GB2501261A

GB2501261A - A method of cleaning water to remove hydrocarbon

Info

Publication number: GB2501261A
Application number: GB201206699A
Authority: GB
Inventors: Svein Viggo Aanesen; Cecilie Fjeld Nygaard; Juliette Diouma Leyris; Karl Erik Larsen; Arne Henriksen; Ba E Bente
Original assignee: Statoil Canada Ltd
Current assignee: Statoil Canada Ltd
Priority date: 2012-04-17
Filing date: 2012-04-17
Publication date: 2013-10-23
Also published as: GB201206699D0

Abstract

A method of cleaning water to remove hydrocarbon, comprises: (i) adding an external diluent to the water; and (ii) removing the hydrocarbon together with the diluents from the water. The method may be used to clean water of a hydrocarbon/water mixture extracting using steam in a subterranean formation, such as an oil well. The external diluent may be added to the water prior to cooling, and may be a mixture of C30-60 hydrocarbons with a boiling point in the range of 30-130ÂºC. Examples of the diluents include naphtha, light crude oil or gas oils, synthetic oil, gas condensates, or mixtures of these. A system for cleaning water by removing hydrocarbon comprises: (a) a means for adding an external diluent 102 to water separated in a bulk separator 105; and (b) a means for removing hydrocarbon together with the diluents from the water, and comprising an outlet for water. The means for adding an external diluent comprises at least one inlet for the external diluent in between the bulk separator and the means for removing hydrocarbon from the water.

Description

Method

INTRODUCTION

The present invention relates to a method and system for cleaning water, typically extracted from a subterranean formation, to remove hydrocarbon therefrom, wherein an external diluent, e.g. liquid hydrocarbon, is added to the water. The invention also relates to a method for separating a mixture comprising hydrocarbon and water wherein the separated water is cleaned by the method herein described.

BACKGROUND TO THE INVENTION

Heavy hydrocarbons, e.g. bitumen, represent a huge natural source of the world's total potential reserves of oil and specialist methods have been developed for recovering such hydrocarbons. A number of these methods utilise steam to mobilise the hydrocarbon, e.g. steam assisted gravity drainage (SAGD), hot solvent extraction, VAPEX, ISC and cyclic steam stimulation (CSS).

The mobilised hydrocarbon recovered at the surface by these methods is in the form of a mixture with water from condensed steam and formation water. A diluent is usually added to this mixture to reduce its viscosity. The diluent used is generally a lighter hydrocarbon such as naphtha, a light crude oil, a gas condensate or synthetic crude. The dilution of the mobilised hydrocarbon/water mixture with a diluent typically reduces its overall API to about 20 degrees enabling it to be pumped to a processing plant.

In the plant the hydrocarbon/water mixture is generally treated to separate the hydrocarbon from the produced water. Ideally the water that is obtained from this separation process is recycled by using it for the generation of further steam in a steam generator. Usually, however, the water must first be cleaned or purified to render it suitable for feeding to a steam generator, e.g. a once through steam generator (OTSG). Otherwise the oil, organics, minerals and inorganic salts present in the water precipitate out to form deposits that stick to the heat surfaces of the boiler in a process often referred to as "fouling". The deposits form a thermal barrier on the heat surfaces and increase the temperature of the surfaces which ultimately reduces the strength of their material and their service lifetime. The deposits also reduce the heat transfer to water to generate steam thus reducing the quantity and quality of the steam subsequently produced by the steam generator. Boilers generally need to be taken out of operation at regular intervals for cleaning and maintenance to remove deposits created by fouling. The higher the degree of fouling the shorter the operational periods between cleaning and maintenance are.

To minimise the amount of fouling that occurs in a steam generator, e.g. an OTSG, valious different methods aie employed to treat watel iecoveied from hydrocarbon production prior to recycling it for steam generation. In the commercial process operated today, the hydrocarbon/water mixture to which diluent is usually added is geneially separated in a bulk separator to yield a hydiocarbon/diluent fiaction and a water fraction. Emulsion breakers are usually added to improve the separation process. The water fraction is generally cooled and then sent to a skim tank, gas floatation tank and/or oil removal filter wherein further hydrocarbon impurities are removed. Flocculants andloi coagulants may optionally be added duiing these latter stages. Further chemical treatments to reduce water hardness and silica content are often carried out on the resulting water prior to its pumping to a boiler.

There are, however, drawbacks associated with this process. Although the watel finally produced by the process meets the cuirent requirements for use in steam generators, e.g. <50 ppm wt hydrocarbon, it still causes some fouling in the generators and they must regulaily be taken out of service for cleaning and maintenance. The coolers used in the processing of the water fraction are also prone to fouling. The water that enters the coolers, namely the water obtained from the initial bulk separation, still tends to contain at least 0.01 %wt hydrocarbon and could potentially contain up to about 4 %wt hydrocaibon, and as a iesult fouling occurs. Relatively high amounts of chemicals such as emulsion breakers, flocculants and coagulants also need to be added to, e.g. the separator, skim tank and flotation tank, to improve the separation piocesses. The use of such chemicals is, howevei, expensive.

Attempts have been made to improve the above-described process to increase the purity of the water recycled for steam generation. A number of attempts have focussed on improving the separation that occurs in the bulk separator, i.e. on the initial separation of the hydiocarbon and water mixtule extracted from a foimation.

W02010/004266 discloses a method wherein a lipophilic liquefied gas is injected into a mixture comprising gas, hydrocarbon, water and solids to be separated by a separator.

The lipophilic liquefied gas is pieferably a gas condensate and is preferably intioduced into the separator in an aqueous carrier.

