WO2013043354A1

WO2013043354A1 - Separating oil and water streams

Info

Publication number: WO2013043354A1
Application number: PCT/US2012/053409
Authority: WO
Inventors: Scott M. WHITNEY; Per-Reidar Larnholm
Original assignee: Exxonmobil Upstream Research Company
Priority date: 2011-09-21
Filing date: 2012-08-31
Publication date: 2013-03-28
Also published as: US20140209465A1

Abstract

Embodiments described herein provide a system and methods for separating oil and water streams. The method includes separating a fluid stream into an oil continuous stream and a water continuous stream using a cyclonic separator, flowing the oil continuous stream to a first gravity separation vessel, and flowing the water continuous stream to a second gravity separation vessel. The method also includes separating the oil continuous stream in the first gravity separation vessel into an oil stream and a water stream and separating the water continuous stream in the second gravity separation vessel into an oil stream and a water stream.

Description

SEPARATING OIL AND WATER STREAMS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of U.S. Provisional Patent Application 61/537,317 filed September 21 , 201 1 entitled SEPARATING OIL AND WATER STREAMS the entirety of which is incorporated by reference herein.

FIELD

[0002] Exemplary embodiments of the subject innovation relate to the separation of oil and water streams in a subsea or topside environment.

BACKGROUND

[0003] Obtaining hydrocarbons from subsea environments is becoming an increasingly important alternative to obtaining hydrocarbons from land-based sources. As long as energy prices continue to increase, this trend is likely to continue. Delivering hydrocarbons from a subsea well to the surface presents technologists with a number of challenges. Water in hydrocarbons can form hydrate clathrates in transportation lines and subsea equipment as the fluid cools, creating flow restrictions. Further, fluid obtained from a subsea well may comprise a large proportion of water relative to hydrocarbons, reducing the efficiency of hydrocarbon transportation from the well. In such situations, it may be desirable to attempt to separate hydrocarbons out of the liquid produced by a subsea well at the sea floor.

[0004] Separating hydrocarbons flowing from a subsurface well from other fluids may present difficulties. While subsea separation is not trivial in shallow waters, for example, fifteen hundred meters or less, it becomes much more challenging in deeper water. As water depth increases, the external pressure on a vessel created by the hydrostatic head increases the required wall thickness for vessels used for subsea processing. At depths in excess of fifteen hundred meters, this wall thickness becomes great enough, that typical gravity separation is not practical because the allowable vessel size is limited in diameter by wall thickness and weight. As a result, deepwater subsea separation of hydrocarbons is relatively difficult because traditional large-diameter separators cannot be used. This disadvantage is further increased if the separation is a heavy oil and water, which may emulsify.

[0005] Typically, separation of heavy oils from water necessitates the use of a large gravity separation vessel that provides long retention times for the oil and water to separate. However, due to size and weight constraints, the use of large gravity separation vessels is not practical for many applications, both on and offshore. Topside applications of large gravity separation vessels can be limited by the space requirements of the vessel. In some instances, the ability to use a smaller, more efficient separation system may be desirable.

[0006] Separation of oil and water is especially difficult when the fluids are in an inversion range, for example, when the watercut of the stream in the range between about 40% and about 60%. The watercut is the ratio of water produced compared to the total volume of liquid produced. In the fluid inversion range, emulsion layers may form and inhibit effective separation of hydrocarbons and water. An emulsion is a physical mixture of two liquids which are immiscible, in which one liquid exists as nearly stable droplets that are dispersed in the other liquid. An emulsion is also known as a colloid. While overcoming this problem is difficult topside, performance can be improved by heat, chemicals, or time. However, in subsea applications, these techniques are often not feasible.

[0007] The current practice to separate oil and water in the inversion range is through separation enhancers. An example of a separation enhancer includes mixing chemicals, such as demulsifiers, with the fluid. However, as the fluid properties change, such as by turning on new wells, the appropriate amount of demulsifier to use becomes difficult to predict. This can lead to using excess amounts of demulsifier, which is expensive and often causes other challenges, such as foaming.

[0008] Another example of a separation enhancer is the application of heat to lower the viscosity of the fluids and ease separation. However, applying heat is expensive, and is very challenging in a subsea environment.

[0009] Another example of a separation enhancer is the injection of water into a separator to raise the watercut beyond the inversion range. This is the most common currently-used method for subsea oil and water separation. However, recirculation can require large amounts of water, which necessitates an increase in the size of equipment, such as vessels, pumps, and piping. Further, deepwater vessels are limited in terms of available vessel volume due to the external pressures. Therefore any waste of space due to added water is a disadvantage.

[0010] Another separation enhancer involves the use of electrostatic coalescers. Electrostatic coalescers place a charge across a fluid or fluid mixture to cause droplets of polar fluids, such as water, to coalesce into larger droplets. Although, electrostatic coalescers are relatively effective, and are currently used in many applications, they will turn off automatically once the fluid mixture approaches a water continuous phase to avoid shorting out. Therefore, since the separation will still be difficult past the operation point of the electrostatic coalescers, the use of electrostatic coalescers may not be relied on as a total solution to the problem of separating oil and water in the inversion range.

[0011] U.S. Patent No. 6,197,095 to Ditria, et al., discloses a method for subsea multiphase fluid separation. The initial step of the method is the separation of solids using a cyclonic solids separator. In a second step, bulk gas is removed from the liquid using a cyclone or auger separator. In a third step, water is separated from the oil using a liquid-liquid hydrocyclone. In a final step, a gravity separator is used to cause further separation of the water from the oil. However, the method is limited by the size of the gravity separator, since a fairly large vessel may be required to cause sufficient separation of the water from the oil using one gravity separator.

