CN115925051A - Hybrid system and method for treating produced water and seawater to be reinjected into subsea oil reservoirs - Google Patents

Abstract

The present invention relates to a water treatment system for producing and treating seawater for secondary recovery in oil wells. The present invention provides a hybrid system for treating produced water and seawater for reinjection into an offshore oil reservoir and an associated hybrid water treatment method. The water to be treated is directed to a treatment module (20) comprising microfiltration/ultrafiltration membranes or to a water treatment module comprising nanofiltration membranes, depending on the quality of the water in relation to the content of oil and solids or the content of sulphate ions. The systems and methods of the present invention allow reinjection of produced water without the need for additional treatment systems on the platform. Other advantages of the present invention include reduced offshore oil handling and reduced installation, operation and maintenance costs associated with additional systems at the offshore installation.

Description

Hybrid system and method for treating produced water and seawater to be reinjected into a subsea oil reservoir

This application is the result of a Chinese patent application with an application date of July 19, 2017, an application number of 201780057450.6, and an invention title of "Hybrid System and Method for Treating Produced Water and Seawater to be Reinjected into Subsea Oil Reservoirs" Divisional application.

technical field

The present invention relates to water treatment systems in offshore oil production units. More specifically, the present invention relates to systems for the treatment of produced water and seawater for secondary recovery in oil wells.

Background technique

It is known that in offshore oil installations, one of the techniques used for secondary oil recovery is the injection of treated seawater. In this context, it is known that seawater contains large amounts of sulfate ions (SO ₄ ⁻² ), approximately 2800 mg/L. When seawater is injected into a region where formation water (homologous water) contains sufficient barium (Ba ⁺² ), strontium (Sr ⁺² ) or calcium (Ca ²⁺ ) ions in solution, contact between these two fronts usually causes Its sulfates: barium sulfate (BaSO ₄ ), strontium sulfate (SrSO ₄ ) or calcium sulfate (CaSO ₄ ) precipitate. These salts are extremely insoluble, and the precipitated salts plug well bores causing formation damage. They also settle in production lines and equipment in processing plants.

Depending on the barium and strontium content of the formation water, it may be necessary to deploy a sulfate removal unit (URS) for the treatment of seawater to be injected into the reservoir, as shown in Figure 1 . In URS, nanofiltration membranes (which can be ceramic or polymeric) are used to remove sulfate ions from seawater. Because seawater has solid particles and marine plant and animal components, it is necessary to install filters upstream of the URS unit to improve its performance. The filtration is performed initially with a coarse filter and subsequently with a cartridge filter of smaller flow diameter.

In URS, water is permeated through nanofiltration membranes and a portion (typically 25%) is concentrated in sulfate ions and separated for future disposal into the ocean. To achieve the design specifications for sulfate ions in treated water, two sets of membranes were used in parallel, followed by a third set in series, according to the diagram in Figure 2.

Once the water has been treated with URS, the necessary specifications have been obtained and can be injected into oil reservoirs for secondary recovery.

Furthermore, it is further known to treat the produced water reaching the treatment unit to remove oil droplets. Conventional techniques for this type of processing have the general and simplified form shown in FIG. 1 .

Specifically, produced water undergoes treatment methods to separate the aqueous phase from the oil phase, which include gravity separation, hydrocyclones, and floatation separation, before being regulated for disposal in the ocean according to current environmental regulations. Water not specified for disposal on some platforms can be directed to tanks called "bad tanks" where it takes longer to separate the oil phase and in some cases can be reprocessed in processing equipment.

However, such produced water treatment devices have reduced removal efficiency of oil droplets and solid particles smaller than 5.0 μm. Such conditions limit the overall efficiency of the treatment, and thus obtain effluent streams with characteristics suitable for more restrictive reservoirs in terms of suspended solids content, oil and grease. So after treatment, it is stipulated that the produced water be disposed of in the sea due to the content of suspended solids, oil and grease, and it is stipulated not to reinject.