EP-A-1,783,1O1 discloses a related method wherein a C13 hydrocarbon is injected into a water, oil and gas mixture in an oil/water separator. In the method of EP-A-1,783,1O1 water is separated from oil and gas in a first separator and the resulting water fraction which still comprises some oil and gas is led to a liquid-liquid/gas separator. A condensed Ci-8 hydrocarbon is added into this separator. The Cl-B hydrocarbon becomes substantially gaseous due to the release of pressure. This is said to lead to a more efficient separation of oil/gas from water. It is thought that the C18 hydrocarbon fluid absorbs the oil in the water and that the gas bubbles of fluid enhance the separation. It is therefore essential that the C1-8 hydrocarbon fluid converts to gaseous form in the separator. In the separator the mixture is separated into a water fraction and an oil/gas fraction. The oil/gas fraction is then separated into a light fraction, which includes the added C1-8 hydrocarbon, and a heavy traction. The light fraction is subsequently condensed in a condensation vessel and the C1-8 hydrocarbon is obtained therefrom. It is then pressurised by a pump and recycled for injection into the separator.

WO01/58813 discloses a different method which targets separation of oil soluble components such as naphthalene and phenantrene (NPD), polyaromatic hydrocarbons (PAH) and benzene/toluene/ethyl benzene/xylene (BTEX), rather than dispersed oil. In the method of WO01/58813 a liquid condensate is separated from the extracted oil that has a lower concentration of water soluble oil components than the water phase and is then injected into the water phase. Since the liquid condensate has a lower concentration of oil soluble components mass transfer occurs from the water to the condensate. The mass transfer is optimised by injecting the liquid condensate through a nozzle to produce a fine distribution having a maximum surface area and therefore contact area with the water. The liquid condensate, along with the oil soluble components that have transferred into it, are then separated from the water in a separator such as a hydrocyclone.

The method disclosed in WOO1/58813 does, however, have some disadvantages. First WO01/58813 highlights the importance of choosing the correct liquid condensate for use in the extraction process. WO01/58813 therefore advocates the inclusion of a device such as a rectifier or fractionater to ensure that a condensate with the required low concentrations of NPD, PAH and BTEX can be obtained.

Alternatively stripping, adsorption or absorption equipment may be employed to remove NPD, PAH and BTEX from condensates. In all cases, however, there is need to include additional equipment specifically for producing suitable liquid condensate for use in the extraction process as well as for piping to route the liquid condensate to the separated water. Second, as noted by WO01/58814, the temperature of the condensate obtained from the, e.g. rectifier or fractionation column, is often lower than that of the separated water to which it is to be added therefore the risk of gas being formed is high. As a result, WO01/58814 advocates the inclusion of a heat exchanger in the liquid condensate line prior to the point at which it is mixed with water. A means to remove any gas produced in the heat exchanger must then also be provided.

A need therefore exists for an alternative method for cleaning separated water that utilises conventional equipment and is straightforward to put into practice.

Methods requiring the use of fewer specialist chemicals such as emulsion breakers, flocculants and coagulants are particularly attractive. Naturally methods that yield water of high purity that reduces steam generator and cooler fouling are especially attractive.

It has now been discovered that a method of cleaning separated water wherein an external diluent is added thereto is highly advantageous. Since the diluent is added to the water after the separation stage in a bulk separator, conventional separation equipment may be used for the bulk separation. The presence of the external diluent during subsequent cleaning steps wherein hydrocarbon is removed from the separated water, however, improves the purity of the final water obtained. The use of an external diluent to perform this function is highly beneficial. The external diluent is readily available at the plant, therefore its use avoids the need to include any equipment in the plant specifically for the production and transport of diluent. A further significant advantage is that the external diluent can be recycled with hydrocarbon without any need to carry out a separation.

SUMMARY OF INVENTION

Viewed from a first aspect, the present invention provides a method of cleaning water to remove hydrocarbon therefrom comprising: (i) adding an external diluent to said water; and (H) removing said hydrocarbon together with said diluent from said water.

Viewed from a further aspect, the present invention provides a method of separating a mixture comprising hydrocarbon and water wherein said method comprises: (i) separating said mixture in a separator to produce separated hydrocarbon and separated water; and (H) cleaning said separated water by a method as hereinbefore described.

Viewed from a still further aspect, the present invention provides a system for cleaning water to remove hydrocarbon therefrom comprising: (a) a means for adding an external diluent to water separated in a bulk separator; and (b) a means for removing hydrocarbon together with said diluent from said water and comprising an outlet for water, wherein said means for adding an external diluent comprises at least one inlet for said external diluent in between said bulk separator and said means for removing hydrocarbon from said water.

Preferred systems further comprise a cooler. Further preferred systems comprise a dispersing device for dispersing said external diluent in said water.

Preferred methods and system remove dispersed and/or dissolved hydrocarbon from the water.

DESCRIPTION OF THE INVENTION

As used herein the term external diluent refers to a diluent that is produced or supplied from outside, or independently to, the methods of the invention. More specifically an external diluent is not produced or supplied from the hydrocarbon extracted from the formation in combination with the water being cleaned. Preferably the diluent is a liquid hydrocarbon.

In the methods of the present invention water is cleaned or purified. More specifically the amount of hydrocarbon present in the water is reduced. Hydrocarbon may also be completely removed from water in the methods of the invention.

The water cleaned in the methods of the present invention is preferably water separated from a hydrocarbon and water mixture, particularly mixtures extracted from a subterranean formation, e.g. an oil well. A number of methods used to extract mixtures utilise steam, e.g. SAGD, CSS, hot solvent extraction, VAPEX, ISC and combinations thereof. The method of the present invention is particularly useful for cleaning water separated from a hydrocarbon and water mixture extracted by steam based methods since the cleaned water can then be recycled for further steam generation. SAGD is the most commonly used extraction process commercially. Preferably therefore the water cleaned is water separated from a hydrocarbon and water mixture extracted from a hydrocarbon formation using SAGD.

When a hydrocarbon and water mixture is extracted from a formation, it generally undergoes a bulk separation in a bulk separator wherein hydrocarbon! water, gas and solids are separated. The cleaning methods of the present invention are preferably carried out on the separated water obtained from this bulk separation in a bulk separator.

