This invention relates to the biocide treatment of seawater treatment membranes, such as reverse osmosis membranes, which may otherwise become clogged with e.g. bacterial matter and a water treatment system therefor.

It has become common practice in the oil industry to utilise membranes to adjust the ionic content of seawater, either for injection into the reservoir for the purposes of enhanced oil recovery or pressure maintenance, or for the flushing of production flowlines. The membranes may be used to remove a substantial proportion of all of the salts present (desalination) or they may selectively remove higher proportions of specific salts such as sulphates. In this process, the seawater to be treated by the membranes is first pre-treated to make it suitable as a membrane feed. Depending on the properties of the water and on the details of the application, pre-treatment may include some or all of the following processes: pumping, chlorination, ozonation, heating, cooling, coagulation, flocculation, straining, filtration, deaeration, dechlorination and scale inhibition. The pressure of the pre-treated water is then raised as shown in Figures 1 and 2 by pump 1 and the water is fed to one or more trains or streams of membranes 2 contained within pressure vessels or housings 3. As shown in Figure 1 , a proportion of the feed water passes through the semi-permeable membrane 2 and leaves the vessel or housing 3 with a lower proportion of the dissolved species than the feed water it entered, this stream being referred to as the permeate 4. The remainder of the feed water leaves the vessel or housing 3

without passing through the semi-permeable membrane. This stream is known as the retentate 5 and it contains the dissolved species removed from the permeate 4, giving it a higher salinity than the feed water. The feed water flowrate and the ratio of permeate flow to retentate flow are set by means of control valves 6, restriction orifices 7 or other devices to maintain the flux through the semi-permeable membrane 2 and the velocity across the membrane surface within acceptable limits to reduce the rate of membrane fouling and maintains the pressure drop across the membrane and along the membrane within acceptable limits. Depending on the application and the product specification required, the water may pass through a single stage of membranes as shown in Figure 1 or through two or more stages of membranes in various configurations as shown in Figure 2 whereby the retentate or the permeate are recycled or re-treated to enhance the product stream quality or to minimise the overall retentate volume. Regardless of the arrangement of the membranes 2, the result will be to produce a product stream 8 comprised of the permeate from some or all of the membranes 2 and a reject stream 9 comprised of the retentate from some or all of the retentate streams. The reject stream 9 may also contain some or all of the permeate or product water 8 that is either out of specification or that is not required for injection or flushing purposes. The product stream 8 may be further treated, for example by deaeration and further pumping prior to injection into the reservoir or production flowlines. The reject stream 9 is usually returned to the sea.

Seawater contains high levels of bacteria and of other biological matter which, if left untreated, establishes living colonies within the membrane

elements, these being a major contributor to the fouling of the membranes. The membranes used for these applications often incorporate cellulose acetate or other organic materials that deteriorate if exposed to chlorine or other strong oxidising agents which would normally be used for the purpose of bacterial and biological control. It is therefore necessary to use biocides which do not degrade the membranes, such as gluteraldehyde or 2,2-dibromo-3-nitrilopropionamide or similar biocides that gain their effectiveness from toxicity rather than by degradation of the cell structure. These biocides are conventionally introduced to the feed stream at some point upstream of the membranes 2 either as a continuous biocide dose, or more often as a high or shock dose introduced for a pre-determined period of time at a pre-determined frequency. The biocide passes into the membrane vessels or housings 3, where a proportion of the biocide may pass through the membranes 2 into the permeate 4, and where some or all of the biocide may remain in the retentate 5. Some or all of the biocide is therefore carried in the reject line to the sea.

Organic biocides are toxic to marine life and can in some cases persist for days or weeks in the sea. As a result, a number of governmental regulatory bodies and a number of oil companies wish to reduce or eliminate completely the discharge of biocides to the environment. This invention describes a method for the elimination of biocide discharges to sea from membrane processes described above.

