: 2016-08-12

Description

SYSTEM AND METHOD FOR PRODUCTION OF HYDROCARBONS IN OFFSHORE SALT CAVERNS

Field of the invention

[0001] The present invention relates to the technical field of production of hydrocarbons. More specifically it concerns the production and storage of hydrocarbons in sub-sea salt caverns. The invention also relates to the separation of liquids and hydrocarbons produced by a well.

Background art

[0002] The process of hydrocarbon or petroleum production is well known in the art.

[0003] In an offshore environment, an oil platform or rig is used both for drilling a wellbore and for extraction of hydrocarbons such as oil and gas. Initially a wellbore has to be established by drilling a hole into the ground. Then, extraction of petroleum takes place. The hydrocarbons can be processed on board the platform or stored temporarily until it is transported to a refinery. The platform may also be directly connected to a refinery with pipelines.

[0004] The hydrocarbons extracted from the well are mixed with water and sediments, and a number of steps comprising filtering and separation have to be completed to arrive at the desired product. For the purpose of process efficiency and costs, filtering and separation usually takes place on board the platform. In this way the volume of the hydrocarbons that have to be transported to the refinery is reduced. Offshore refiners are also used in some regions.

[0005] To reduce the need for infrastructure for the transportation and storage of hydrocarbons it has been proposed to use underground salt caverns. Such caverns can be established by e.g. salt cavern leaching.

[0006] US patent application US 2014/0338921, discloses a method for production of hydrocarbons that utilizes underground salt caverns for phase separation of hydrocarbons into an aqueous phase and an organic phase, and for storage of the hydrocarbons.

[0007] Using salt caverns for storage of hydrocarbons is also known in the art from US 5,129,759 and US 8,714,874.

[0008] US 6,277,286 discloses a method for separation of hydrocarbons where the applied phase separation leads to smaller pressure loss in the well compared to traditional methods because less water is transported together with the hydrocarbons out from the well. It also means that simpler equipment can be used to keep the well pressure up and simpler downstream equipment can be used for additional separation.

[0009] However, methods according to background art rely on aquifiers connected to the salt cavern, and requires a lot of equipment for processing and transporting water out of the salt cavern. Another problem is that water reaching the salt cavern may potentially affect the integrity of the cavern, and that water leaving the salt cavern may impact downstream transportation pipes or processing facilities.

[0010] Subsea separators have been proposed in e.g. US 2015/0021235, describing a sub-sea arranged three phase separator obtaining a crude oil stream, and a crude natural gas stream, separating acid gasses and/or water from the crude natural gas, pressurizing the gas stream sub-sea, adding at least part of the pressurized gas to the crude oil stream subsea and transporting the added gas together with the oil to the topside.

[0011] The downhole subsea separator is a specific type of subsea separators, where the separator is installed in the wellbore itself. WO 2014/0152585 discloses an example of such a separator.

[0012] Another example is given in US 6,277,286, where gravity separation is used for separating water from the hydrocarbons in the mostly horizontal section of the well.

[0013] Subsea equipment such as separators and flow lines require external power to operate. US 8,904,792 discloses a method and system for storing energy and generating power heat in a subsea environment.

Short summary of the invention

[0014] A main object of the present invention is to disclose a method for production of hydrocarbons in offshore salt caverns that solves the problems identified with prior art.

[0015] The invention is a method for subsea production of hydrocarbons from a wellbore, comprising the steps of; - separating a first wellbore fluid from said wellbore in a first separator, to obtain a second wellbore fluid comprising mainly hydrocarbons, - transporting said second wellbore fluid in a flow to an offshore salt cavern via a first fluid line, - separating said second wellbore fluid in said salt cavern into at least oil and water, by allowing the second wellbore fluid to rest in the salt cavern, and - transporting said oil via a second fluid line to an oil processing or storage facility.

[0016] The invention is also a subsea hydrocarbon production system (1) comprising; - a first separator arranged for separating a first wellbore fluid from said wellbore (16), to obtain a second wellbore fluid comprising mainly hydrocarbons, - a salt cavern arranged for storage of said second wellbore fluid to separate said second wellbore fluid in said salt cavern into at least oil and water, - a first fluid line connecting an output of said first separator and said salt cavern

arranged for transporting said second wellbore fluid in a flow to said salt cavern,

- a second fluid line connected in one end to said salt cavern and arranged for transporting said oil to an oil processing or storage facility.