In this way, currently the only destination of produced water after treatment in offshore oil production facilities is disposal. The inefficiency of conventionally produced water treatment equipment used to achieve the required solids and oil content according to the requirements imposed by re-injection into a more restrictive reservoir, among other factors, renders such re-injection impractical. Therefore, this option is still ignored in recent secondary recycling schemes.

However, it is to be noted that the development of treatment systems that allow for the re-injection of produced water is a very attractive option for the oil producing sector, mainly due to the trend towards increasingly stringent environmental regulations and the tendency to increase this role Sustainability of industrial practices in the field.

In this sense, microfiltration/ultrafiltration membrane separation technology (using ceramic membranes) has proven to be an interesting option for this challenge, because when applied to treat produced water, it produces oil with low and solids content of water.

In microfiltration/ultrafiltration membrane separation methods, as known in the prior art, water permeates through the membrane, while a part of the supplied volume collects non-permeated oil and returns it to the system in recirculated form.

Ashaghi, K. Shams et al., entitled "Ceramic Ultra-and Nanofi l trat ion Membranes for Oilfield Produced Water Treatment: A Mini Review" disclose information on the use of microfiltration/ultrafiltration ceramic membranes to treat produced water (removal of solids and A review of oil particles). Several techniques using microfiltration/ultrafiltration ceramic membranes are proposed in this scientific paper, so their descriptions are hereby incorporated by reference.

The paper entitled "Evaluation of membranes for the treatment of water from the oi lextract ion process" by Weschenfelder, Silvio E. et al. (one of the inventors of the present invention) discloses a study by using real effluent Long-term tests evaluate the performance of membranes used to treat produced water, taking into account permeate flow formation and the characteristics of the resulting effluent. The results show that by using a membrane with a pore size equal to 0.1 mm, a permeate stream with a solids content of less than 1 mg L ^-1 and an oil and grease content of 1-3 mg L ^-1 can be obtained. Furthermore, this document discloses that it is possible to restore 95% of the initial permeability of microfiltration/ultrafiltration ceramic membranes using chemical regeneration methods. The disclosure of this document is also incorporated herein by reference.

In the present scheme, if it is determined that re-injection can be achieved by supplementing conventional produced water treatment with microfiltration/ultrafiltration membrane separation methods, additional systems will be required, for example in the treatment plant, as described in the above-mentioned prior art documents described in . This brings significantly higher configuration, operating and maintenance costs and greater operational difficulty, as well as greater weight and footprint on the platform.

Thus, it is apparent that the prior art lacks a produced water treatment system that allows re-injection without the need for additional treatment systems as known in the prior art.

As will be better described below, the present invention seeks to solve the above-mentioned problems of the prior art in a practical, efficient and cost-effective manner.

Contents of the invention

The main objective of the present invention is to provide a hybrid system and method for seawater treatment and production that allows re-injection of produced water without the need for additional treatment systems on the platform.

To achieve the above objects, the present invention provides a hybrid system for treating produced water and seawater for re-injection into an offshore oil reservoir comprising (i) at least one inlet for water to be treated, (ii) At least two microfiltration/ultrafiltration water treatment modules, each module comprising (ii-a) at least one set of microfiltration/ultrafiltration membranes for removing oil and solids from the water to be treated, or (ii-b) at least one A set of nanofiltration membranes for removing sulphate ions from the water to be treated, (iii) at least one outlet for treated water, wherein said volume of water to be treated is directed to comprise microfiltration/ultrafiltration membranes Depending on the quality of the water in relation to the oil and solids content or the sulfate ion content, the water treatment module is directed or directed to a water treatment module comprising nanofiltration membranes.

The present invention further provides a hybrid method for treating produced water and seawater for re-injection into an offshore oil reservoir, essentially comprising the steps of: (i) directing the water to be treated to at least one set of microfiltration/ultrafiltration A water treatment module of membranes for removing oil and solids from the water to be treated, or (ii) directing the water to be treated to a water treatment module comprising at least one set of nanofiltration membranes for removing Sulfate ion, wherein the volume of water to be treated is directed to a water treatment module comprising microfiltration/ultrafiltration membranes or to a water treatment module comprising nanofiltration membranes, depending on the oil and solids content of the water or Quality related to sulfate ion content.