[0012] International Patent Publication No. WO2004/007908 by Gulbraar, et al., discloses an apparatus for separating water from oil. The apparatus includes an electrostatic coalescer for the separation of the water droplets from the oil droplets within a stream. After the water has been partially separated from the oil by the electrostatic coalescer, the stream is sent to an oil/water separation arrangement for further separation of the water from the oil. However, while the electrostatic coalescer may help to avoid the formation of emulsions, the coalescer may not be sufficient to ensure the avoidance of separation in the inversion range. In addition, the use of only one oil/water separation arrangement may increase the required size of the apparatus and limit the effectiveness of the system. SUMMARY

[0013] An embodiment provides a method for separating oil and water streams. The method includes separating a fluid stream into an oil continuous stream and a water continuous stream using a cyclonic separator, flowing the oil continuous stream to a first gravity separation vessel, and flowing the water continuous stream to a second gravity separation vessel. The method also includes separating the oil continuous stream in the first gravity separation vessel into an oil stream and a water stream and separating the water continuous stream in the second gravity separation vessel into an oil stream and a water stream.

[0014] Another embodiment provides a system for separating oil and water streams. The system includes a cyclonic separator configured to separate a fluid stream into an oil continuous stream and a water continuous stream, a first gravity separation vessel configured to separate the water continuous stream into a first oil stream and a first water stream, and a second gravity separation vessel configured to separate the oil continuous stream into a second oil stream and a second water stream.

[0015] Another embodiment provides a method for separating two immiscible phases from a fluid stream. The method includes sending the fluid stream into a cyclonic separator, generating radial acceleration within the cyclonic separator using a swirl element, and controlling the radial acceleration at a value at which the two immiscible phases separate into two continuous phases. The method also includes removing the two continuous phases from the cyclonic separator into two lines using a vortex finder and sending the two continuous phases to two separate downstream vessels for further separation of the two immiscible phases.

DESCRIPTION OF THE DRAWINGS

[0016] The advantages of the present techniques are better understood by referring to the following detailed description and the attached drawings, in which:

[0017] Fig. 1 is an illustration of a subsea hydrocarbon field that uses subsea separation techniques prior to sending materials to the surface;

[0018] Fig. 2 is a schematic of a system for separating oil and water using a cyclonic separator upstream of two gravity separation vessels; [0019] Fig. 3 is a schematic of a complete system for separating gas, oil, water, and sand;

[0020] Fig. 4 is a schematic of a complete system, including an electrostatic coalescer, for separating gas, oil, water, and sand;

[0021] Fig. 5 is an illustrative view of a cyclonic separator that may be used to separate oil and water streams;

[0022] Fig. 6 is an illustrative view of the swirl element that may be used in the cyclonic separator; and

[0023] Fig. 7 is a process flow diagram showing a method for the separation of oil and water streams.

DETAILED DESCRIPTION

[0024] In the following detailed description section, specific embodiments of the present techniques are described. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the techniques are not limited to the specific embodiments described below, but rather, include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

[0025] At the outset, for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims.

[0026] A "facility" as used herein is a representation of a tangible piece of physical equipment through which hydrocarbon fluids are either produced from a reservoir or injected into a reservoir. In its broadest sense, the term facility is applied to any equipment that may be present along the flow path between a reservoir and the destination for a hydrocarbon product. Facilities may comprise drilling platforms, production platforms, production wells, injection wells, well tubulars, wellhead equipment, gathering lines, manifolds, pumps, compressors, separators, surface flow lines, and delivery outlets. In some instances, the term "surface facility" is used to distinguish those facilities other than wells. A "facility network" is the complete collection of facilities that are present in the model, which would include all wells and the surface facilities between the wellheads and the delivery outlets.

[0027] The term "gas" is used interchangeably with "vapor," and means a substance or mixture of substances in the gaseous state as distinguished from the liquid or solid state. Likewise, the term "liquid" means a substance or mixture of substances in the liquid state as distinguished from the gas or solid state. As used herein, "fluid" is a generic term that may include either a gas or vapor.

[0028] A "hydrocarbon" is an organic compound that primarily includes the elements hydrogen and carbon although nitrogen, sulfur, oxygen, metals, or any number of other elements may be present in small amounts. As used herein, hydrocarbons generally refer to organic materials that are transported by pipeline, such as any form of natural gas or crude oil. A "hydrocarbon stream" is a stream enriched in hydrocarbons by the removal of other materials, such as water.

[0029] The terms "inversion range" or "fluid inversion range" refer to a range between about 40%-60% watercut in a stream comprising water and hydrocarbons. The inversion relates to a change of phase in which the stream changes or "inverts" between a water continuous stream and an oil continuous stream.

[0030] "Liquefied natural gas" or "LNG" is natural gas that has been processed to remove impurities (for example, nitrogen, and water and/or heavy hydrocarbons) and then condensed into a liquid at almost atmospheric pressure by cooling and depressurization.

[0031] The term "natural gas" refers to a multi-component gas obtained from a crude oil well (termed associated gas) or from a subterranean gas-bearing formation (termed non-associated gas). The composition and pressure of natural gas can vary significantly. A typical natural gas stream contains methane (CH₄) as a significant component. Raw natural gas will also typically contain ethylene (C₂H₄), ethane (C₂H₆), other hydrocarbons, one or more acid gases (such as carbon dioxide, hydrogen sulfide, carbonyl sulfide, carbon disulfide, and mercaptans), and minor amounts of contaminants such as water, nitrogen, iron sulfide, wax, and crude oil.

[0032] "Pressure" is the force exerted per unit area by the fluid on the walls of the volume. Pressure can be shown as pounds per square inch (psi). "Atmospheric pressure" refers to the local pressure of the air. "Absolute pressure" (psia) refers to the sum of the atmospheric pressure (14.7 psia at standard conditions) plus the gage pressure (psig). "Gauge pressure" (psig) refers to the pressure measured by a gauge, which indicates only the pressure exceeding the local atmospheric pressure (i.e., a gauge pressure of 0 psig corresponds to an absolute pressure of 14.7 psia).