[0017] Since a large portion of the water is separated from the hydrocarbons in the first stage of separation, the water that is separated in the first stage can be reused in the well to keep the well pressure high without håving to pass this water further to and through the salt cavern. This reduces the need for aquifers connected to the salt cavern.

[0018] It further reduces the need for equipment that processes water or transports water out from the salt cavern.

[0019] An additional benefit is that less water reaches the downstream salt cavern, where it could potentially enlarge the salt cavern or in other ways affect the integrity of the salt cavern.

[0020] Another benefit is that less water also comes out from the salt cavern, where it could negatively impact downstream transportation pipes or processing facilities, by for example clogging them with solids transported with the water.

[0021] The present invention provides a low-cost hydrocarbon separation solution that is especially useful in regions where separation equipment is expensive to deploy or where such equipment could have an adverse environmental impact. Examples of such regions can be regions with extreme weather conditions, deep waters, frozen water surfaces, sensitive eco-systems, or multiple smaller or stranded hydrocarbon accumulations that otherwise would have been prohibitively expensive to exploit.

[0022] Subsea separation according to the invention reduces the need for topside processing capacity and can further contribute to more efficient production of hydrocarbons.

[0023] Since the salt cavern has the dual functionality of both a separator and a storage facility, gas and/or oil can be offloaded from the cavern based on a cost and revenue consideration, e.g. offload when the rent for the LNG/gas/oil tanker is low and when oil/gas prices are high.

Figure captions

[0024] The attached figures illustrate some embodiments of the claimed invention.

[0025] Fig. 1 illustrates in a simplified drawing and embodiment of the invention with a two-step separation process, where the first separator is arranged on the sea floor.

[0026] Fig. 2 illustrates in a simplified drawing and embodiment of the invention a two-step separation process, where the first separator is a downhole separator.

[0027] Fig. 3 illustrates in a simplified drawing and embodiment of the invention where two or more wellbores with downhole separators are connected via flow lines on or near the seafloor to deliver separated fluid to the salt cavern.

[0028] Fig. 4 illustrates in a simplified drawing and embodiment of the invention where two or more wellbores with downhole separators are connected via flow lines, such as horizontal wells, below the seafloor to deliver separated fluid to the salt cavern.

[0029] Fig. 5 illustrates in a simplified drawing and embodiment of the invention where the oil and gas resulting from the separation process in the first salt cavern is transferred to a second and third cavern for long time storage.

Embodiments of the invention

[0030] The invention will in the following be described and embodiments of the invention will be explained with reference to the accompanying drawings.

[0031] All the figures 1 to 5 illustrate various embodiments of the invention installed in a possible environment. The various layers illustrated are; the layer containing the hydrocarbon or oil (15) reservoir, the salt layer (13) which is a pre-requisite for creating the salt caverns, the sea floor (12) and the sea level (11). In some geographical regions, such as outside the coast of Brazil, there is a pre-salt (14) layer below the salt layer. Large reservoirs of oil are situated between the pre-salt layer and the salt layer, making this invention of specific commercial interest for such areas. However, the layer of pre-salt carbonates is not necessary for the invention to work, and it may also be used in other regions where there is a salt layer suitable for creating salt caverns.

[0032] According to the invention, separation is performed in a two-step process, in two different sub-sea locations as illustrated in Fig. 1 and Fig. 2.

[0033] The first step is to reduce the amount of water in the fluid from the wellbore by using a first separator (101, 201) arranged for removing a substantial portion of the water in the fluid from the wellbore (16).

[0034] In an embodiment, illustrated in Fig. 1, the subsea hydrocarbon production system (1) comprises a first separator (101) arranged for separating a first wellbore fluid

(A) from said wellbore (16), to obtain a second wellbore fluid (B) comprising mainly hydrocarbons, where the subsea separator is arranged on the seafloor (12). Thus, the

second wellbore fluid (B) contains substantially less water than the first wellbore fluid

(A), which is the fluid obtained from the wellbore (16).

[0035] In another embodiment, illustrated in Fig. 2, the subsea hydrocarbon production

system (1) comprises in its first separation stage a horizontal downhole separator (201) as illustrated in Fig. 2.

[0036] With regard to the subsea separators described under prior art, the downhole separator (201) can be completely fitted in a 9 5/8" or 10 3/4" casing, and the downhole separation equipment may complete separation within 10 to 300 seconds in a 7 to 10 meter section for 25 to 40 degree API oils with up to 50 percent water cut.