Description of drawings

The detailed description given below refers to the drawings and their respective reference numerals.

Figure 1 shows a schematic diagram of a seawater treatment system and for injection and disposal of produced water respectively, as known in the prior art.

Figure 2 shows a schematic diagram of an example of seawater treatment for injection into an oil reservoir by means of a sulphate removal unit (URS), as known in the prior art.

Figure 3 shows a schematic diagram of a processing module comprising nanofiltration or microfiltration/ultrafiltration membranes according to a preferred embodiment of the present invention.

Figure 4 shows a schematic diagram of a hybrid seawater treatment system and one for reinjection of produced water according to a preferred embodiment of the present invention.

Figure 5 shows a schematic diagram of a complete system for treating seawater and for reinjection of produced water including the hybrid system of the present invention.

Detailed ways

In the foregoing, it is to be understood that the following description will depart from the preferred embodiments of the invention. However, it will be apparent to those skilled in the art that the present invention is not limited to specific embodiments.

Figure 4 shows a simplified schematic diagram of a hybrid seawater treatment system and further re-injection of produced water according to a preferred embodiment of the present invention. The figure basically comprises two inlets for the water to be treated, namely one inlet 2 for produced water, with a high content of oil and solids, and one inlet 4 for seawater, with a high content of sulphate ions.

Produced water is preferably stored in at least one tank 10 prior to diversion for disposal or treatment by the hybrid system of the present invention.

Seawater collected for treatment and subsequent injection is preferably passed through a series of filters, the first filter being provided with a filter element with a coarse mesh and followed by a filter element with a fine mesh. Preferably the first filter 12 retains particles to 500 μm, the second filter 14 to 25 μm and the third filter to 5 μm.

Preferably both the produced water and the collected seawater respectively arrive at at least one header 18 consisting of a plurality of water control valves, which join each treatment module 20 .

Each treatment module 20 contains at least one suitable set of microfiltration/ultrafiltration membranes (ceramic membranes) to remove oil and solids from the produced water, or at least one set of nanofiltration membranes (ceramic or polymeric membranes) used for for the removal of sulfate ions from seawater. Thus, at least one header 18 directs produced water through its control valves to the modules containing the micro/ultrafiltration membranes and pumps seawater into the modules containing the nanofiltration membranes. Preferably said at least one header is subdivided into two headers, one for controlling the inlet of produced water in modules comprising microfiltration/ultrafiltration membranes, and the other for controlling the entry of seawater into modules comprising nanofiltration membranes .

Preferably at least one header 18 is fluidly connected to two inlet conduits for the water to be treated, ie one for produced water 2 and one for seawater 4 . These inlet conduits are each subdivided into a plurality of parallel secondary conduits for the secondary conduits of each process module. Before entering each treatment module 20, secondary conduits of produced water and seawater flow into a single inlet conduit from each module downstream of each control valve.

The control valves are located upstream of each treatment module 20 such that each valve controls the entry of one type of water to be treated, ie produced water or seawater from each secondary pipeline.

Preferably, there is no mixing between produced water and seawater prior to entering the treatment module 20 . That is, if the produced water inlet control valve is opened, the seawater inlet control valve should preferably be closed.

Preferably, each treatment module 20 contains only one type of membrane, ie nanofiltration or micro/ultrafiltration. It is therefore preferred that if a specific treatment module 20 comprises only nanofiltration membranes, only seawater is directed therein, and the inlet control valve for produced water is closed. Likewise, produced water will be directed to a treatment module 20 containing only microfiltration/ultrafiltration membranes.