[0033] "Production fluid" refers to a liquid and/or gaseous stream removed from a subsurface formation, such as an organic-rich rock formation. Produced fluids may include both hydrocarbon fluids and non-hydrocarbon fluids. For example, production fluids may include, but are not limited to, oil, natural gas, and water.

[0034] "Substantial" when used in reference to a quantity or amount of a material, or a specific characteristic thereof, refers to an amount that is sufficient to provide an effect that the material or characteristic was intended to provide. The exact degree of deviation allowable may in some cases depend on the specific context.

[0035] The term "watercut" refers to the proportion of water present in a stream that comprises both water and other components such as hydrocarbons. For example, a stream having a 20% watercut comprises about 20% water and 80% other components.

[0036] "Well" or "wellbore" refers to a hole in the subsurface made by drilling or insertion of a conduit into the subsurface. The terms are interchangeable when referring to an opening in the formation. A well may have a substantially circular cross section, or other cross-sectional shapes (for example, circles, ovals, squares, rectangles, triangles, slits, or other regular or irregular shapes). Wells may be cased, cased and cemented, or open-hole well, and may be any type, including, but not limited to a producing well, an experimental well, and an exploratory well, or the like. A well may be vertical, horizontal, or any angle between vertical and horizontal (a deviated well), for example a vertical well may comprise a non-vertical component. [0037] "Clathrate hydrates" (hereinafter clathrate or hydrate) are composites formed from a water matrix and a guest molecule, such as methane or carbon dioxide, among others. Clathrates may form, for example, at the high pressures and low temperatures that may be found in pipelines and other hydrocarbon equipment. For any particular clathrate composition involving water and guest molecules, such as methane, ethane, propane, carbon dioxide, and hydrogen sulfide, at a particular pressure there is a specific clathrate equilibrium temperature, above which clathrates are not stable and below which they are stable. After forming, the clathrates can agglomerate, leading to plugging or fouling of the equipment. Further, many hydrocarbons, such as crude oil, may contain significant amounts of wax, e.g., in the form of paraffinic compounds that may precipitate as temperatures are lowered. These paraffinic compounds can form layers along cold surfaces, such as the inner wall of a subsea pipeline, and can cause fouling or plugging of equipment.

[0038] A "piston motor valve" (PMV) is a type of valve that uses the linear motion of a piston to open or close the valve. A PMV is used when a fully open or fully closed valve is desirable for flow control.

[0039] A "diaphragm motor valve" (DMV) is a type of device or component that may be used to control the flow of a fluid through a pipe or tube by moving a valve though a range of positions from fully closed to fully open. A DMV is generally used to throttle fluid flow in a line.

Overview

[0040] Current separation systems, including both subsea and topside separation systems, encounter difficulty in the separation of oil and water once the fluid mixture approaches the inversion range. The inversion range of an oil and water fluid mixture is typically around 40% to 60% watercut. Below the lower watercut limit, the mixture is usually oil continuous; and the water droplets are dispersed into the oil. Above the upper watercut limit, the mixture is usually water continuous and the oil is the dispersed phase. However, emulsions can be formed in the intermediate range of watercuts where the two phases are switching from continuous to dispersed phase. The formation of stable emulsions makes separation of oil and water in the inversion range very challenging. [0041] Embodiments disclosed herein provide methods and systems that allow for the separation of gas, oil, water, and sand throughout all watercuts, including those in the inversion range. The method lowers the likelihood of separating mixtures in the inversion range through the utilization of a cyclonic separator upstream of two gravity separation vessels. Accordingly, the system may function effectively without the use of separation enhancers for gravity separation in the fluid inversion range, since gravity separation techniques are not applied while the fluid is in the inversion range.

[0042] The cyclonic separator is placed upstream of the two gravity separation vessels to provide an initial separation of the fluid into a water continuous stream and an oil continuous stream. A fixed swirl element inside the cyclonic separator creates a radial acceleration in the fluid, e.g., by generating a cyclone action. The fixed swirl element is designed to have a low pressure drop to lower turbulence and fluid mixing. The cyclonic action generates a centripetal force that causes the heavier fluid phase, water, to move towards the outside wall of the pipe and the lighter fluid phase, oil, to move towards the center. In this manner, the swirl element performs a bulk phase separation of the fluids.

[0043] The streams may be split by means of a vortex finder in the center of the cyclonic separator or by a horizontal flow split with a baffle running parallel to the cyclonic separator. Some amount of the other component will remain in each stream. The two different streams may be sent to separate gravity separation vessels, e.g., pipe separators, for further separation of oil and water phases. Thus, separation of fluids in a gravity separation vessel is not attempted while the mixture is in the inversion range.

[0044] After the two different streams enter into the separate gravity separation vessels, further separation is performed, and each vessel has an oil outlet and a water outlet. The oil streams from the gravity separation vessels may remain separate or may be blended together. The same is true for the water streams from the gravity separation vessels. The particular application or system configuration may be used to determine whether the fluid streams from multiple gravity separation vessels should be combined or remain separated. Further, a number of the separation systems may be utilized in one area, with the like streams combined into single streams. [0045] In an embodiment, the present techniques may be used in any transportation or production environment that is susceptible to clathrate, including subsea to shore pipelines, on-shore pipelines, wells, oil from oil sands, natural gas, or any number of liquid or gaseous hydrocarbons from any number of sources. For example, a specific application of the present techniques may include the protection of subsea lines from a production field.

[0046] The system described herein may be used on the seafloor to separate oil and water mixtures that are in or near the inversion range by avoiding gravity separation while fluids are in the inversion range. This system does not depend on the use of separation enhancers, which can be costly and may limit the capacity of the system.

[0047] Fig. 1 is an illustration of a subsea hydrocarbon field 100 that uses a cyclonic separation technique. The field 100 can have a number of wellheads 102 coupled to wells 104 that harvest hydrocarbons from a formation (not shown). As shown in this example, the wellheads 102 may be located on the ocean floor 106. Each of the wells 104 may include single wellbores or multiple, branched wellbores. Each of the wellheads 102 can be coupled to a central pipeline 108 by gathering lines 110. The central pipeline 108 may continue through the field 100, coupling to further wellheads 102, as indicated by reference number 1 12.