[0037] When the horizontal section of the well (16) consists of pipes, gravity separation is in an embodiment used for separating water from the hydrocarbons in the mostly horizontal section of the well. An example of such separation has been given in prior art.

[0038] This form of phase separation leads to smaller pressure loss in the well compared to traditional methods because less water is transported together with the hydrocarbons out from the well. It also means that simpler equipment can be used to keep the well pressure up and simpler downstream equipment can be used for additional separation.

[0039] In an embodiment, the subsea separator (101, 201), in any of the embodiments above also separates oil from gas. This gas can be transported back to the well to keep the well pressure up, while the oil is transported to at least one salt cavern.

[0040] In both the embodiments described above, the subsea hydrocarbon production system (1) comprises a salt cavern (3) arranged for storage of said second wellbore fluid (B) to separate said second wellbore fluid (B) in said salt cavern (3) into at least oil (22) and water (21).

[0041] A process for creating salt caverns may start with the step of finding a suitable

location. In an embodiment, salt body composition and geometry is survey ed via seismic imaging. Once the location for the salt cavern has been chosen, it needs to be formed. In an embodiment, salt caverns are formed by pumping fresh water through a drill pipe into the salt body, dissolving the salt, and pumping the saturated saltwater out from the

formed cavity, e.g. out near the sea bottom floor.

[0042] After håving been separated in the horizontal well (16), the hydrocarbons are then transported to at least one salt cavern (3) via a first fluid line (102) connecting an output of said first separator (101) and said salt cavern (3) arranged for transporting said second wellbore fluid (B) to said salt cavern (3).

[0043] In an embodiment, as illustrated in Fig. 1 the first fluid line (102) is implemented as one or more flow lines on or near the seafloor (12).

[0044] In an embodiment, the first fluid line (102) is implemented as one or more underground pipes, i.e. arranged below the seafloor (12) as illustrated in Fig. 2.

[0045] Fig. 4 illustrates another embodiment, where a horizontal well (302) is produced between a reservoir (15) and the salt cavern (3) to enable direct transportation of hydrocarbons from the reservoir (15) to the salt cavern (3).

[0046] A horizontal well can be formed by using horizontal drilling equipment.

[0047] The various fluid-line types can be used independently or combined, e.g. where the fluid line (102, 202, 302) comprises different sections of pipes above or below the sea floor or wells.

[0048] When the desired amount of hydrocarbons has been transported into the salt cavern, it is desirable to terminate the flow of the hydrocarbons into the cavern, to allow the passive in-cavern separation to occur without unnecessary noise and turbulence from the incoming flow.

[0049] In an embodiment, the oil is transported via the second fluid line to the oil processing or storage facility at least one solar day after said flow of said second wellbore fluid into said salt cavern has terminated.

[0050] Flow termination means, such as valves connected to the fluid lines (102, 202, 302) may be used to terminate the flow. Depending on the flow termination means used, such flow termination may not necessarily be abrupt, and a certain leakage may therefore occur after termination has been initiated. What is considered important in this respect, is that the incoming fluid does no create any significant turbulence in the cavern.

[0051] In an embodiment, the salt cavern (3) can be used for accumulating separated hydrocarbons from multiple wells (16, 17) as illustrated in Fig. 4. One or more subsea separators (101) and flow lines (102) arranged above sea floor (12) could be used in a similar embodiment (not illustrated).

[0052] In the salt cavern (3), the hydrocarbons undergo an in-cavern phase separation.

[0053] In an embodiment, the in-cavern phase separation of water (21), oil (22) and gas (23) is carried out passively by the influence of gravity on the hydrocarbons in the salt cavern (3).

[0054] In an embodiment, the hydrocarbons are left in the salt cavern (3) for at least 1 day.

[0055] In other embodiments, the hydrocarbons are left in the salt cavern for at least 2 days or at least 3 days.

[0056] Oil (22) in a salt cavern typically reaches sale quality after 2 to 3 days.

[0057] The salt cavern (3) can also be used for storage of the separated hydrocarbons.

[0058] In an embodiment, the in-salt cavern separation can be combined with additional in-salt cavern separation in one or more additional salt caverns. In some embodiments, the additional salt caverns are directly connected with the first salt cavern. In other embodiments, the salt caverns are indirectly connected via a shared well to which several salt caverns are connected.

[0059] Using multiple salt caverns for in-cavern separation is especially useful if conditions in a salt cavern slows down or disturbs the phase separation in that cavern. An example of such conditions is a high amount of turbulence, which can be caused if the flow of fluid into the first salt cavern is high, or if there are several inlets into the salt cavern that together form conditions for turbulence.