S

The water cleaned in the methods of the invention predominantly comprises water. Preferably at least 90 % by weight, more preferably at least 95 % by weight, yet more preferably at least 97 % by weight, still more preferably at least 98 % by weight, e.g. at least 99 % by weight of the water cleaned is water. The maximum concentration of water may be, for example, 99.9 % by weight.

In the methods of the present invention the water cleaned may comprise up to 5 %wt hydrocarbon. The amount of hydrocarbon present in the water may be, for example, 0.0002 to 5 %wt. More typically, however, the water cleaned in the method of the invention comprises 0.01 to 3 % wt hydrocarbon, more preferably 0.015 to 1 % wt hydrocarbon, still more preferably 0.02 to 0.1 % wt hydrocarbon, e.g. about 0.025 to 0.05 % wt hydrocarbon. Amounts up to 3 or 4 %wt of hydrocarbon can, however, be present. This occurs, for instance, when there is a problem such as the presence of an unstable emulsion in the bulk separation process.

The hydrocarbon present in the water is generally a mixture of different types of hydrocarbon having a range of molecular weights. The hydrocarbon present in the water may be dispersed therein or dissolved therein. The method of the present invention is, however, aimed at removal of hydrocarbon dispersed in the water.

Preferably therefore the hydrocarbon present in the water is not water soluble.

As mentioned above, the methods of the present invention are particularly useful for cleaning water produced by steam assisted extraction methods. Such methods are typically used for extraction of heavy hydrocarbons. Thus in many cases the hydrocarbon present in the water to be cleaned will comprise heavy hydrocarbon.

Heavy hydrocarbons are often characterised by their API gravity. A heavy hydrocarbon preferably has an API gravity of less than about 20°, preferably less than about 15°, more preferably less than 12°, still more preferably less than 10°, e.g. less than 8°.

Generally a heavy hydrocarbon has an API of about 5° to about 15°, more preferably from about 6° to about 12°, still more preferably about 7° to about 12°, e.g. about 7.5-9°.

A key step in the method of the present invention is that an external diluent, especially an external hydrocarbon diluent, is added to the water to be cleaned. The external diluent used in the method of the invention is preferably a liquid. By a liquid is meant herein that the diluent is in liquid form at 20 °C and at atmospheric pressure.

Still more preferably the external diluent is in liquid form throughout the method of the invention. Preferably therefore the diluent does not undergo an evaporation and/or compression process.

Preferably the external diluent has a boiling point in the range 30 to 130°C and more preferably 50 to 95 °C. Preferably the external diluent has a density in the range 700-900 kg/rn3 and rnore preferably 725-850 kg/rn3. Preferably the external diluent has a flash point in the range -10 to -50 °C. Preferably the external diluent is completely stable in the water and does not cause, e.g. precipitation of asphaltenes.

The external diluent added to the water in the methods of the invention is preferably a hydrocarbon diluent. Preferred external hydrocarbon diluents comprise a mixture of C630 hydrocarbons, particularly C1028 hydrocarbons and rnore preferably C12÷ hydrocarbons. External diluents comprising longer hydrocarbons, e.g. C6+ or C1o+ are preferred since they are less likely to cause flashing when they are added to the water. Preferred diluents have an API ot 20-80°, more preferably 30-70°.

Preferred external diluents comprise aromatic hydrocarbons. These hydrocarbons are generally better at removing hydrocarbon impurities than aliphatic hydrocarbons. Preferably the concentration of BTEX in the external diluent is in the range 0 to 3 wt% and more preferably 0.5 to 2 wt%.

Representative examples of suitable external diluents include naphtha, light crude oil or gas oils, synthetic oil, gas condensates and mixtures thereof. External diluent cornprising naptha, light crude oil or gas oil and synthetic oil are generally preferred. External diluent rnay, for example, cornprise 0-100 %wt naptha, 0-70 %wt light crude oil or gas oil, 0-25% gas condensates, 0-3 %wt butane and 0-3 %wt BTEX.

Particularly preferably the external diluent added to the water is the same diluent that is added to the hydrocarbon and water rnixture after it has been extracted from the subterranean forrnation and prior to a bulk separation. As described in more detail below, this facilitates the recycling of the external diluent and the capture of the hydrocarbon and avoids the need for any separation equipment.

The addition of external diluent, especially external hydrocarbon diluent, to the water to be cleaned is counter intuitive since the objective of the rnethod is to rernove hydrocarbon. It has been found, however, that the addition of external diluent after the bulk separation is completed has a number of beneficial effects. First it improves the removal of further hydrocarbon, especially heavy hydrocarbon, from water in subsequent steps, e.g. by skimming in a skirn tank. Second, in addition to rernoving dispersed hydrocarbon, it improves removal of dissolved hydrocarbon. Third it may reduce the amount of specialty chemicals that need to be used in the cleaning process.

Fourth it increases the overall purity of the cleaned water by increasing the amount of hydrocarbon, especially heavy hydrocarbon, removed therefrom, thus facilitating recycling of the cleaned or purified water to a steam generator.

The amount of diluent added to the water to be cleaned varies and depends, for example, on the level of separation achieved in the bulk separator, the nature of the hydrocarbon impurities and the level of purity that the final water must achieve.

Typically, however, the amount of diluent added is 0.05-10 %wt, more preferably 0.1 to %wt and still more preferably 0.5 to 1.5 %wt, e.g. about 0.75 %wt based on the total weight of the water to be cleaned (including any impurities such as hydrocarbon present therein). The diluent is preferably added to the water in neat form, i.e. in the absence of a carrier.

An advantage of the method of the invention is that the diluent may be added to the water to be cleaned over a wide range of temperatures, e.g. in the range 0 to 160 "0. In preferred methods of the invention, however, the water is cooled to a temperature of 80-140 00 and more preferably 90-130 °C. Cooling of the water is preferably carried out in at least one heat exchanger. Still more preferably the water is cooled in a plurailty of heat exchangers connected in series and/or parallel. In some preferred methods the water is cooled prior to removing hydrocarbon (e.g. after bulk separation). In other preferred methods the water is cooled after a first step of removing hydrocarbon. When the latter approach is used, preferably a further hydrocarbon removal step follows cooling (i.e. cooling occurs in between hydrocarbon removal steps).