[0048] In an embodiment, a cyclonic separation system 114 is used for the separation of gas, oil, water, and sand from the central pipeline 108. Three lines 116, 118, and 120 may couple the cyclonic separation system 114 to a platform 122 at the ocean surface 124. The three lines 116, 118, and 120 may be flexible to allow movement of the platform 122. The flexible lines 116, 118, and 120 may carry gas, oil, and water, respectively, to the platform 122. The platform 122 may be, for example, a floating processing station, such as a floating storage and offloading unit (or FSO), that is anchored to the sea floor 106 by a number of tethers 126.

[0049] Any number of other types of platforms or rigs may be used. For example, the platform 122 may be a production platform with equipment for dehydration, purification, oil and water separation, oil and gas separation, and the like, such as a storage vessel or separation vessel 128. The platform 122 may be a drilling platform that includes drilling equipment, such as a tower or derrick 130. The platform 122 may transport the processed hydrocarbons to shore facilities by pipeline (not shown). The separation of the hydrocarbons in a cyclonic separation system 114 may prevent the formation of hydrate plugs in transportation lines to the surface, as the oil lines 118, and gas lines 118, are separate from the water lines 120. Further, the separation at the sea floor, or at a well site in a surface field, may provide water for reinjection into the formation to enhance production.

Cyclonic Separation Apparatus

[0050] Fig. 2 is a schematic of a system 200 for separating oil and water using a cyclonic separator 202 upstream of two gravity separation vessels 204 and 206. The cyclonic separator 202 is discussed further with respect to Fig. 5. The cyclonic separator 202 produces an oil continuous stream 208 and a water continuous stream 210. The concentration of oil and water, respectively, in each of the two streams may remain above 60%. Therefore, the two streams may remain outside of the inversion range and be easier to separate.

[0051] In an embodiment, the flow of liquid out of the cyclonic separator 202 may be controlled by a level control or a back pressure control, or any combination thereof, located on the cyclonic separator 202. The level control or back pressure control may allow for the extraction of the appropriate amount of liquid from the cyclonic separator 202 according to the application or the operation of the gravity separation vessels 204 and 206 downstream of the cyclonic separator 202. In another embodiment, any other type of controls may be used in conjunction with the system 200 to regulate the flow of liquid from one component of the system 200 to another.

[0052] The oil continuous stream 208 flows into a first gravity separation vessel 204, and the water continuous stream 210 flows into a second gravity separation vessel 206. The cyclonic separator 202 separates mixtures of oil and water that may be in the inversion range prior to the gravity separation by the gravity separation vessels 204 and 206.

[0053] Once the oil continuous stream 208 has entered the gravity separation vessel 204, the oil and the water within the fluid may be separated using conventional gravity separation techniques. The less dense oil may float to the top and exit as oil stream 212, while the denser water may sink to the bottom and exit as water stream 214. The same process may occur once the water continuous stream 210 has entered the gravity separation vessel 206. The oil may exit as oil stream 216 at the top of the gravity separation vessel 206, and the water may exit as water stream 218 at the bottom of the vessel. The pressure, fluid level, and temperature within the gravity separation vessels 204 and 206 may be monitored using sensors 220, 222, and 224, respectively.

[0054] The oil streams 212 and 216 containing the oil continuous phase may be joined into one oil stream 226, while the water streams 214 and 218 containing the water continuous phase may be joined into one water stream 228. Diaphragm motor valves (DMV) 230 may be used as control valves to adjust the amount of flow of streams 226 and 228. For example, the DMVs 230 may be partially opened or partially closed to adjust the velocity and pressure of the streams 226 and 228. In addition, the oil-in-water concentration of the stream 228 may be monitored by an OIW sensor 232. The amount of oil in the water continuous phase at any point in time may be used to determine the appropriate action to take with respect to the fluid. For example, if a large amount of oil is left in the water continuous phase, the fluid may be passed through another cyclonic separator, or de-oiler, to salvage as much oil as possible.

[0055] The system 200 may also include additional control features. For example, a fluid level sensor 222 may determine the fluid level in the pipes and send the information regarding the fluid level to a computing device. The computing device may include a programmable logic controller (PLC), distributed control system (DCS), or a direct digital controller (DDC), among others. The computing device may use the fluid level information to control the DMV 226, as indicated by the dotted line 234. The DMV 226 may act as an effector by adjusting the position of the valve to allow for increased or decreased fluid flow. In another embodiment, the DMV 226 may be a smart valve, which may act as both the computing device and the effector. In addition, it should be noted that multiple DMVs or other effectors, for example, pumps or PMVs, may be connected to one computing device and controlled based on changes to multiple different sensors.

[0056] In an embodiment, any number of additional gravity separation vessels may be used in conjunction with the system 200. For example, the gravity separation vessels 204 and 206 may be configured to operate in parallel with two additional gravity separation vessels to allow for a higher degree of separation of the oil from the water. As another example, two additional gravity separation vessels may be configured to operate in series with and downstream of the gravity separation vessels 204 and 206.

[0057] In another embodiment, additional cyclonic separators may be used downstream of the first cyclonic separator 202. The additional cyclonic separators may be placed upstream or downstream of the gravity separation vessels 204 and 206, or may replace the gravity separation vessels 204 and 206. In yet another embodiment, the cyclonic separator 202 may be replaced with a bundle of multiple cyclonic separators arranged in series or parallel. The use of multiple cyclonic separators may allow for a more efficient separation process.