Membranes in this duty require cleaning with acids, alkali's or detergents from time to time in order to remove organic or inorganic fouling that accumulates on the surface. Though it is possible to remove the membranes

from their housings to clean them, the membranes are more usually cleaned in place (CIP). The period that a membrane can be operated before CIP depends on a number of factors such as the type of membrane, the degree of pre- treatment and specifically pre-filtration, but is significantly influenced by the flux or specific flowrate through the membranes and by the fluid velocity across the membranes. If the flux is too high, suspended solids and scale particles are forced into the membrane surface. If the surface velocity is too low, the cross flow over the surface is insufficient to entrain and suspend any deposited particles that would otherwise be carried out with the retentate. These two factors mean that in this type of cross-flow membrane, there is a relatively small flowrate envelope in which the membranes can be operated without reducing intervals between CIP. This means that during CIP, a train or stream is normally taken offline and the throughput of the plant is correspondingly reduced. For example, a plant having 4 trains would be reduced to 75% capacity during CIP. Adding a fifth train or stream to such a plant to allow full throughput with one train isolated would mean exceeding the maximum flux during CIP or operating below the minimum flux when all trains are on line. Difficulties in preserving offline membranes make it undesirable to operate with one stream off-line other than during CIP. It would be possible to break the trains or streams up into very small streams or sub-streams, so that both criteria can be met with on-line CIP, but this has proven to be uneconomic for the small gain in availability achieved.

The present invention is derived from the realisation that it would be preferable to provide a water treatment stream with a plurality of sub-streams,

each of which can be selectively closed and cleaned whilst still maintaining a desired flux within the overall water treatment stream.

According to the invention there is provided a method of treating semipermeable membranes with biocides, the method including the steps of providing a water treatment stream and sub-dividing it into a plurality of selectively closable sub-streams, each sub-stream containing one or more semipermeable membranes for providing a treated water product sub-stream forming part of a main product stream, each sub-stream being selectively connectable to a common biocide treatment stream, closing a chosen sub-stream for cleaning with biocide, connecting the sub-stream to the biocide treatment stream to thereafter treat and clean the or each membrane, thereafter opening the sub- stream such that the used biocide is discharged or dischargeable into the main water treatment product stream, either directly or indirectly, the process continuing as required for other sub-streams in the water treatment stream. With this arrangement a water treatment stream can be designed which has a dedicated biocide treatment facility which is accessible to each sub-stream in the main water treatment stream and whereafter the used biocide can simply be discharged e.g. into the main product stream or some other desirable repository, such as an alternative injection stream such as a depleted reservoir, as opposed to being discharged to the sea as in conventional practice. For example, as well as injecting sea water into hydrocarbon reservoirs for the purposes of secondary oil recovery or pressure maintenance, it is common practice within the industry to inject produced water for similar purposes. Produced water, as well as drill cuttings produced during oil well drilling

operations, is also commonly injected into depleted hydrocarbon reservoirs or other sub-surface disposal formations in the vicinity of the production site rather than being discharged to the surface environment.

Accordingly, in another embodiment of the invention, used biocide is discharged or dischargeable into a produced water stream or drill cutting stream, either directly or indirectly, for injection into a sub-surface formation or reservoir.

Alternatively, used biocide may be discharged or dischargeable, either directly or indirectly, into a tank for separate disposal, prior to opening the sub- stream. Optionally, used biocide may be retained in the tank and reused in the treatment of one or more sub-streams.

Preferably, the number of sub-streams will be at least three. The number of streams and the capacity of each stream will be determined by the need to maintain the flux and surface velocity within the membranes within acceptable limits, while one sub-stream is removed from operation.

If full production is required during biocide treatment, then the number and capacity of the sub-streams must be chosen to maintain flux and surface velocity within acceptable limits while producing the full product capacity when one sub-stream is removed from operation. If reduced production is acceptable during biocide treatment, then the number and capacity of the sub-streams must be chosen to maintain flux and surface velocity within acceptable limits while producing the reduced product capacity when one sub-stream is removed from operation.

In the preferred method of implementation, the number and capacity of the sub-streams must also be chosen to maintain flux and surface velocity within acceptable limits when all sub-streams are operating simultaneously.

In an alternative method of implementation, one sub-stream is held off line following biocide treatment until such time as the next sub-stream is treated with biocide. In such a system, intervals between biocide treatments is short enough to avoid the necessity of preserving the membranes in the off-line sub- stream using chemical preservation solutions such as sodium metabisulfite solution. According to the invention there is also provided a water treatment system comprising a membrane system and a biocide treatment system, wherein the membrane system comprises an inlet, an inlet control valve, a feed pump, a plurality of membranes in vessels or housings, and the biocide treatment system comprises a biocide make up and recirculation tank, a biocide recirculation pump, a biocide return pump and one or more semi-permeable membrane filters, wherein the membrane system is configurable such that the or each membrane process stream is split into a number of sub-streams, each of said sub-streams being independently connectable to the biocide treatment stream, the biocide treatment system configurable such that there is no external discharge of used biocide.