In some preferred methods of the invention the external diluent is added to the water to be cleaned prior to cooling (e.g. after it is obtained by separation in the bulk separator and prior to cooling). In this case the temperature of the water may be in the range 100-160 °C and more preferably 120-145 °C when the diluent is added. In other preferred methods the external diluent is added to the water after cooling but prior to removal of hydrocarbon therein. In this case the temperature of the water may be in the range 80140 00 and more preferably 90-1 30 00. The addition of external diluentto the water prior cooling is advantageous as may decrease the level of fouling in the coolers. Otherwise the decrease in temperature that occurs in the coolers tends to result in the precipitation of previously dispersed or dissolved components from the water.

The pressure of the water at the point of addition of external diluent may be in the range 0-20 barg and more preferably 3-10 barg. Preferably the pressure of the water is similar to the operational pressure of the bulk separator from which the water is obtained. Adjustments may, however, be made to the pressure of the water in order to keep the water and/or diluent in liquid form.

The amount of external diluent added to the water is so small that the temperature and pressure of the diluent has little influence on the temperature and pressure of the resulting mixture. Prior to the addition the external diluent is preferably stored in a tank on site. An advantage of the use of an external diluent is that it has a wide window of operation. In other words the external diluent can effectively be used in a wide range of temperature and pressure conditions. Preferably the temperature of the external diluent in its storage tank and therefore just prior to addition to water is in the range -15 to 25 °C and more preferably -20 to 20 °C. Preferably the external diluent is stored in its storage tank and therefore just prior to addition to water at atmospheric pressure. To facilitate addition it may be pumped, therefore the pressure of the diluent at the point of addition may be up to 100 kPa.

The addition of the external diluent to water is preferably carried out by adding diluent into the line transporting the water to the cooler and/or the means for removing hydrocarbon therefrom. This may be achieved, for example, by the use of a suitable inlet valve. Preferably a controlled dosing system is used. The resulting diluent-containing water is preferably forced through a dispersing device, e.g. valves, nozzles or mixers, to distribute the external diluent throughout the water. The addition of external diluent in this way is advantageous in that the water and diluent are thoroughly mixed and contact between the diluent and hydrocarbon present in the water is achieved. Often a pressure drop, e.g. of up to 0.5 bar, occurs during the dispersing process.

In a preferred method of the present invention, the hydrocarbon and external diluent present in the water to be cleaned are removed together in a separator. Any conventional separator may be used, e.g. a cyclone, gravity separator, skim tank, flotation tank (e.g. a gas flotation tank) or oil removal filter. The method of the present invention may comprise one or more cleaning steps, e.g. two, three or four cleaning steps. When multiple cleaning steps are used, each step may be carried out in the same or different types of separator. Preferably, however, the method of the invention comprises one or two cleaning steps and still more preferably one cleaning step.

In a preferred method the hydrocarbon and external diluent are removed together in a skim tank. The majority of external diluent added to the water is removed in this process. Any conventional skim tank may be used. Such tanks are commercially available. In the skim tank the hydrocarbon and the majority of added diluent are captured by the skimming material. The external diluent will generally have a significantly lower density than the hydrocarbon present in the water and as a result will float to the surface much faster than, e.g. heavy hydrocarbon droplets. The fast floating diluent droplets also capture heavy hydrocarbon droplets and transport them to the top of the tank. The conditions in the skim tank are those conventionally used.

Preferably the temperature is 70 to 95 °C. Preferably the pressure is atmospheric pressure.

In another preferred method the hydrocarbon and diluent are removed together in a flotation tank. The majority of external diluent added to the water is removed in this process. Any conventional flotation tank, e.g. gas flotation tank, may be used.

Such tanks are commercially available. In the flotation tank the hydrocarbon and the majority of added diluent are captured. The external diluent may improve the performance of the flotation tank because the diluent mixed into the hydrocarbon droplets will start forming bubbles inside oil droplets and thereby induce strong buoyancy thereto. The hydrocarbon and external diluent floating on the surface of the water can then be removed. The conditions in the flotation tank are those conventionally used. Preferably the temperature is 70 to 95 °C. Preferably the pressure is atmospheric pressure.

In a further preferred method the hydrocarbon and external diluent are removed together in a separator, e.g. gravity separator or cyclone. The majority of the external diluent added to the water is removed in this process. Any conventional separator may be used and such separators are commercially available.

The water obtained from the first cleaning step, e.g. after gravity separation, skimming or flotation, preferably comprises 0-20 ppm wt hydrocarbon and more preferably 0-10 ppm wt hydrocarbon. In some methods, the water may be suitable for recycling to a steam generator. Often, however, the water is treated to remove dissolved organics and/or inorganics prior to recycling to a steam generator.

Other preferred methods comprise the further step of removing hydrocarbon present in the water obtained in the first cleaning step in a further separator, skim tank, flotation tank or oil removal filter. In one preferred method the first cleaning step is in a skim tank and the second cleaning step is in a flotation tank. In another preferred method the first cleaning step is in a separator and the second cleaning step is in a skim tank.

The water obtained from the second cleaning step, e.g. the flotation tank preferably comprises 0-20 ppm wt hydrocarbon and more preferably 0-10 ppm wt hydrocarbon. As above, in some methods, the water may be suitable for recycling to a steam generator. Often, however, the water is treated to remove dissolved organics and/or inorganics prior to recycling to a steam generator.

Other methods, however, comprise the further step of removing hydrocarbon present in the water obtained in the second cleaning step in a further separator, skim tank, flotation tank or oil removal filter. Preferably an oil removal filter is used. After filtration the resulting water preferably comprises 0-5 ppm wt hydrocarbon and more preferably 0-1 ppm wt hydrocarbon.

A preferred method of the invention comprises cooling the water, removing the hydrocarbon together with the external diluent from cooled water in a skim tank, flotation tank and/or oil removal filter, preferably connected in series. Another preferred method comprises removing hydrocarbon together with external diluent from the water, cooling the resulting water and removing further hydrocarbon and diluent from the cooled water.