[0058] Fig. 3 is a schematic of a complete system 300 for separating gas, oil, water, and sand. In Fig. 3, like numbered items are as discussed with respect to Figs. 1 and 2. Liquid may flow into the system through an initial control valve 302 from the central pipeline 108. A series of sensors 304, 306, 308, and 310 may be used to measure the fluid pressure, temperature, multiphase flow rate, and sand content, respectively, as the fluid flows through the initial control valve 302 and into the gas liquid separator 312. The gas liquid separator 312 may be used for bulk separation of the gas phase from the liquid phase. The pressure, temperature, and fluid levels within the gas liquid separator may be monitored using sensors 314, 316, and 318, respectively. Once the gas phase and the liquid phase have been separated by the gas liquid separator 312, the gas may flow out as gas stream 320, and the liquid may flow out as liquid stream 322.

[0059] A DMV 323 may be controlled by the feedback from the sensors 314, 316, and 318. The feedback may be used to determine whether the DMV 323 should be opened, closed, or partially opened or closed, as indicated by the dotted line 324. The flow of the gas stream 320 may also be monitored using a sensor 326. When the DMV 323 is open, the gas stream 320 may flow into the gas polisher 328. The gas polisher 328 may be used to purify the natural gas. The differential pressure within the gas polisher 328 may be monitored using a differential pressure sensor 330. The value measured by the differential pressure sensor 330 may be used to control the position of the DMV 332, which controls the flow of an outlet gas stream 334, as indicated by the dotted line 336. When the DMV 332 is open, the gas stream 334 from the gas polisher 328 may be sent to the collection platform 122 (not shown), as discussed with respect to Fig. 1 . In addition, the flow of the gas stream 334 may be controlled by an orifice plate 335. The orifice plate 335 may be used to control the pressure of gas stream 334 in order to reduce the possibility of backflow of the gas stream 334 into any downstream lines. It should be noted that any number of additional orifice plates 335 may be located within the system 300 and may be used to control the pressure of various streams within the system 300.

[0060] A level sensor 338 may also be used to measure the fluid level within the gas polisher 328 and may be used to control a DMV 340, as indicated by the dotted line 342. When the DMV 340 is open, the liquid that has been separated from the natural gas by the gas polisher 328 may flow into the system 200 as liquid stream 344. The liquid stream 344 may be coupled to liquid stream 346 as it enters system 200.

[0061] The flow rate of the liquid stream 322 from the gas liquid separator 312 may be monitored by a sensor 348. The liquid stream 322 may flow into a desander 350, for example, to cyclonically separate the sand from the liquid stream 322. A DMV 352 may be used to control the outflow of sand from the desander 350 as stream 354. The DMV 352 may be controlled by feedback from a differential pressure sensor 356, which measures the differential pressure between streams 322, 346, and 354. When the DMV 352 is open, stream 354 may flow into a sand accumulator 358.

[0062] From the sand accumulator 358, the sand may take several routes. The sand may be released as stream 360 through a PMV 362. The sand may be ejected as stream 360 into the outflowing water stream. The PMV 362 may be either entirely open or entirely closed, depending on the specification of the operator or specific parameters of system 300. The sand content and pressure within the sand accumulator 358 may be monitored using sensors 364 and 366, respectively. The values measured by the sensors 364 and 366 may be used to control the position of PMV 362.

[0063] In addition, the DMV 367 may be used to control the flow of sand out of the sand accumulator 358. In an embodiment, the DMV 367 may remain closed as the sand accumulator 358 becomes pressurized as it fills with sand. Once the sand accumulator 358 has reached a certain pressure level, the DMV 367 may open to allow the sand accumulator 358 to be emptied. [0064] For a safety measure, a stream 368 may be allowed to flow out of the top of the sand accumulator 358 if the sand accumulation level becomes too high. The primary purpose of stream 368 is to prevent the failure of system 300 through the clogging of the sand accumulator 358 in the case of the failure of PMV 362. In addition, any remaining liquid in the sand accumulator 358 may be released as stream 370 through a PMV 372. In an embodiment, stream 370 may include the liquid (mostly water) from which the sand has settled and may be released from the top of the sand accumulator 358. Stream 370 may allow for the maintenance of mass balance within the sand accumulator 358 as stream 354 flows into the accumulator 358.

[0065] The liquid stream 346 from desander 350 may flow out as to be input into the separation cyclone 202. A DMV 374 may regulate the flow of liquid stream 346 into the system 200. The DMV 374 may be controlled using feedback from the level sensor 318, which determines the fluid level within the gas liquid separator 312, as indicated by the dotted line 376.

[0066] The cyclone 202 may receive incoming liquid streams 344 and 346. The system 200 may be used to separate the oil from the water, as discussed with respect to Fig. 2. The cyclonic separator 202 may be used to create two fluid streams. The cyclonic separator 202 may send an oil continuous stream 208 to the gravity separation vessel 204 and a water continuous stream 210 to the gravity separation vessel 206. The oil from the gravity separation vessels 204 and 206 may be collected into oil streams 212 and 216, while the water may be collected into water streams 214 and 218, respectively.

[0067] As discussed with respect to Fig. 2, the pressure, fluid level, and temperature within the gravity separation vessels 204 and 206 may be monitored using sensors 220, 222, and 224, respectively. The fluid level information may be used to control the position of the DMV 230, as indicated by the dotted line 234. In addition, the information from the sensors 220, 222, and 224 may be used to control the positions of various other control valves, including DMV 376 and DMV 378, as indicated by dotted lines 380 and 382, respectively. The DMV 376 may control the flow of stream 384 into the gravity separation vessel 204. The DMV 378 may control the flow of a gas stream 386 from the gravity separation vessel 204 to the gas outlet stream 334. An additional stream 388 may also be directed to the gravity separation vessel 204 from stream 368. The flow of stream 388 may be controlled by the DMV 390. The stream 388 may include any fluid that was remaining the in the stream 368.

[0068] Once the oil streams 212 and 216 are combined into one oil stream 226, the DMV 230 may control the flow of the oil stream 226. When the DMV 230 is open, the oil stream 226 may flow to an oil pump 392. After the oil has passed through the oil pump 392, it may be flow as stream 394 to the platform 122 (not shown), as discussed with respect to Fig. 1 .