Each processing module 20 is designed to allow interchange between nanofiltration membranes and microfiltration/ultrafiltration membranes. In other words, each module can replace its nanofiltration membrane with microfiltration/ultrafiltration (and vice versa), depending on each water treatment requirement.

As an example, it will be appreciated that shortly after implementation of the hybrid system of the present invention, seawater will only need to be treated through nanofiltration membranes, since no water will still be produced. Thus, practically all treatment modules 20 can be equipped with nanofiltration membranes only. As produced water is produced, the need to treat seawater is reduced. In that case, the nanofiltration membranes of the treatment module 20 are replaced by microfiltration/ultrafiltration membranes.

Figure 3 shows a schematic detail of a processing module 20 according to the invention. As mentioned, the treatment module 20 may contain nanofiltration membranes or micro/ultrafiltration membranes, depending on the type of water (produced water or seawater) that will pass through the particular module. Each module contains at least one set of 20 microfiltration/ultrafiltration or nanofiltration membranes. Preferably as in the prior art URS, each module comprises two sets of membranes 20a, 20b connected in parallel, followed by a third set of membranes in series 20c.

Preferably, in case the treatment module 20 is provided with nanofiltration membranes, in order to remove sulphate ions from seawater, the water to be treated passes through a first two sets of nanofiltration membranes connected in parallel, so that the largest part of the treated volume of water becomes to a low sulfate ion concentration and sent for injection into the reservoir.

The remainder of the water concentrated by sulfate ions passing through the first set of membranes is directed to a third set of membranes 20c in series with the first two sets. This third set treats this more concentrated water and also produces a larger fraction with a lower concentration of sulfate ions (which will mix with the water treated by the first two sets of membranes) and the sulfate ion extremes. A concentrated smaller fraction (which is usually discarded in the sea).

Water with low sulfate ion concentration from nanofiltration membrane stack treatment is used for injection into the reservoir, but may undergo additional treatment steps.

In the case of the treatment module 20 provided with microfiltration/ultrafiltration membranes, for the removal of oil and solids from the produced water, the procedure is very similar to that described above. The water to be treated is preferably passed through the first two sets of parallel membranes 20a, 20b so that the largest part of the treated volume contains low concentrations of oil and solids and is directed for reinjection into the reservoir.

The oil and solids sent through the first set of membranes are concentrated and the remaining water is directed to a third set of membranes 20c in series with the first two sets. This third set processes this more concentrated water and also produces a larger fraction with a lower concentration of oil and solids (which will mix with the water processed by the first two sets of membranes). Water with low concentrations of oil and solids from all three sets of microfiltration/ultrafiltration membranes was used for reinjection into the reservoir.

Depending on the quality of water to be treated, each treatment module 20 may contain more or fewer series and/or parallel membrane stacks. Therefore, it is pointed out that the present invention is not limited to the membrane stack configuration shown in FIG. 3 .

Still in the case of a treatment module 20 provided with microfiltration/ultrafiltration membranes, a smaller fraction of the oil and solids concentration from the third set of membranes 20c can be directed to the inlet of the treatment module 20 as shown in FIG. 3 .

Alternatively, as shown in Figure 5 (full view of the offshore unit), this oil and solids concentrated water (oily recycle) can be sent to a water treatment system to separate the oil phase. Preferably the oil and solids concentrated water can be sent to treatment tanks as shown in Figure 5 (treatment tank 24). This tank may for example be a substandard tank, which is normally already used in production water treatment plants. Optionally, additional slots other than the out-of-spec slots may be provided for this step.

Optionally, at least one water outlet is provided in the lower part of the treatment tank 24 to draw water with a low concentration of oil, which will concentrate at the top after a period of time since the oil is less dense than water. Water drawn through the water outlet in the lower part of the treatment tank 24 (which has a relatively low or medium concentration of oil) can be discarded (if specified) or directed to a hybrid treatment system according to the invention where it will be sent to a A treatment module 20 of filtration/ultrafiltration membranes is used for new treatment to remove oil and solids. After removing some of the water, the oily concentrate remaining in the treatment tank 24 is preferably directed to an oil-water separation system 23 to utilize the recovered oil. This facilitates the minimization of oil discharge into the ocean and better utilization of the oil present in the produced water in the overall production of the well.