The membrane process stream may be split into a number of sub- streams before or after the feed pump, or before or after the inlet flow control valve.

Preferably, each sub-stream has three or more connections to the biocide treatment system.

Even more preferably, a first connection is to a sub-stream feed valve, a second connection is to a sub-stream permeable product output valve, and a third connection is to a sub-stream outlet valve. The connection could be in the form of a multi-port valve, single isolating valve, double block and bleed valve arrangement, mechanical linkage, hose connection or the like, and the valves may be manually operated, fitted with actuators or a combination thereof.

Optionally, the biocide treatment system may include a valve or flow limiting device, or have a filter or combination of filters installed in the recirculation loop.

The biocide may be added to the make up and recirculation tank by means of a transfer pump, barrel pump, gravity draining or pouring through an open lid. Optionally, the biocide make up and recirculation tank may have an agitator or stirrer, or actuated valves with flow and level metering and control devices.

In a further preferred embodiment, the water treatment system may be incorporated into a clean in place (CIP) system. The invention will now be described, by way of example only, in which:

Figure 1 shows a conventional single stage water treatment stream as described above,

Figure 2 shows a conventional multi-stage water treatment stream as described above,

Figure 3 shows a water treatment stream according to the invention,

Figure 4 shows a biocide treatment system for use with the water stream of Figure 3,

Figure 5 shows a alternative biocide treatment system for use in the water stream of Figure 3, and

Figure 6 shows an alternative embodiment of biocide treatment system in which biocide solution is retained and re-used.

The treatment process of the invention is comprised of a membrane system divided into sub-streams as shown in Figure 3 and a biocide treatment system as shown in Figures 4 and 5.

Pre-treated seawater 10 is fed to the membrane process stream, which may be comprised of one or more trains or streams, each of which may comprise one or more stages of membranes 2 in vessels or housings 3 to treat the incoming seawater and where applicable re-treat the permeate 4 or retentate 5. When in normal operation, the seawater is treated as described above, with a reduced salinity product stream being injected into the oil or gas reservoir or into the production flowlines, and an increased salinity reject stream 11 being returned to the sea.

Each train or stream is split into a number of sub-streams 12. The sub- streams 12 may be split before the feed pump 13, but will usually be split after the feed pump. The sub-streams may also be split before the inlet flow control valves 14, or after, as shown in Figure 3.

Each of the sub-streams 12 has three or more connections to a biocide treatment system. One connection is made to the sub-stream feed valve 14, one

connection is made to the sub-stream permeate or product outlet valve 15 while a third connection is made to the sub-stream retentate or reject outlet valve 16. Additional connections to the biocide treatment system may be made depending on the configuration of membranes selected. Each connection will be arranged in such a way that the sub-stream 12 can be connected to the pre-treatment system while being isolated from, i.e. not connected to, the other sub-streams 12 in the system. This may be accomplished by using multi-port valves 17, single isolating valves of various types 18, double block and bleed valve arrangements 19, mechanical linkages or hose connections 20 or a variety of other established means to divide one part of a production process from another.

Where valves are used to make the connections, they may be manually operated or they may be fitted with actuators of various types to allow the system to be operated remotely or automatically.

In normal operation, the water treatment system operates with all sub- streams 12 in operation, or with one sub-stream held off line. When a biocide dose is required, a sub-streams 12 is isolated or closed from the production system using the means described above and is connected to the biocide treatment system.

Two possible embodiments of the biocide treatment system are shown in Figures 4 and 5. The biocide treatment system is typically comprised of a biocide make-up and recirculation tank 21 , a biocide circulation pump 22, a biocide return pump 23 and one or more filters 24.

The biocide tank 21 is first filled with a pre-determined volume of product water taken from the product stream 4 leaving the membranes 2 introduced

through line 25. The tank 21 may be filled prior to isolating the sub-stream or after the sub-stream is isolated.

A pre-determined volume of biocide is then added to the tank 21 in order to make up the desired biocide treatment concentration, typically between 50 and 500 parts per million by volume. The biocide may be added to the tank 21 by various means including transfer pumps, barrel pumps, gravity drainage or pouring through an open lid.

Biocide effectiveness may be improved by mixing the biocide and water in the tank 21 prior to or during use, either by means of an agitator or stirrer 26 or by returning a portion of the pumped flow, such as from a dedicated pump return line through a restrictor 27.