[0069] Once the water streams 214 and 218 are combined into one water stream 228, the DMV 230 may control the flow of the water stream 228. When the DMV 230 is open, the water stream 228 may flow into a bulk de-oiler 396. The bulk-de-oiler 396 may be a type of cyclonic separator that is used to separate oil droplets from water droplets. Any remaining oil in the water stream 228 may be separated from the water by the bulk de-oiler 396 and sent as oil stream 398 to be combined with the main oil stream 226 upstream of the oil pump 392. The flow of oil stream 398 may be controlled by the DMV 400. Once the water has been separated from the oil in the bulk de-oiler 396, the water stream 402 may be sent to a second de-oiler 404. In addition, the differential pressure between oil stream 398 and water stream 402 may be measured by the differential pressure sensor 406. The differential pressure value measured by the sensor 406 may be used as feedback to control the DMV 400, as indicated by the dotted line 408. The DMV 400 may be used to control the flow of oil stream 398.

[0070] The second de-oiler 404 may be used to ensure an even higher degree of oil and water separation by repeating the separation process one more time. The oil which is separated from the water within the second de-oiler 404 may be sent as oil stream 410 to be combined with oil stream 398. A differential pressure sensor 412 may measure the differential pressure between oil streams 402 and 410 and value of the measurement may be used to control the position DMV 414, as indicated by dotted line 416. The DMV 414 may control the flow of oil stream 410. Once the remaining oil has been separated from the water by the second de-oiler 404, the water stream 418 may be sent to a water injection pump 420. The sensor 422 may be used to measure the final oil-in-water concentration of water stream 418. In addition, water stream 370 from the sand accumulator 358 may flow into water stream 418. In an embodiment, if the oil-in-water concentration is considered to be low enough, the water injection pump 420 may be omitted, and the water stream 418 may be released into the ocean. In some circumstances, additional purification of the water may also take place before releasing the water stream 418 into the ocean. In another embodiment, the water stream 424 exiting the injection pump 420 may be sent to the platform 122, as discussed with respect to Fig. 1 . The sand streams 360 and 368 may also flow into stream 424 to be injected, released into the ocean, or sent to the platform 122 (not shown).

[0071] It should be noted that the system 300 is not limited to the configuration shown but, instead, may be arranged in any number of different ways using any number of different components. For example, additional DMVs, PMVs, and sensors may be added to the system 300 to improve the functioning of the system 300. As another example, more de-oilers may be added to the system in addition to the de- oilers 396 and 404.

[0072] Fig. 3 illustrates a four-phase separation system that may achieve the separation of gas, oil, water, and sand on the sea floor. The majority of the components of system 300 may be designed based on pipe code such that the required wall thicknesses are greatly reduced while still being useful in deepwater operations. In addition, the use of system 300 as a subsea separation system may allow for the separation of oil and water in the inversion range without the use of separation enhancers, which are costly and may limit capacity.

[0073] Fig. 4 is a schematic of a complete system 426, including an electrostatic coalescer 428, for separating gas, oil, water, and sand. The system 426 is the same as system 300, except for the addition of the electrostatic coalescer 428 upstream of the cyclonic separator 202. Thus, in Fig. 4, like numbers are as discussed with respect to the previous figures. An "electrostatic coalescer" is a device that may be used to separate an emulsion into its components, e.g., water and oil, in this case, by subjecting the emulsion to a high-voltage electrical field. The electrical field causes the water droplets in the emulsion, which are conductive, to separate from the oil droplets, which are non-conductive, by combining. The separation of the water from the oil in the fluid may help to avoid the inversion range.

[0074] As shown in Fig. 4, the electrostatic coalescer 428 may be positioned immediately upstream of the cyclonic separator 202. The location of the electrostatic coalescer upstream of the cyclonic separator 202 may enhance the coalescence and separation of the dispersed phase, e.g., water, in the fluid. While the radial acceleration of the swirl element within the cyclonic separator 202 may be sufficient for separating the water and oil phases, the use of the electrostatic coalescer 428 may be beneficial, particularly in the case of emulsion formation or the presence of high-viscosity oil within the fluid.

[0075] In another embodiment, the electrostatic coalescer 428 may be positioned downstream of the cyclonic separator 202 and upstream of the gravity separation vessel 204. In this case, the electrostatic coalescer 428 may be used for the same purpose as in the previous embodiment. However, it should be noted that an electrostatic coalescer 428 will turn off automatically if the fluid mixture approaches water continuity in order to avoid a short out and to conform to safety standards. Therefore, an electrostatic coalescer may not be positioned upstream of gravity separation vessel 206, since the water continuous stream 210 flows into gravity separation vessel 206.

Cyclonic Separator

[0076] Fig. 5 is an illustrative view of a cyclonic separator 500 that may be used to separate oil and water streams. In Fig. 5, like numbers are as discussed with respect to the previous figures. The stream may enter the vessel 502 of the cyclonic separator 500 as stream 346, as discussed with respect to Figs. 2 and 3. As the fluid enters the vessel 502, a swirl element 504 within the vessel 502 may impart a radial acceleration and a tangential velocity component to the fluid through the rotation of twisted swirl vanes. The swirl vanes may be arranged parallel or in series on the swirl element 504. The swirling of the fluid using the swirl element may be controlled to maintain the radial acceleration at a value at which the two phases separate into two continuous phases, while minimizing the turbulence to avoid shearing of the fluid. If shearing occurs within the fluid, an emulsion of oil and water may form. Once an emulsion has formed, it becomes even more challenging and costly to separate the oil and water. Therefore, in order for the cyclonic separator 202 to be effective, the centrifugal force that is generated should be enough to effect bulk separation of the oil and water, but not enough to cause significant shearing effects within the fluid. To minimize the shearing, the radial acceleration of the fluid may be maintained at a value which does not cause a total pressure drop exceeding 1 bar in the fluid.