The invention further provides the possibility to perform a backwashing procedure on the membranes used in the treatment modules, in particular microfiltration/ultrafiltration membranes. Such a procedure can be performed, for example, by pumps (not shown) or by manipulating timed valves in the treated water lines and supply lines of each group. This procedure allows periodically reversing the flow in the membrane, cleaning and maintaining its performance.

Optionally, at least a first degasser unit 28 is provided upstream or downstream of the treatment module 20 for seawater degassing, if desired, prior to reinjection into the reservoir.

The present invention further provides a hybrid method for treating produced water and seawater for re-injection into an offshore reservoir, essentially comprising the steps of:

a) directing the water to be treated to a water treatment module comprising at least one set of microfiltration/ultrafiltration membranes for removing oil and solids from the water to be treated; or

b) directing the water to be treated to a water treatment module comprising at least one set of nanofiltration membranes for removing sulfate ions from the water to be treated, wherein said volume of water to be treated is directed to comprise microfiltration/ultrafiltration membranes A water treatment module or a water treatment module comprising nanofiltration membranes, depending on the quality of the water in relation to oil and solids content or sulfate ion content.

It is further emphasized that all process steps detailed herein are applicable to both the system and method of the present invention.

Therefore, based on the above description, the present invention provides a system and method for treating seawater and production that allows re-injection of produced water without the need for additional treatment systems on the platform. Still further advantages are achieved by the present invention, such as reduced offshore oil disposal through more efficient treatment of produced water, and reduced installation, operation and maintenance costs associated with additional systems at offshore installations.

Numerous variations within the scope of the invention are permissible. Accordingly, emphasis is placed on the fact that the present invention is not limited to the specific settings/embodiments described above.

In particular, the present invention also relates to the following items:

Item 1. A hybrid system for treating produced water and seawater to be reinjected into an offshore oil reservoir characterized by comprising:

At least one inlet for water to be treated;

At least two processing modules (20), each containing:

At least one set of microfiltration/ultrafiltration membranes (20a, 20b, 20c) for removing oil and solids from the water to be treated; or

at least one set of nanofiltration membranes (20a, 20b, 20c) for removing sulfate ions from the water to be treated; and

At least one outlet for treated water, wherein the volume of water to be treated is directed to a treatment module (20) comprising microfiltration/ultrafiltration membranes or to a water treatment module comprising nanofiltration membranes, depending on the water Content Quality related to oil and solids content or sulfate ion content.

Item 2. The system according to item 1, characterized in that each treatment module (20) comprises at least two sets of membranes (20a, 20b) in parallel.

Item 3. The system according to item 1 or 2, characterized in that each treatment module (20) comprises at least one set of membranes (20c) in series with other sets of membranes (20a, 20b).

Item 4. The system according to any one of items 1-3, characterized in that the at least one inlet for water to be treated comprises two water inlets, i.e. one for produced water (2) and one for seawater (4 ).

Item 5. The system according to item 4, characterized in that it further comprises at least one header (18) provided with a plurality of valves for controlling the type of water entering each water treatment module (20), i.e. produced water Or sea water.

Item 6. The system according to item 5, characterized in that said inlet conduits (2, 4) are each subdivided into a plurality of parallel secondary conduits for the secondary conduits of each treatment module (20).

Item 7. The system according to any one of items 1-6, characterized in that each treatment module (20) contains only one type of membrane, namely nanofiltration or micro/ultrafiltration.

Item 8. The system according to any one of items 1-7, characterized in that the membrane of each treatment module (20) is interchangeable with another type of membrane.

Item 9. The system according to any one of items 1-7, characterized in that it additionally comprises at least one water treatment tank (24) for separating the water phase from the oil phase by density difference.