[0060] In an embodiment, a salt cavern is used as a drainage point for different types of hydrocarbons, for example oil and natural gas. The oil and natural gas may come from the same reservoir or reservoirs or from different reservoirs.

[0061] In an embodiment, the salt cavern is equipped with at least one pressure inlet that hinders backflow of hydrocarbons back to a reservoir.

[0062] In an embodiment, hydrocarbons from a plurality of reservoirs (15) are transported into the first salt cavern (3) through different pressure inlets.

[0063] After separation in the cavern (3) for a time as indicated above, the oil (22) and/or gas (23) is transported via a second fluid line (103) connected in one end to said salt cavern (3) and arranged for transporting said oil (22) and/or gas (23) to an oil processing or storage facility.

[0064] In an embodiment, after oil has been separated and accumulated in one or more salt caverns (3, 3a), the oil is offloaded from the salt caverns to a floating vessel, e.g. a floating production, storage and offloading (FPSO) unit (110, 110a). The oil (22) may be offloaded from the oil cavern (3, 3a) via a flexible riser system (105, 105a) and a mooring buoy (104, 104a). Oil (22) may further be offloaded from the floating vessel (110, 110a) to a shuttle tanker (111), as illustrated in Fig. 5 for transportation to land.

[0065] Advantageously, oil is offloaded from the one or more salt caverns to a shuttle tanker when the price of renting the shuttle tanker is lower than the price of renting the shuttle tanker during separation of the oil in the salt caverns. The oil can also be offloaded from the one or more salt caverns to a shuttle tanker when the market price of oil is higher than the market price of oil during separation of the oil in the salt caverns.

[0066] If natural gas has been separated and accumulated in one or more salt caverns (3) as described above, the natural gas may in an embodiment be offloaded from the salt cavern to a floating LNG plant (110b) via e.g. a flexible riser system.

[0067] Advantageously, natural gas is offloaded from the one or more salt caverns to a floating LNG plant when the price of renting the floating LNG plant is lower than the price of renting the floating LNG plant during separation of the natural gas in the salt caverns. The natural gas can also be offloaded from the one or more salt caverns to the floating LNG plant when the market price of natural gas is higher than the market price of natural gas during separation of the natural gas in the salt caverns.

[0068] The depth of the riser system into the cavern will depend on the type of hydrocarbons to offload. Due to the result of the separation process, gas will be stored above the oil, so an oil riser will be terminated in the oil layer, while a gas riser will terminate above, in the gas layer.

[0069] In an embodiment, oil separated during the in-cavern phase separation is transported from the first salt cavern (3) via a fourth fluid line (103a) to at least one second salt cavern (3a) dedicated to the long-term storage of oil. In some embodiments, long-term storage of oil means storage of oil for at least 30 days.

[0070] The fourth fluid line above (103a) is connecting an output of said first salt cavern

(3) and an input of said second salt cavern (3a) as illustrated in Fig. 5.

[0071] In an embodiment, natural gas (23) separated during the in-cavern phase separation is transported from the first salt cavern (3) via a fifth fluid line (103b) to at

least one salt cavern dedicated to long-term storage of gas (3b). This is especially useful if the first salt cavern accumulates hydrocarbons from reservoirs with a high gas/oil ratio. In some embodiments, long-term storage of natural gas means storage of natural gas for at least 30 days.

[0072] The fifth fluid line above (103b) is connecting an output of said first salt cavern (3) and an input of said salt cavern (3b) as illustrated in Fig. 5.

[0073] Further separation and processing of the hydrocarbons can be handled by downstream processing facilities. Examples of such facilities can be a subsea processing facility, a processing facility on a ship or platform on the water surface, or an on-land processing facility connected with one or more of the salt caverns.

[0074] In an embodiment, subsea processing facilities can be powered by subsea heat and/or power generation methods and arrangements as described under prior art. This embodiment is advantageous in remote areas.

[0075] Such subsea heat and/or power generation methods can be applied in connection with salt caverns, for facilitating the transportation of hydrocarbons to or from the salt cavern, or between salt caverns if multiple salt caverns are used.

Claims

1. A method for subsea production of hydrocarbons from a wellbore (16), comprising the steps of; - separating a first wellbore fluid (A) from said wellbore (16) in a first separator (101, 201), to obtain a second wellbore fluid (B) comprising mainly hydrocarbons, - transporting said second wellbore fluid (B) in a flow to an offshore salt cavern (3) via a first fluid line (102, 202, 302), - separating said second wellbore fluid (B) in said salt cavern (3) into at least oil (22) and water (21), by allowing the second wellbore fluid (B) to rest in the salt cavern (3), and - transporting said oil (22) via a second fluid line (103) to an oil processing or storage facility.