[0077] The radial acceleration imparted to the fluid may cause the fluid to begin swirling through the vessel 502 due to the generated centrifugal force. The heavier and denser water droplets may migrate to the outer rim of the vessel 502 and begin traveling in a wide circular path, while the lighter and less dense oil droplets may migrate towards the center of the vessel 502 and begin traveling in a narrow circular path. As the fluid continues to move through the vessel 502, the fluid may be separated into two phases, an oil continuous phase, and a water continuous phase. As the fluid nears the end of the vessel 502, a vortex finder 506 may be used to capture the oil continuous phase and send it out as oil stream 208, while an outlet 508 may be used to capture the water continuous phase and send it out as water stream 210.

[0078] An antiswirl device (not shown) may be positioned downstream of the cyclonic separator 500. The antiswirl device may be used to reduce the tangential velocity component of the oil stream 208 or the water stream 210 perpendicular to the flow path. The antiswirl device may help to align the flow path of a stream before the fluid enters a gravity separation vessel, lessening the likelihood the tangential velocity may cause mixing in the gravity separation vessel.

[0079] Fig. 6 is an illustrative view 600 of a swirl element 504 that may be used in the cyclonic separator 500. In Fig. 6, like numbers are as discussed with respect to the previous figures. The swirl element 504 may be affixed inside the cyclonic separator pipe 502, near the inlet of stream 346. The swirl element may include several twisted swirl vanes 602 that are used to swirl the stream 346 within the cyclonic separator pipe 502. The swirl vanes 602 may be arranged parallel or in series on the swirl element 504 and may be positioned at a particular angular orientation in order to effectively control the swirling of the fluid. The radial acceleration may be maintained at a value which causes the separation of the two phases while preventing the generation of shearing forces within the fluid. If shearing of the fluid occurs, an emulsion may form. Emulsion generation may make it more difficult to separate two phases, due to the strong interaction forces between the individual particles of the two phases. [0080] As the fluid flow rotates downstream of the swirl element 602, the oil continuous phase, indicated by the dark area in Fig. 6, moves to the core of the cyclonic separator pipe 502, while the water continuous phase, indicated by the light area in Fig. 6, moves toward the outer wall of the pipe 502. The swirl element 504 creates a gentle rotation within the fluid, thereby utilizing the centrifugal force of the rotation to move the heavier, denser water droplets within the fluid toward the outer wall. The ultimate effect is to increase the number of water droplet interactions and oil droplet interactions and, thus, coalescing the droplets in the stream and removing the water phase from the oil phase.

Method for Separation

[0081] Fig. 7 is a process flow diagram showing a method 700 for the separation of oil and water streams. The method 700 may be useful for the harvesting of hydrocarbons from an oil well in both subsea and topside environments. In addition, method 700 may separate oil and water streams efficiently by avoiding the gravity separation of the two phases in the inversion range.

[0082] At block 702, the stream may be separated into an oil continuous stream and a water continuous stream using a cyclonic separator. The cyclonic separator may use a number of swirl vanes arranged parallel or in series to generate radial acceleration within the stream, as discussed with respect to Figs. 5 and 6. The radial acceleration within the cyclonic device may also be controlled to ensure effective separation of the oil continuous phase and the water continuous phase.

[0083] At block 704, the oil continuous stream may be allowed to flow into a first gravity separation vessel. The oil continuous stream may be directed from the cyclonic separator to the first gravity separation vessel using a vortex finder extended through the center of the cyclonic separator pipe. An antiswirl device may be used to straighten the flow of the oil continuous stream upstream of the first gravity separation vessel.

[0084] At block 706, the water continuous stream may be allowed to flow into a second gravity separation vessel. The water continuous stream may be directed from the cyclonic separator to the second gravity separation vessel through an outlet on the bottom of the cyclonic separator pipe. The outlet may capture the water continuous stream as it flows in a wide circular path around the rim of the cyclonic separator pipe. An antiswirl device may be used to straighten the flow of the water continuous stream upstream of the second gravity separation vessel.

[0085] At block 708, the oil may be separated from the water in the first gravity separation vessel using gravity separation techniques. Because water is heavy and denser than oil, the water will settle at the bottom of the vessel, while the oil will float to the top. At block 710, the oil may be separated from the water in the second gravity separation vessel using the same gravity separation techniques as those discussed with respect to block 708.

[0086] It should be noted that the process flow diagram is not intended to indicate that the steps of method 700 must be executed in any particular order or that every step must be included for every case. Further, additional steps may be included which are not shown in Fig. 7. For example, the two oil streams may be combined into a single oil stream, and the two water streams may be combined into a single water stream downstream of the first and second gravity separation vessels.

[0087] While the present techniques may be susceptible to various modifications and alternative forms, the exemplary embodiments discussed above have been shown only by way of example. However, it should again be understood that the technique is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

Embodiments

[0088] Embodiments of the invention may include any of the following methods and systems, among others, as discussed herein. This is not to be considered a complete listing of all possible embodiments, as any number of variations can be envisioned from the description above.

[0089] An exemplary embodiment provides a method for separating oil and water streams. The method includes separating a fluid stream into an oil continuous stream and a water continuous stream using a cyclonic separator, flowing the oil continuous stream to a first gravity separation vessel, and flowing the water continuous stream to a second gravity separation vessel. The method also includes separating the oil continuous stream in the first gravity separation vessel into an oil stream and a water stream and separating the water continuous stream in the second gravity separation vessel into an oil stream and a water stream.

[0090] In some embodiments, the method may include combining the oil streams into a single oil stream and combining the water streams into a single water stream.

[0091] In some embodiments, the method may include using a swirl element within the cyclonic separator to impart radial acceleration to the fluid stream.

[0092] In some embodiments, the method may include controlling a radial acceleration to avoid forming an emulsion.

[0093] In some embodiments, the method may include controlling the radial acceleration using a plurality of swirl vanes arranged in parallel or in series on the swirl element.

[0094] In some embodiments, the method may include generating the radial acceleration within the fluid stream with a total pressure drop of less than about 1 bar.