Item 10. The system according to item 9, characterized in that the water treatment tank (24) is in fluid communication with at least two treatment modules (20) downstream and upstream thereof, completing a cycle.

Item 11. The system according to item 9 or 10, characterized in that the water treatment tank (24) is furthermore in fluid communication with at least one outlet for marine waste treatment and a water inlet conduit for separation of water and oil.

Item 12. A hybrid method for treating produced water and seawater for reinjection into an offshore oil reservoir, characterized in that it comprises the steps of:

directing the water to be treated to at least one water treatment module (20) comprising at least one set of microfiltration/ultrafiltration membranes for removing oil and solids from the water to be treated; or

leading the water to be treated to at least one treatment module (20) comprising at least one set of nanofiltration membranes for removing sulfate ions from the water to be treated,

Therein, depending on the type of water to be treated, i.e. produced water or seawater, said volume of water to be treated is directed to at least one treatment module (20) comprising microfiltration/ultrafiltration membranes or to a treatment module (20) comprising nanofiltration membranes. At least one water treatment module.

Item 13. The method according to item 12, characterized in that the step of directing the water to be treated to the treatment module (20) further comprises treating the water through at least two sets of parallel membranes (20a, 20b).

Item 14. The method according to item 12 or 13, characterized in that the step of directing the water to be treated to the treatment module (20) further comprises treating the water through at least one set of membranes (20c) in series with other sets of membranes (20a, 20b) .

Item 15. The method according to any one of items 12-14, characterized in that the water to be treated is produced water concentrated in oil and solids and seawater concentrated in sulfate ions.

Item 16. The method according to item 15, characterized in that it additionally comprises the step of: controlling the type of water entering each treatment module (20) through at least one header (18) provided with a plurality of valves, i.e. produced water or seawater .

Item 17. The method according to any one of items 12-16, characterized in that it further comprises the step of directing the concentrated water portion of oil and solids from at least one treatment module (20) comprising at least one set of micromembranes/ultrafiltration to at least A treatment tank (24).

Item 18. The method according to item 17, characterized in that it further comprises the step of: separating a less thick oily phase and a thicker aqueous phase by density difference within a given period of time in the treatment tank (24).

Item 19. The method of item 18, characterized in that it further comprises the step of discharging the separated aqueous phase through at least one water outlet provided in the lower part of the treatment tank (24).

Item 20. The method according to item 19, characterized in that it further comprises the step of directing the discharged aqueous phase to marine waste treatment or to at least one treatment module (20).

Item 21. The method according to any one of items 18-20, characterized in that it further comprises the step of directing the remaining oily concentrate in the treatment tank (24) to the water and oil separation system (23) after the aqueous phase discharge step .

Item 22. The method according to any one of items 14-24, characterized in that it further comprises at least one step of deaerating the treated water by means of at least one deaerator unit (28).

Item 23. The method according to any one of items 12-22, characterized in that it further comprises at least one step of backwashing the membranes of at least one treatment module (20) by reversing the flow of water therein.

Claims (17)

1. A hybrid system for treating produced water and seawater for reinjection into an offshore oil reservoir, characterized by comprising:

at least one inlet for water to be treated, having a produced water inlet (2) and a seawater inlet (4);

at least one header (18) provided with a plurality of valves;

at least two processing modules (20), each module comprising:

at least one set of microfiltration/ultrafiltration membranes (20a, 20b, 20c) adapted to remove oil and solids from the produced water to be treated; and

at least one set of nanofiltration membranes (20a, 20b, 20c) adapted to remove sulfate ions from the seawater to be treated,

wherein the microfiltration/ultrafiltration membrane assembly membrane (20a, 20b, 20c) and the nanofiltration membrane assembly membrane (20a, 20b, 20c) are interchangeable; and

at least one outlet for treated water,

wherein the water to be treated is directed through at least one header (18), the at least one header (18) directing the produced water stream to a treatment module (20) comprising a microfiltration/ultrafiltration membrane or directing the seawater stream to a treatment module (20) comprising a nanofiltration membrane,

the at least one header is subdivided into two headers, one for controlling the produced water to enter the module comprising the microfiltration/ultrafiltration membrane and the other for controlling the seawater to enter the module comprising the nanofiltration membrane, and

there is no mixing between the produced water and the seawater before entering the treatment module (20), an