2. The method of claim 1, wherein the first separator (101) is a subsea separator arranged on the seafloor.

3. The method of claim 1, wherein the first separator (201) is a downhole separator arranged in a horizontal part of the wellbore (16).

4. The method of any of the claims above, comprising the step of offloading the oil (22) from the salt cavern (3) to a floating vessel (110) via a flexible riser system (105) and a mooring buoy (104).

5. The method of any of the claims above, comprising the step of transporting said oil (22) via a fourth fluid line (103a) to a second salt cavern (3a) for long time storage.

6. The method of any of the claims above, further comprising: - separating a first portion of water from the first wellbore (A) fluid in the first separator (101, 102), and - injecting said first portion of water back into the wellbore (16).

7. The method of any of the claims above, wherein the salt cavern (3) comprises at least one pressure inlet arranged to prevent backflow of hydrocarbons from the salt cavern (3) to the first fluid line (102, 202, 302).

8. The method of any of the claims above, further comprising: - separating wellbore fluid (A) from a second wellbore (16) in a second separator (101, 201), to obtain additional wellbore fluid (B) comprising mainly hydrocarbons, - transporting said additional wellbore fluid (B) to the offshore salt cavern (3) via an additional first fluid line.

9. The method of claim 8, wherein the additional first fluid line is connected to the first fluid line, such that the separated additional wellbore fluid from the second wellbore (16) enters the salt cavern (3) via the first fluid line (102).

10. The method of claim 8, wherein the first fluid line and the additional first fluid line from the respective first and second wellbores are connected to separate fluid inlets of the salt cavern.

11. The method of any of the claims above, wherein the first fluid line (102) is a horizontal well connecting said first separator (101, 201) to the salt cavern (3).

12. The method of any of the claims above where the oil processing or storage facility is a Floating Production, Storage and Offloading unit (FPSO), said method further comprising: - separating spill water from the oil in a separator on-board said FPSO, - releasing the spill water from the FPSO into the sea, and - offloading the oil from the FPSO to a shuttle tanker.

13. The method of any of the claims above, further comprising: - separating gas (23) from the second wellbore fluid (B) in the salt cavern (3), - transporting said gas to a third salt cavern (3b) for long term storage of gas (23), and - offloading said gas (23) from said third salt cavern (3b) to a floating LNG plant (110b) via a flexible riser system (105b) and a mooring buoy (104b).

14. A subsea hydrocarbon production system (1) comprising; - a first separator (101) arranged for separating a first wellbore fluid (A) from said wellbore (16), to obtain a second wellbore fluid (B) comprising mainly hydrocarbons, - a salt cavern (3) arranged for storage of said second wellbore fluid (B) to separate said second wellbore fluid (B) in said salt cavern (3) into at least oil (22) and water (21), - a first fluid line (102, 202, 302) connecting an output of said first separator (101) and said salt cavern (3) arranged for transporting said second wellbore fluid (B) in a flow to said salt cavern (3), - a second fluid line (103) connected in one end to said salt cavern (3) and arranged for transporting said oil (22) to an oil processing or storage facility.

15. The system according to claim 14, wherein the first separator (101) is a subsea separator arranged on the seafloor.

16. The system according to claim 14, wherein the first separator (101) is a downhole separator arranged in a horizontal part of the wellbore (16).

17. The system according to any of the claims 14 to 16, comprising a flexible riser system (105, 105a) and a mooring buoy (104, 104a) arranged to offloading the oil (22) from the salt cavern (3) to a floating vessel (110, 110a).

18. The system according to any of the claims 14 to 17, comprising a flexible riser system (105, 105b) and a mooring buoy (104, 104b) arranged to offloading gas (23) from the salt cavern (3) to a floating LNG plant (110, 110b).

19. The system according to any of the claims 14 to 18, comprising a second separator (107) and a third fluid line (108), wherein the third fluid line (108) is connecting an output of said second separator (108) and said salt cavern (3).

20. The method according to any of the claims 14 to 19, comprising a second salt cavern (3a) for long time storage of oil and a fourth fluid line (103a), wherein the fourth fluid line (103a) is connecting an output of said second salt cavern (3) and an input of said salt cavern (3a).