[0095] In some embodiments, the method may include using a vortex finder within the cyclonic separator to remove the oil continuous stream.

[0096] In some embodiments, the method may include using an electrostatic coalescer upstream of the cyclonic separator to create larger water droplets.

[0097] In some embodiments, the method may include using an electrostatic coalescer downstream of the cyclonic separator and upstream of the first gravity separation vessel.

[0098] In some embodiments, the method may include automatically shutting off the electrostatic coalescer if the fluid stream approaches a water continuous phase.

[0099] In some embodiments, the method may include using an additional cyclonic separator downstream of the first gravity separation vessel or the second gravity separation vessel, or both, for further separation of oil from water.

[0100] Another exemplary embodiment provides a system for separating oil and water streams. The system includes a cyclonic separator configured to separate a fluid stream into an oil continuous stream and a water continuous stream, a first gravity separation vessel configured to separate the water continuous stream into a first oil stream and a first water stream, and a second gravity separation vessel configured to separate the oil continuous stream into a second oil stream and a second water stream.

[0101] In some embodiments, the system includes an electrostatic coalescer upstream of the cyclonic separator.

[0102] In some embodiments, the system includes an electrostatic coalescer on the oil continuous stream.

[0103] In some embodiments, the system includes a swirl element within the cyclonic separator comprises a plurality of swirl vanes arranged parallel or in series.

[0104] In some embodiments, the system includes an antiswirl device for straightening a flow path of the water continuous stream or the oil continuous stream, or both, downstream of the cyclonic separator.

[0105] Another exemplary embodiment provides a method for separating two immiscible phases from a fluid stream. The method includes sending the fluid stream into a cyclonic separator, generating radial acceleration within the cyclonic separator using a swirl element, and controlling the radial acceleration at a value at which the two immiscible phases separate into two continuous phases. The method also includes removing the two continuous phases from the cyclonic separator into two lines using a vortex finder and sending the two continuous phases to two separate downstream vessels for further separation of the two immiscible phases.

[0106] In some embodiments, the method includes controlling the radial acceleration of the fluid stream by selecting an angular orientation of at least one swirl vane on the swirl element.

[0107] In some embodiments, the method includes decreasing the tangential velocity component of the fluid stream perpendicular to a flow path using an antiswirl device downstream of a point at which the radial acceleration was generated.

[0108] In some embodiments, the method includes controlling the swirling of the fluid stream to maintain the radial acceleration at a value at which shearing of the two immiscible phases does not cause an emulsion to form.

Claims

CLAIMS What is claimed is:

1 . A method for separating oil and water streams, comprising:

separating a fluid stream into an oil continuous stream and a water continuous stream using a cyclonic separator;

flowing the oil continuous stream to a first gravity separation vessel;

flowing the water continuous stream to a second gravity separation vessel; separating the oil continuous stream in the first gravity separation vessel into an oil stream and a water stream; and

separating the water continuous stream in the second gravity separation vessel into an oil stream and a water stream.

2. The method of claim 1 , further comprising:

combining the oil streams into a single oil stream; and

combining the water streams into a single water stream.

3. The method of claim 1 , comprising using a swirl element within the cyclonic separator to impart radial acceleration to the fluid stream.

4. The method of claim 2, comprising controlling a radial acceleration to avoid forming an emulsion.

5. The method of claim 4, comprising controlling the radial acceleration using a plurality of swirl vanes arranged in parallel or in series on the swirl element.

6. The method of claim 3, comprising generating the radial acceleration within the fluid stream with a total pressure drop of less than about 1 bar.

7. The method of claim 1 , comprising using a vortex finder within the cyclonic separator to remove the oil continuous stream.

8. The method of claim 1 , comprising using an electrostatic coalescer upstream of the cyclonic separator to create larger water droplets.

9. The method of claim 1 , comprising using an electrostatic coalescer downstream of the cyclonic separator and upstream of the first gravity separation vessel.

10. The method of claim 8, comprising automatically shutting off the electrostatic coalescer if the fluid stream approaches a water continuous phase.

1 1 . The method of claim 1 , comprising using an additional cyclonic separator downstream of the first gravity separation vessel or the second gravity separation vessel , or both, for further separation of oil from water.

12. A system for separating oil and water streams, comprising:

a cyclonic separator configured to separate a fluid stream into an oil continuous stream and a water continuous stream;

a first gravity separation vessel configured to separate the water continuous stream into a first oil stream and a first water stream; and

a second gravity separation vessel configured to separate the oil continuous stream into a second oil stream and a second water stream.

13. The system of claim 12, comprising an electrostatic coalescer upstream of the cyclonic separator.

14. The system of claim 12, comprising an electrostatic coalescer on the oil continuous stream.

15. The system of claim 12, wherein a swirl element within the cyclonic separator comprises a plurality of swirl vanes arranged parallel or in series.

16. The system of claim 12, comprising an antiswirl device for straightening a flow path of the water continuous stream or the oil continuous stream, or both, downstream of the cyclonic separator.

17. A method for separating two immiscible phases from a fluid stream, comprising:

sending the fluid stream into a cyclonic separator;

generating radial acceleration within the cyclonic separator using a swirl element;

controlling the radial acceleration at a value at which the two immiscible phases separate into two continuous phases;

removing the two continuous phases from the cyclonic separator into two lines using a vortex finder; and sending the two continuous phases to two separate downstream vessels for further separation of the two immiscible phases.

18. The method of claim 17, comprising controlling the radial acceleration of the fluid stream by selecting an angular orientation of at least one swirl vane on the swirl element.

19. The method of claim 17, comprising decreasing the tangential velocity component of the fluid stream perpendicular to a flow path using an antiswirl device downstream of a point at which the radial acceleration was generated.

20. The method of claim 17, comprising controlling the swirling of the fluid stream to maintain the radial acceleration at a value at which shearing of the two immiscible phases does not cause an emulsion to form.