The preparation of the biocide solution may be an automatic operation using actuated valves with flow and level metering and control devices or it may be a manual operation or a combination thereof. Once the solution is ready, the biocide recirculation pump 22 is used to pump the biocide solution through the isolated sub-stream via line 28. The flowrate may be controlled by a valve or other flow limiting device or may be set by the pump hydraulics.

The pumped supply will normally be introduced to the stream through the connection 14 adjacent to the sub-stream inlet and will flow over and through each membrane 2 in the sub-stream 12, returning to the tank 21 via the lines 29 and 30 adjacent to the permeate 15 and retentate 16 outlets. A flow or pressure control valve or orifice plate 31 may be placed in the retentate return line 30 to ensure that a proportion of the biocide solution is passed through the

membranes to treat the permeate side. Permeate treatment is not always required and this control device may be omitted when it is not required.

Alternatively, the biocide recirculation pump discharge may flow to the retentate connection adjacent to the retentate outlet 16 of the sub-stream for return to the tank 21 via the connections adjacent to the feed water inlet 14 and permeate outlet 15 of the sub-stream.

Where the membrane design is tolerant of back-flow, the biocide recirculation pump discharge may be connected to the connection adjacent to the permeate outlet 15 , for return to the tank 21 via the connections adjacent to the feed water inlet 14 and retentate outlet 16 of the sub-stream.

In an alternative embodiment as shown in Figure 5, the tank 21 may be filled with product water only. The biocide is metered into the pumped flow from a container 32 by means of a dosing or metering pump or by means of an eductor 33 until such time as sufficient biocide has been added to the system. Biocide is circulated through the membranes 2 of the sub-stream until the biocide treatment is considered to be complete.

During biocide treatment of a sub-stream, a proportion of the bacterial colonies and other deposited solids are lifted off the surfaces of the membranes

2, housings 3, pipework and fittings. This material may be removed by a suitable filter 24 or combination of filters installed in the recirculation loop either on the feed or return side of the loop.

When the biocide treatment is complete, the contents of the make-up and recirculation tank 21 are pumped into the membrane product stream at some point downstream of the membranes so that the biocide solution is injected into

the reservoir or production flowlines. The pump used may be a dedicated biocide return pump 23 or the biocide recirculation pump 22 may also be used for this purpose.

In cases where the target reservoir or production process is sensitive to the presence of solids, the solution returned to the product stream is first passed through a suitable filter 34 or combination of filters, which may be dedicated to this service, or which may be the same filters 24 used to clean the solution during recirculation.

As well as injecting seawater into hydrocarbon reservoirs for the purposes of secondary oil recovery or pressure maintenance, it is common practice within the industry to inject produced water for similar purposes. Produced water, as well as drill cuttings produced during oil well drilling operations, is also commonly injected into depleted hydrocarbon reservoir or other sub-surface disposal formations in the vicinity of the production site rather than being discharged to the surface environment. In another embodiment of the invention, the used biocide solution may therefore be added to the produced water stream or to the drill cuttings by means of biocide return pump 23 or biocide recirculation pump 22 for subsequent injection into hydrocarbon reservoir, depleted reservoir or disposal formation. Following the emptying of the tank, additional product water 25 is fed through the tank 21 and the recirculation system and the sub-stream in order to flush out the residual biocide and freed solids. This flushing water is returned to the product stream as described above, and the flushing sequence is repeated until the system is considered to be adequately flushed.

Following the flushing step, the sub-stream is disconnected from the biocide treatment system and is put back into service. Biocide treatment of the next sub-stream may then begin when required.

In order to reduce biocide consumption, the step of emptying the biocide make-up and recirculation tank into the product stream may be omitted and the partially used biocide solution retained for use in a later biocide treatment, with connections provided as shown in Figure 6 to allow the biocide to be flushed out of the membranes into the product line 35 without passing through the tank 21.

Many of the components of the biocide treatment system described above are common with the Clean In Place (CIP) system used to clean membrane trains from time to time. The biocide treatment system of the invention may therefore be incorporated into the CIP system, making use of common equipment to achieve the same operation as described above. Such common components might include for example recirculation tank 21 , recirculation pump 22, and/or cartridge filter 23.

CIP is usually performed with an entire stream or train isolated, though other trains or stream may be in production. In accordance with the invention, the sub-division of the membrane trains or streams described above allows for the design of a common CIP and biocide treatment system whereby CIP is also conducted on one sub-stream at a time while the remaining sub-streams continue to operate. This reduces the size of the CIP tank and CIP pump. Multiple CIP tanks may then be used to minimise consumption of CIP chemicals.