The hydrocarbon and external diluent removed from the water, e.g. in a separator, skim tank, flotation tank and/or oil removal filter, are preferably recycled, particularly preferably without separation. Preferably the hydrocarbon and external diluent removed from the water are recycled to a separator (e.g. a bulk separator used to produce the water to be cleaned) and/or to a hydrocarbon treater. An advantage of the method of the present invention is that the hydrocarbon and external diluent removed during cleaning does not need to be separated and instead can be recycled together. This benefit arises from the fact that the external diluent used to enhance the removal of hydrocarbon from the water is an external diluent which is preferably similar, or still more preferably identical, to the diluent used to decrease the viscosity of the hydrocarbon and water mixture extracted from a hydrocarbon formation. The hydrocarbon separated in the bulk separation will therefore comprise similar or identical diluent, hence the recycling of the hydrocarbon and diluent removed from the water to the separator and/or treater does not add anything new to be separated downstream, e.g. at the treater. This significantly simplifies recycling of the external diluent and avoids any need for an additional separation steps or equipment to regenerate diluent.

Preferably the method of the present invention yields water that meets the requirements with respect to dispersed hydrocarbon for use in a steam generator. e.g. OTSG, in terms of its hydrocarbon content. Preferably the method of the invention yields water comprising 0 to 50 ppm wt dispersed hydrocarbon, still more preferably 0 to 25 ppm wt dispersed hydrocarbon and especially 0 to 10 ppm wt dispersed hydrocarbon, e.g. 0 to 5 ppm wt dispersed hydrocarbon. Preferably the water obtained is recycled to a steam boiler to generate steam, e.g. for SAGD. Preferably the water obtained is sent to a water treatment facility prior to recycling to a steam boiler.

The water produced by the method of the invention may comprise dissolved organic components, e.g. PAH, NPD and BTEX. Preferred processes of the invention further comprise a step of removing dissolved organic components. Similarly the water produced by the method of the invention may comprise dissolved inorganic components, e.g. salts, silica. Preferred processes of the invention further comprise a step of removing dissolved inorganic components. Conventional processes that are well know to the skilled man may be used, e.g. warm lime softening, media filtration and ion exchangers (WAC).

The method of cleaning water according to the present invention may be incorporated into a method of separating a mixture comprising hydrocarbon and water.

As mentioned above, the hydrocarbon produced at the surface of a formation by extraction, e.g. using steam, comprises water. The amount of water present in the hydrocarbon and water mixture is highly variable and depends, for example, on the type of formation, the type of recovery operation being carried out, the quality of the steam injected into the formation and the length of time for which the operation has been carried out. The amount of water present in the hydrocarbon and water mixture may be, for example, 30-90% by volume. Correspondingly the amount of hydrocarbon present in the hydrocarbon and water mixture is variable and may be, for example, 70- 10% by volume.

When the hydrocarbon and water mixture is extracted from the subterranean formation, despite its mobilisation by steam, it is typically still viscous and therefore difficult to pump. Preferably therefore a diluent is added to the mixture comprising hydrocarbon and water prior to separation in a bulk separator. Preferably the diluent added is similar or identical to the external diluent added during the cleaning method.

Preferably therefore the diluent is as described above in relation to the cleaning method.

The diluent may be added to the mixture comprising hydrocarbon and water prior to its entry to the bulk separator or may be added to the bulk separator.

Preferably, however, the diluent is added to the mixture prior to its entry to the bulk separator, e.g. shortly after the mixture is brought to the surface of the formation from which it is extracted. This improves the pumpability and separability of the mixture.

The addition of the diluent is preferably carried out by adding diluent into the line transporting the mixture of hydrocarbon and water to the separator. This may be achieved, for example, by the use of a suitable inlet valve.

The bulk separator used to carry out the bulk separation on the hydrocarbon and water mixture may be any conventional separator, e.g. a gravity separator, a cyclone separator or a vortex separator. Preferably, however, the separator is a gravity separator. The separator optionally includes means for separation of gas from the mixture. The separator optionally includes means for separation of solids from the mixture. The separator is operated under conditions that are conventional in the art.

The separator may be operated in a continuous, semi-continuous or batchwise manner.

In the bulk separator the hydrocarbon and water mixture is separated to yield separated hydrocarbon and separated water. The mixture is fed into the bulk separator and allowed to separate out to a gas phase, a hydrocarbon phase, a water phase and a solids phase in vertically descending order. Optionally chemicals such as emulsion breakers may be added to the separator to improve the separation. The separated hydrocarbon predominantly comprises hydrocarbon. Preferably at least 75 % by volume, more preferably at least 85 % by volume and still more preferably at least 95 % by volume of the separated hydrocarbon is hydrocarbon.

The separated hydrocarbon is preferably removed from the bulk separator via a hydrocarbon outlet. The majority of the diluent added to the mixture prior to its entry into the bulk separator will be present in this separated hydrocarbon. Preferably the separated hydrocarbon is transported to a treater for processing.

The separated water is preferably cleaned by the method as hereinbefore described.

An advantage of the methods of the present invention is that it can be carried out using conventional equipment, i.e. conventional separators, coolers, skim tanks, floatation tanks, filters etc. Such equipment is all commercially available. The only modifications required to carry out the method of the present invention is that an inlet for external diluent and optionally a dispersing device be provided in the system prior to the means for removing hydrocarbon from the water. Such an inlet can conveniently be provided in the form of a suitable valve in the line transporting the water to be cleaned. Preferred systems comprise a means for dispersing the external diluent in the water, e.g. valves, nozzles or mixers etc. Valves are generally preferred. Suitable valves, nozzles and mixers are commercially available.

Preferred systems of the invention further comprise a cooler comprising an inlet for water and an outlet for cooled water. In preferred systems of the present invention, the cooler is at least one heat exchanger. In particularly preferred systems, a plurality of heat exchangers are present, preferably connected in series and/or parallel.

