The present invention relates to fluid treatment apparatus, and a method and system for injecting water treated by for example the apparatus into oil resorvoirs.

There is a need to reduce the cost of exploration and production activity in off-shore oil fields including the North Sea. Problems associated with the use of injected sea water to assist the sweep of oil and pressure maintenance in the reservoir has received particular attention.

Generally an oil production platform is provided with a processing plant for sea water, the output of that processing plant being conveyed through pipelines from the platform to the formation, for example to each of a series of satellite injection wells located on the sea bed. The sea water processing plant typically comprises in series a coarse filter in the form of a body of sand, a fine filter, typically again of sand, a chlorination unit, a de-aeration unit and a de- oxygenation treatment. Chlorination has been thought necessary to avoid biological activity in sea water handling facilities, although it has been found that residual chlorine in injected sea water can cause considerable corrosion in the sea water distribution piping. Chlorination increases the redox potential of the sea water to a potential at which microbiological souring of the seawater handling facilities is not considered possible. Operators injecting anaerobic sea water at negative redox potentials, that is with residual bisulphite oxygen scavenger, run the risk of microbiological souring although corrosion of equipment is significantly reduced at lower redox potentials.

The conventional deaeration equipment has been provided to prevent rapid corrosion of the water distribution pipeline connecting the platform to the injection wells. The pipeline has generally been fabricated from carbon steel. Deaeration equipment is relatively large and heavy, and therefore providing accommodation on a platform for such equipment demands substantial expenditure. Materials are now available however which can be used for the distribution pipeline which are corrosion resistant and the cost of which can be offset by savings resulting from dispensing with the conventional deaeration equipment. Thus the injection of aerobic sea water can be considered providing appropriate materials are used for the distribution pipeline.

Although it is known in conventional systems to prevent microbiological souring of seawater handling facilities by chlorination. microbiological souring of the oil bearing formation itself is a problem even if the injected seawater contains

traces of residual chlorine. This is because sulphate reducing bacteria (SRB) can grow in anaerobic seawater which has a redox potential of about -lOO V or more negative. SRB cannot grow in aerobic seawater however which has a redox potential of about +250mV. Accordingly, if aerobic seawater was injected it might be expected that SRB growth would not be a problem. Any aerobic microbiological activity could be controlled by conventional chlorination of the sea water handling facilities. However, monitoring of oil bearing formations into which sea water has been injected has revealed that the physical boundary between injected sea water and displaced oil is in advance of the thermodynamic front (the region of the formation where the cooling effect of the injected sea water is pronounced) and is in advance of the physiochemical front (the region of the formation where the chemical environment changes from approximately that of the original formation to approximately that of the injected seawater. This means that microbiological activity the nature of which can be difficult to predict can occur within the formation in the region between the physical boundary of the injected sea water and the physiochemical front.

If aerobic sea water is to be injected, as indicated above it will be necessary to use corrosion resistant materials for the distribution pipeline, for example corrosion resistant alloys or plastics coated steel. Providing a sea water distribution pipeline network from a platform to a series of satellite wells involves considerable expense, particularly if simple carbon steel cannot be used. In addition, it would be desirable to avoid the need for any sea water handling facilities on the platform.

It is an object of the present invention to provide a fluid treatment apparatus which can significantly reduce the costs associated with the known sea water processing plant described above, but which may find application in fields other than that of sea water processing in offshore oil installations. It is also an object of the present invention to provide a method for operating a water injection system, and the components of such a system.

According to the present invention, there is provided a fluid treatment apparatus comprising a membrane formed from first and second electrically conductive fluid permeable layers which are separated and electrically isolated from each other by a fluid permeable electrically insulating layer, means for applying an electrical potential between the first and second electrically conductive layers, and means for applying a pressure differential across the membrane to drive fluid therethrough.

The apparatus may be used in the off-shore oil industry to process sea water which is injected into sub-sea oil wells. In such an application, the membrane may define a housing which encloses a sea water pump, the housing being located in use on the sea bed. The pump has an inlet communicating with the interior of the housing and an outlet connected to a sea water injection string. The housing may be provided with a releasable connector which connects the pump outlet to the injection string. When the connector is released, the housing may be lifted from the sea bed by connecting appropriate lifting apparatus to an appropriate coupling provided on the housing.

The electrically insulating layer may be a consolidated body of sand several centimetres thick, for example 2 to 10 centimetres thick. The first and second electrical conductor layers may be perforated metal sheets. The metal sheet on the side of the membrane which is at the higher pressure may be coated with titanium of a similar material which does not self-consume at the electrochemical potential at which chlorine species are generated, or alternatively it may be coated with a graphite-loaded polymer coating.

The chemical characteristics of fluid emerging from the membrane may be controlled by monitoring the chemical condition of the apparatus and controlling the potential applied between the first and second layers accordingly. A variety of chemical conditions could be monitored in isolation or combination, including redox potential monitoring, corrosion potential noise monitoring, zero-resistance ammetry and the like.

The invention also provides a method for operating a water injection system to lift oil from an oil bearing formation, wherein aerobic water is injected into the formation, the water being chlorinated such that the redox potential of the injected water is sufficiently high to inhibit anaerobic bacterial activity within regions of the formation flooded by the injected water.

The injected water may be chlorinated such that the redox potential of regions of the formation flooded by the injected water is more positive than -lOOmV.

The invention also provides an oil bearing formation water injection system comprising an injector extending into the formation, means for pumping aerobic water into the injector, and means for chlorinating the injected water such that the redox potential of the injected water is sufficiently high to inhibit anaerobic bacterial activity within regions of the formation flooded by the injected water.