Each treatment module (20) comprises only one type of membrane, i.e. nanofiltration or microfiltration/ultrafiltration, and

shortly after implementation of the hybrid system, the seawater will only need to be treated by nanofiltration membranes, and as produced water is produced, the nanofiltration membranes of the treatment module (20) are replaced with microfiltration/ultrafiltration membranes.

2. The system according to claim 1, characterized in that each processing module (20) comprises at least two sets of membranes (20a, 20b) in parallel.

3. The system according to claim 1 or 2, characterized in that each processing module (20) comprises at least one set of membranes (20 c) in series with the other sets of membranes (20a, 20b).

4. System according to claim 1, characterized in that each inlet duct (2, 4) is subdivided into a plurality of parallel secondary ducts, one for each processing module (20).

5. System according to claim 1, characterized in that it additionally comprises at least one water treatment tank (24) adapted to separate the aqueous phase from the oil phase by means of a density difference.

6. The system of claim 5, characterized in that the water treatment tank (24) is in fluid communication with at least two treatment modules (20) downstream and upstream thereof to complete a cycle.

7. System according to claim 5 or 6, characterized in that the water treatment tank (24) is furthermore in fluid communication with at least one outlet for offshore disposal and one inlet conduit for produced water separated between water and oil.

8. A hybrid method for treating produced water and seawater for re-injection into an offshore oil reservoir, the method using the system defined in claim 1, characterized in that the method comprises the steps of:

directing the water to be treated to at least one water treatment module (20) comprising at least one set of microfiltration/ultrafiltration membranes adapted to remove oil and solids from the produced water to be treated; or

Directing the water to be treated to at least one treatment module (20) comprising at least one set of nanofiltration membranes adapted to remove sulphate ions from the seawater to be treated; and

backwashing the membranes of at least one treatment module (20) by reversing the flow of water therein,

wherein the water to be treated is directed through at least one header (18), the at least one header (18) directing the produced water stream to a treatment module (20) comprising a microfiltration/ultrafiltration membrane or directing the seawater stream to a treatment module (20) comprising a nanofiltration membrane.

9. A method according to claim 8, characterized in that the step of directing the water to be treated to the treatment module (20) further comprises treating the water by at least two sets of membranes (20a, 20b) in parallel.

10. A method according to claim 8 or 9, characterised in that the step of directing the water to be treated to the treatment module (20) further comprises treating the water by at least one set of membranes (20 c) in series with the other sets of membranes (20a, 20b).

11. A method according to claim 8 or 9, characterized in that the water to be treated is produced water concentrated with oil and solids and seawater concentrated with sulphate ions.

12. Method according to claim 8 or 9, characterized in that it further comprises the step of: a portion of the oil and solids-enriched water is directed from at least one treatment module (20) containing at least one set of micro/ultrafiltration membranes to at least one treatment tank (24).

13. Method according to claim 12, characterized in that it further comprises the step of: the less thick oil phase is separated from the thicker water phase by a density difference in a given period of time in a treatment tank (24).

14. The method as recited in claim 13, characterized in that it further comprises the step of: the separated aqueous phase is discharged through at least one water outlet provided in the lower part of the treatment tank (24).

15. Method according to claim 14, characterized in that it further comprises the step of: the discharged aqueous phase is directed for offshore disposal or to at least one treatment module (20).

16. Method according to claim 13, characterized in that it further comprises the step of: after the aqueous phase discharge step, the remaining oily concentrate in the treatment tank (24) is directed to a water and oil separation system (23).

17. Method according to claim 13, characterized in that it further comprises at least one step of: the treated water is degassed by at least one degasser unit (28).

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