Suitable heat exchangers are commercially available.

In further preferred systems the means for removing hydrocarbon from the water is a conventional separator, e.g. a separator (e.g. gravity separator or cyclone), skim tank, a flotation tank and/or oil removal filter. Preferred systems comprise at least two of a separator (e.g. gravity separator or cyclone), a skim tank, a flotation tank and/or an oil removal filter. Particularly preferred systems comprise a skim tank and a flotation tank or a separator (e.g. gravity separator) and a skim tank. The system preferably comprises an outlet for water that is fluidly connected, directly or indirectly, to the water supply tank of a steam generator.

In some preferred systems the cooler is in between the bulk separator and the means for removing hydrocarbon. In this case the cooler is fluidly connected to a water outlet of a bulk separator and comprises a cooled water outlet fluidly connected to the means for removing hydrocarbon. In other preferred systems, the cooler is after the means for removing hydrocarbon. In this case the water outlet of the means for removing hydrocarbon is fluidly connected to the water inlet of said cooler. Preferably the outlet of the cooler is fluidly connected to a second means for removing hydrocarbon.

The inlet for external diluent is after the bulk separator. The inlet for external diluent may be prior to or after the cooler. Preferably it is before the cooler. When the means for removing hydrocarbon is before the cooler, the inlet for diluent is preferably prior to the means for removing hydrocarbon, i.e. in between the bulk separator the means for removing hydrocarbon.

In one preferred system of the invention, particularly a system wherein a cooler is in between the bulk separator and the means for removing hydrocarbon, the means for removing hydrocarbon is a skim tank or flotation tank, particularly a skim tank, fluidly connected to the cooled water outlet of the cooler and comprising an outlet for water. Such systems optionally further comprise a flotation tank fluidly connected to the outlet for water of said means for removing hydrocarbon from the water and comprising an outlet for water. Such systems further optionally comprise an oil removal filter fluidly connected to the outlet for water of the means for removing hydrocarbon or the flotation tank and comprising an outlet for further purified water.

Still more preferably the outlet for water and/or further purified water is fluidly connected to a water treatment system feeding the water supply tank of a steam generator.

In other preferred systems of the invention, particularly a system wherein a cooler is after the means for removing hydrocarbon, the means for removing hydrocarbon is a separator, e.g. gravity separator or cyclone. Such systems optionally further comprise a skim tank, a flotation tank and/or an oil filter removal fluidly connected to said cooled water outlet ot said cooler. It is preferable in some systems to remove dispersed or dissolved hydrocarbon and external diluent in a first means for removing hydrocarbon, e.g. a separator, and then cool the mixture prior to removing further dispersed or dissolved hydrocarbon and external diluent. The cooling process affects the equilibria in action in the mixture and, e.g. causes more hydrocarbon to come out of solution into dispersion and reduces the solubility of the dissolved hydrocarbon.

The means for removing hydrocarbon from the cooled water, e.g. the separator, skim tank, flotation tank and/or oil removal filter, preferably further comprises an outlet for hydrocarbon and external diluent that is fluidly connected to the bulk separator and/or treater (both described below). This enables the hydrocarbon and external diluent removed to be recycled through the process.

Further preferred systems of the invention enable a hydrocarbon and water mixture to be separated as well as cleaned. Such systems further comprise a bulk separator, preferably a bulk gravity separator, a bulk cyclone separator or a vortex separator, especially a bulk gravity separator. The bulk separator comprises an inlet for a mixture comprising hydrocarbon and water, an outlet for hydrocarbon and an outlet for water. The bulk separator optionally includes means for separation of gas from the mixture. The bulk separator optionally includes means for separation of solids from the mixture. Suitable bulk separators for use in the invention are commercially available.

Preferred systems of the present invention further comprise a treater fluidly connected to the hydrocarbon outlet of the separator and comprising an outlet for treated hydrocarbon.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention will now be described with reference to the following non-limiting Figures and examples wherein: Figure 1 shows a schematic of a method and system for cleaning water according to the present invention; Figure 2 shows a schematic of another method and system for cleaning water according to the invention; Figure 3 shows a schematic of a further method and system for cleaning water according to the invention. In Figures 1-3 the method of cleaning begins with separated water 109. The additional steps involved in a method of separating a hydrocarbon and water mixture comprising the cleaning method of the invention is described in relation to 101-108; Figure 4 shows a schematic of the experimental set up used to test the method of the invention; and Figure 5 shows the effect of addition of external diluent on the removal of hydrocarbon from simulated separated water.

Referring to Figure 1, a mixture 101 is extracted from an oil well which comprises hydrocarbon, water, gas and solids. External diluent 102 is generally added via line 103 to mixture 101 to lower its viscosity and improve its ability to be pumped and separated. The diluent is preferably a liquid. The mixture 101 further comprising diluent 102 is led via line 104 to a bulk separator 105 wherein an initial separation of the mixture into a separated hydrocarbon, a separated water and gas is carried out. This bulk separator 105 may be any conventional three-phase separator known in the art, e.g. a gravity separator or a cyclone separator. Any gas present is removed via line 106.

The separated hydrocarbon is led via line 107 to a treater 108. Most of the diluent 102 added is present in the separated hydrocarbon. Further external diluent 102 is optionally added to the separated hydrocarbon in line 107. The treater produces dilbit (diluted bitumen).

The separated water is led via line 109 to at least one cooler 112a. Prior to its entry to cooler 1 12a, external diluent 102 is added to the separated water via line 111.

The separated water comprising external diluent is then passed through a dispersing device 110. As shown in Figure 1 this may be a valve that disperses the external diluent throughout the water. Preferably the external diluent 102 is identical to the external diluent added to the original mixture 101 extracted from the well. Optionally the cooled water comprising external diluent 102 passes through a further cooler 112b.

The cooled water comprising external diluent is then led to skim tank 113 via line 114.