The redox potential of the injected sea water may be maintained at a very high level, for example 50mg/litre of residual free chlorine, although in many

applications a level of 2mg/litre should be sufficient to suppress microbiological activity. Some corrosion resistant alloys are subject to crevice attack if exposed to high levels of residual free chlorine for prolonged periods, and if such alloys are used for the distribution pipeline it would be preferable to vary the residual free chlorine level over time, for example using a level of 50mg/litre for one hour per day, and a level of zero for the rest of each day.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a highly schematic illustration of a sub-sea housing embodying the present invention;

Figure 2 is a schematic illustration of a portion of the wall of the housing shown in Figure 1 ; and

Figure 3 is a schematic sectional view through the wall of the housing of Figure 1 together with a schematic representation of a potential applying and chemical condition monitoring system.

Referring to Figure 1 , in the illustrated case the tube 1 represents an injection string which extends downwards through the sea bed. The housing 2 rests on the sea bed and comprises a substantially planar base 3 and a hemispherical domed roof 4. A cylinder 5 schematically represents a sea water pump enclosed within the housing and a cylinder 6 schematically represents a coupling device which would enable the housing to be connected to lifting apparatus (not shown). A portion of the roof 4 of the housing is shown as being cut-away for the purposes of illustration. It will be appreciated that in practice the housing will completely enclose the sea water pump 5.

The sea water pump 5 will be mounted on a connector extending through the base of the housing to enable it to mate with the injection string 1. When so positioned and energised the sea water pump will pump fluid from inside the housing into the injection string 1 and as a result the pressure within the housing will be less than that outside the housing.

Referring to Figure 2, this illustrates the structure of the wall of the housing forming the roof 4. An upper perforated metal sheet 7 which is located on the outside of the housing and a lower perforated sheet 8 which is located on the inside of the housing has sandwiched between it a consolidated layer of sand 9. The perforated metal sheets 7 and 8 may be of any appropriate structure providing they are permeable to sea water. For example they could be in the form of a simple metallic mesh. A power source which would generally be an AC rectifier but which is represented schematically in Figure 2 by a battery 10

maintains a DC potential difference between the sheets 7 and 8 and thus applies a DC potential across the layer 9.

Referring to Figure 3, this is a schematic sectional view through the structure of Figure 2. An arrow 1 1 represents the flow of sea water through the structure. The conductive layer 7 forms an anode and the conductive layer 8 forms cathode. As sea water is drawn through the structure, chlorine species are evolved. At the pH of sea water, the chlorine appears as hypochlorite. This evolution of chlorine is represented in Figure 3 by the arrow 12. The cathode 8 will give off hydrogen and will be subject to some metal plating. The potential difference between the anode and cathode will determine the nature of the chemical processes that take place. If both layers 7 and 8 were simple steel electrodes, one of the electrodes would self-consume and therefore it is necessary to make the structure of materials that will ensure that oxidation reactions are at a potential more negative than would cause self-consumption. For example titanium, ruthenium oxide coated titanium and graphite loaded polymers can be considered as suitable materials for coating the metal layers 7 and 8.

As sea water is drawn through the membrane structure, the sand layer 9 effects coarse filtration. The amount of hydrochlorite in the sea water inside the housing is a function of both the rate at which sea water enters the housing and the potential applied between the layers 7 and 8. Generally the rate of flow will be determined by the management requirements of a particular installation, whereas the chemical nature of the sea water reaching the pump 5 should be constant. Accordingly it is desirable to monitor the chemical nature of the sea water reaching the pump and control the voltage applied between the layers 7 and 8 to stabilise that chemical nature. Accordingly as illustrated in Figure 3, a sensor 13 may be arranged to monitor the chemical nature of the sea water passing through the membrane structure, the output of the sensor 13 controlling a power supply 14 which in turn controls the potential applied between layers 7 and 8.

The redox potential of aerobic sea water is typically +250mV. The illustrated apparatus may be operated such that the redox potential of the sea water reaching the pump will be +600mV. This is sufficient to ensure that microbiological activity within the formation flooded by the injected sea water is suppressed.

In a typical installation, a number of housings such as that illustrated in Figure 1 will be linked to a single production platform, each housing being positioned directly over a respective injection string or positioned in the vicinity

of a respective injection string to which it will be connected by appropriate piping. Electrical power for each of the housings would be delivered from the production platform and an appropriate DC rectifier would be provided for each housing to maintain the desired potential difference across the membrane. In a typical case, each unit would be expected to deliver 30,000 barrels of water per day. A throughput of this volume could be achieved having a housing in the form of a six metre diameter hemisphere. In such an arrangement the velocity of sea water passing through the membrane would be of the order of five metres per day. This flow rate is less than that of ocean currents and so blinding of the filter by a build-up of contaminants is not expected to be a serious problem. The sand layer would be no more than a coarse filter removing solids with a diameter of for example greater than 100 microns.

In the case illustrated in Figure 3, a single chemical parameter of the sea water emerging through the membrane is monitored by a sensor mounted on the inner surface of the membrane. It will be appreciated however that monitoring probes could be embedded within the sand layer, or simply located inside the housing. All that is required is the capability to detect chemical changes in the flowing sea water to enable appropriate control of the potential applied across the membrane. This enables a constant concentration of hypochlorite and other generated species to be maintained independent of the flow rate.

It will be appreciated that with the apparatus illustrated in Figures 1 to 3, chlorinated aerobic sea water is injected into the injection string 1 . The level of residual free chlorine in the injected sea water can be controlled by controlling the flow rate and the potential between the electrodes 7 and 8. Alternative apparatus could be used to achieve the same objective however, for example a conventional electrochlorinator connected in series with a conventional filter such as a sand filter or a hydrocyclone. It will also be appreciated that the apparatus illustrated in Figure 1 to 3 could be used for purposes other than the chlorination of sea water, for example any application in which large volumes of a fluid must be filtered and electrochemical ly treated.