In the skim tank 113 the water is cleaned by removing the hydrocarbon and external diluent. Any gas generated in the process is removed via line 115. The external diluent improves the cleaning process by coalescing with the dispersed hydrocarbon thereby generating larger droplets. The external diluent will also generally have a lower density than the dispersed hydrocarbon therefore the coalesced droplets generally also float better than the droplets solely comprising hydrocarbon. The hydrocarbon and diluent droplets are collected by the skimming media and leave the skim tank via line 116. As shown the hydrocarbon and external diluent may be recycled together via slop tank 117 to the treater 108 via line 11 8b and/or to separator 105 via line 118a. A significant advantage of the method of the present invention is therefore that the external diluent 102 used is already on site and is, in a sense, simply "borrowed' from the hydrocarbon treating processes.

The water is led via line 119 to a gas floatation tank 120 wherein further hydrocarbon and external diluent are removed. Again the external diluent improves the cleaning process by dissolved gas flotation and continued coalescence with the dispersed hydrocarbon to form larger droplets having a lower density than the dispersed hydrocarbon droplets that aie easier to remove. The hydrocarbon and external diluent are recycled via line 121 to line 116 from where it is eventually returned to separator 105 and treater 108.

The water is led via line 122 to an oil removal filter 123. The water 124 obtained therefrom preferably meets the specification required for use in a steam generator. If necessary, however, the water may undergo further treatments, for example, to remove inorganic components. Preferably it is not necessary to remove any further dissolved organics from the water. The water 124 is stored for future use in steam generation.

Referring to Figure 2, it shows a similar method and system to that of Figure 1 except that the separated water 109 is cooled in heat exchangers 112a, 112b prior to addition of external diluent 102 via line 111. Otherwise the method and system are the same. The water in line 119 may optionally be further cleaned in a flotation tank and/or oil removal filter as described in relation to Figure 1.

Referring to Figure 3, the method and system shown therein comprises gravity separator 150. Thus the separated water 109 to which external diluent 102 has already been added as described above in relation to Figure 1, is transported into separator 150 and separation occurs. Any gas produced in the process is removed via line 151. The separated hydrocarbon and external diluent is removed via line 152 and is recycled to the bulk separator 105. Although not shown it could alternatively or additionally be recycled to the treater 103. The presence of the external diluent 102 in the separator 150 improves the separation process.

The separated water 153 is transported to coolers 112a, 112b. The cooled water is then led to skim tank 113 via line 114. In the skim tank 113 the water is cleaned by removing the hydrocarbon and external diluent present. Although a significant proportion of the hydrocarbon and external diluent is removed in the separator 150, the cooling process reduces the solubility of hydrocarbon in the water and more dispersed and less soluble hydrocarbon results.

The water 124 produced by the skim tank preferably meets the specification required for use in a steam generator. If necessary, however, the water may undergo further treatments, for example, to remove inorganic components. Preferably it is not necessary to remove any further dissolved organics from the water. The water 124 is led by line 119 to storage for future use in steam generation.

The method of the present invention at least provides the following advantages: * The method introduces an external diluent during transportation of the water to be cleaned from the bulk separator to the means for removing hydrocarbon, e.g. skim tank or separator. The only modification needed to the system to carry out the method is the addition of one or more valves on the transportation line to enable the addition of external diluent and optionally dispersing means (e.g. valves or nozzles) to disperse the diluent throughout the water.

* The method allows for removal of both dispersed and dissolved hydrocarbons.

* The external diluent used is a liquid, rather than a condensed gas.

Consequently there is no need for separation equipment, e.g. fractionating column or rectifier, or compression equipment to facilitate recycling of the external diluent.

* The external diluent used is preferably the same diluent added to the hydrocarbon mixture obtained from the oil well to reduce its density and viscosity. This is highly beneficial. The diluent is readily available on site and it opens up the possibility of recycling the diluent to various stages of the process without separation from the hydrocarbon. For instance, the hydrocarbon and diluent may be recycled to the separator or to the treater. A huge advantage is gained by the fact that the hydrocarbon and diluent do not need to be separated to enable recycling to occur.

* Since the external diluent used in the method is the same as that added to the crude hydrocarbon mixture obtained from the well, no instability issues due to incompatibility of the hydrocarbon and the diluent is expected.

* The method may utilise one three-phase separator in a conventional manner and does not therefore require any modification to the separator or the addition of extra equipment.

* The method allows for efficient removal of hydrocarbons without the presence of conventional water treatment chemicals

EXAMPLES

A series of experimental laboratory tests have been performed in the water rig in Statoil Porsgrunn. The objective of the testing was to demonstrate efficient hydrocarbon removal by dispersing a diluent (OSN) into water simulating water produced by SAGD. Batch sample testing has been performed in the water rig, giving a 5-30 fold improvement of oil removal with 1% diluent compared to the same process without diluent, depending on the oil droplet size. The conclusion of the tests performed is that the results indicate a significant improvement of de-oiling efficiency in SAGD produced water.

The batch sample testing was done according to the schematic in figure 4. A water tank (3 m3) heated to 80 °C and applying diluted seawater to 3500 mg/L TDS functioned as a water reservoir. A pump from a loop close to the oil addition point gives a flow of approximately 150 L/min through the system. Leismer dilbit was dosed into the water stream as the hydrocarbon to be removed and exposed to a shod pressure drop across a ball valve to control the droplet size (8-25 pm). The external diluent (OSN) was added in a 0.2, 0.5 and 1.0% of total volume ratio and was intermixed with the oil containing stream across a second valve (S2) with a given pressure drop. Batch samples of the water stream were then sampled in 2L pyrex bottles with a given settling time (0-60 minutes). The pyrex bottles simulates the skim tank in the cleaning process. After a given settling time, the water samples were tapped from the bottom of the pyrex bottle and analyzed. During the settling process, the pyrex bottles were kept warm in a heating cabinet at 70°C. The results are shown in Figure 5.

As Figure 5 shows, the hydrocarbon removal is most efficient for the largest droplets, which is logical with reference to Stoke's law, where the larger oil droplets (25 pm) have a larger rising velocity than the smaller ones (8.5 pm). The effect of adding external diluent has a strong positive impact, which increases with increased amount of added diluent up to 1%. When adding 1% external diluent, the extraction and separation of hydrocarbon from the water phase happens much more quickly than with no or lower diluent concentrations.

Claims

CLAIMS: 1. A method of cleaning water to remove hydrocarbon therefrom comprising: (i) adding an external diluent to said water; and (U) removing said hydrocarbon together with said diluent from said water.
2. A method as claimed in claim 1, wherein said water is water separated from a hydrocarbon and water mixture.
3. A method as claimed in claim 1 012, wherein said water comprises up to 5 %wt hydrocarbon.
4. A method as claimed in any preceding claim, wherein said hydrocarbon is dispersed in said water.
5. A method as claimed in any preceding claim, wherein said water is cooled to a temperature of 60-90 °C.
6. A method as claimed in claim 5, wherein said external diluent is added to said water prior to cooling.
7. A method as claimed in claim 5 or 6, wherein said external diluent is added to said water after cooling.
8. A method as claimed in claim 5 or 6, wherein cooling is carried out after removing hydrocarbon together with said diluent from said water and a further step of removing hydrocarbon together with diluent is carried out.
9. A method as claimed in any preceding claim, wherein said external diluent comprises C630 hydrocarbons.
10. A method as claimed in any preceding claim, wherein said external diluent comprises naphtha, light crude oil, gas condensate, synthetic crude or a mixture thereof.
11. A method as claimed in any preceding claim, wherein the amount of external diluent added to said water is 0.1-5 %wt based on the total weight of the resulting mixture.
12. A method as claimed in any preceding claim, wherein said hydrocarbon and diluent are removed together in a separator, a skim tank, a flotation tank and/or an oil removal filter.
13. A method as claimed in claim 12, wherein said hydrocarbon and external diluent are removed together in a skim tank.
14. A method as claimed in claim 12 or 13, wherein said hydrocarbon and external diluent are removed together in a separator.
15. A method as claimed in any one of claims 12 to 14, wherein said hydrocarbon and external diluent are removed together in a gas flotation tank.
16. A method as claimed in any preceding claim which yields water comprising 0 to ppm wt dispersed hydrocarbon.
17. A method as claimed in any preceding claim, wherein said hydrocarbon and external diluent removed from said water are recycled without separation.
18. A method as claimed in claim 17, wherein hydrocarbon and diluent removed from said water is recycled to a separator used to produce the water to be cleaned and/or to a treater.
19. A method of separating a mixture comprising hydrocarbon and water wherein said method comprises: (i) separating said mixture in a bulk separator to produce separated hydrocarbon and separated water; and (ii) cleaning said water by a method as claimed in any one of claims ito 18.
20. A method as claimed in claim 19, wherein said bulk separator is a gravity separator, a cyclone separator or a vortex separator.
21. A method as claimed in any preceding claim, wherein the water obtained is fed directly or indirectly to a steam generator to generate steam.
22. A system for cleaning water to remove hydrocarbon therefrom comprising: (a) a means for adding an external diluent to water separated in a bulk separator; and (b) a means for removing hydrocarbon together with said diluent from said water and comprising an outlet for water, wherein said means for adding an external diluent comprises at least one inlet for said external diluent in between said bulk separator and said means for removing hydrocarbon from said water.
23. A system as claimed in claim 22, further comprising a dispersing device for dispersing said external diluent throughout the water.
24. A system as claimed in claim 22 or 23, further comprising a cooler comprising an inlet for water and an outlet for cooled water.
25. A system as claimed in any one of claims 22 to 24, wherein said means for removing hydrocarbon from said water is a separator, skim tank, a flotation tank and/or an oil removal filter.
26. A system as claimed in claim 24 or 25, wherein said cooler is fluidly connected to a water outlet of a bulk separator and comprises a cooled water outlet fluidly connected to said means for removing hydrocarbon.
27. A system as claimed in any one of claims 22 to 26, wherein said inlet for external diluent is prior to said cooler.
28. A system as claimed in any one of claims 22 to 27, wherein said inlet for external diluent is in between said cooler and said means for removing hydrocarbon from said water.
29. A system as claimed in any one of claims 22 to 28, wherein said means for removing hydrocarbon from said water is a skim tank.
30. A system as claimed in any one of claims 22 to 29, further comprising a flotation tank fluidly connected to said outlet for water of said means for removing hydrocarbon from said water and comprising an outlet for water.
31. A system as claimed in any one of claims 22 to 30, further comprising an oil removal filter fluidly connected to said outlet for water of said means for removing hydrocarbon or said flotation tank and comprising an outlet for further purified water.
32. A system as claimed in any one of claims 22 to 25, wherein said means for removing hydrocarbon is fluidly connected to a water outlet of a bulk separator and comprises a water outlet.
33. A system as claimed in claim 32, wherein said water outlet of said means for removing hydrocarbon is fluidly connected to said water inlet of said cooler.
34. A system as claimed in claim 32 or 33, wherein said means for removing hydrocarbon from said water is a separator.
35. A system as claimed in any one of claims 32 to 34, further comprising a skim tank, a flotation tank and/or an oil filter removal fluidly connected to said cooled water outlet of said cooler.
36. A system as claimed in any one of claims 22 to 35, wherein said outlet for water is fluidly connected to the water supply tank of a steam generator.
37. A system as claimed in any one of claims 22 to 36, wherein said means for removing hydrocarbon from said water further comprises an outlet for hydrocarbon and external diluent that is fluidly connected to said separator and/or a treater.
38. A system as claimed in any one of claims 22 to 37, further comprising a bulk separator with an inlet for a mixture comprising hydrocarbon and water, an outlet for hydrocarbon and an outlet for water.
39. A system as claimed in claim 38, wherein said bulk separator is a gravity separator, a cyclone separator or a vortex separator.
40. A system as claimed in claim 38 or 39, further comprising a treater that is fluidly connected to said bulk separator.