WO2007138327A1

WO2007138327A1 - Method of providing a supply of water of controlled salinity and water treatment system

Info

Publication number: WO2007138327A1
Application number: PCT/GB2007/002020
Authority: WO
Inventors: Robert Weston; John Dale Williams
Original assignee: Natco Uk Limited; Bp Exploration Operating Company Limited
Priority date: 2006-06-01
Filing date: 2007-06-01
Publication date: 2007-12-06

Abstract

This invention relates to a water treatment system and a method of providing a supply of water of controlled salinity suitable for injection into an oil bearing reservoir including the steps of : substantially desalinating a frist feed supply of water to provide a first supply of treated water of low salinity; treating a second feed supply of water to provide a second supply of treated water having a reduced concentration of divalent ions in compartison to the second feed supply and a higher salinity than the first supply of treated water; and mixing the first supply of treated water and the second supply of treated water to provide a supply of mixed water having a desired salinity suitable for injection into an oil bearing reservoir. The first feed supply is preferably treated by reverse osmosis. The second feed supply is preferably treated by nanofiltration.

Description

Method of Providing a Supply of Water of Controlled Salinity and Water Treatment System

This invention relates to methods of providing a supply of water of controlled salinity and water treatment systems, with particular reference to the provision of water for injection into an oil bearing reservoir.

It is known to inject water of low salinity into an oil bearing reservoir in order to enhance the oil recovery from such reservoirs. It has been proposed that the enhanced oil recovery is likely to be due to an increase in pH caused by minerals contained in the rock matrix which is dissolved in the low salinity water. Several mechanisms have been proposed, such as the generation of surfactants, changes in wettability and reduction in interfacial tension. A problem associated with this method is that currently available desalination techniques yield water of a salinity that may be lower than the optimal salinity for improved oil recovery. In fact, the desalinated water produced by such techniques may actually be damaging to the reservoir and inhibit oil recovery, for example by causing swelling of clays. There is an optimal range of salinity, and the optimum values will vary from reservoir to reservoir. One way in which the salinity of a water supply of overly low salinity might be increased is by blending with water of higher salinity. However, the present inventors have found that such blended water has the potential to cause damage to the oil production system. In particular, the present inventors have recognised that the presence of divalent ions in the water above a certain concentration level can have negative effects, such as through the precipitation of "generated" surfactants.

The present invention, in at least some of its embodiments, addresses the above named problems, and enables a supply of water of controlled salinity to be produced which is suitable for injection into an oil bearing reservoir in order to enhance the oil recovery from same.

According to a first aspect of the invention there is provided a method of providing a supply of water of controlled salinity suitable for injection into an oil bearing reservoir including the steps of: substantially desalinating a first feed supply of water to provide a first supply of treated water of low salinity; treating a second feed supply of water to provide a second supply of treated water having a reduced concentration of divalent ions in comparison to the second feed supply and a higher salinity than the first supply of treated water; and mixing the first supply of treated water and the second supply of treated water to provide a supply of mixed water having a desired salinity suitable for injection into an oil bearing reservoir.

For the avoidance of doubt the salinity of a sample of water is herein equated with the total dissolved solids (TDS) in the sample of water.

In addition to providing a supply of mixed water of a desired salinity value, the present invention provides water having relatively low concentrations of divalent ions, such as divalent cations, and in particular calcium ions. The present inventors have recognised that if the concentration of divalent ions in the water supply injected into an oil bearing reservoir is above a certain level, then there is a risk that damage may be caused.

Preferably, the step of treating a second feed supply of water is performed by nanofiltration.

Nanofiltration is commonly used in the oil industry to remove sulphate ions from a source fluid. In such cases, the treated water can be injected into a reservoir without forming sulphate salts with barium/strontium and calcium, which might be present in the formation water in the reservoir. It is common in such instances in the prior art to treat the whole of the water injection stream with nanofiltration. There is no suggestion in the prior art of combining a water stream treated by nanofiltration with a substantially desalinated water stream in order to produce a mixed supply of water of a desired salinity. Furthermore, it should be noted that the present invention enables the capability of nanofiltration to remove divalent ions other than sulphate ions to be exploited.

Alternatively, the step of treating a second feed supply of water may be performed by performing filtration with an ion exchange membrane or a loose reverse osmosis membrane. In a preferred embodiment, the first feed supply is substantially desalinated by a reverse osmosis process.

Alternatively, the first feed supply may be substantially desalinated by an ion exchange process. Other processes, such as thermal desalination or electro-dialysis, might be envisaged. Preferably, the salinity of the supply of mixed water is in the range 100 to

2500 mg r¹.

Preferably, the salinity of the first supply of treated water is less than 100 mg 1-1

Typically, the first supply of treated water and the second supply of treated water are mixed in a volume ratio (volume of first supply to volume of second supply) of 9:1 or greater. The skilled reader will appreciate that the mixing employed will depend on the desired salinity of the supply of mixed water, which in turn may be dependent on the nature of the oil bearing reservoir. The mixing may be affected by the nature of the first and second supplies of treated water and by the concentration of divalent ions in the supply of mixed water.

The first feed supply of water and/or the second feed supply of water may be derived from seawater, aquifer water, river water, brackish water, water obtained from an oil bearing reservoir, synthetic saline water, waste water produced by the desalination or waste water produced by the treatment of the second feed supply of water

Preferably, a source of water is split to provide both the first feed supply of water and the second feed supply of water. The water is the first feed supply and/or the second feed supply may have undergone at least one of: filtration to remove particulate matter; chlorine scavenging; dosing with a biocide; and scale inhibition. These treatments may be performed on the first and/or second feed supplies themselves, or may be performed on a single feed source prior to splitting same into the first and second feed supplies.

Advantageously, the mixing of the first supply of treated water and the second supply of treated water is controlled in accordance with a measured variable. The control may be automatic, and a feedback control system may be employed. The measured variable may be a property of the supply of mixed water, in which instance the measured variable may relate to the salinity of the supply of mixed water, and preferably is the conductivity of the supply of mixed water. The conductivity is a measure of the TDS content of the supply of mixed water. Alternatively or additionally, the measured variable may relate to the divalent ion concentration in the supply of mixed water, or the concentration of selected divalent ions, such as calcium. For example, the mixing may be controlled to prevent a divalent ion concentration in the supply of mixed water exceeding an upper limit. The flow rate of the supply of mixed water may be controlled in accordance with a measured variable.

According to a second aspect of the invention there is provided a method of enhancing oil recovery from an oil bearing reservoir including the steps of: substantially desalinating a first feed supply of water to provide a first supply of treated water of low salinity; treating a second feed supply of water to provide a second supply of treated water having a reduced concentration of divalent ions in comparison to the second feed supply and a higher salinity than the first supply of treated water; mixing the first supply of treated water and the second supply of treated water to provide a supply of mixed water having a desired salinity suitable for injection into an oil bearing reservoir; and injecting the supply of water into the oil bearing reservoir.

According to a third aspect of the invention there is provided a water treatment system for providing a supply of water of controlled salinity suitable for injection into an oil hearing reservoir including: means for providing a first feed supply of water; means for substantially desalinating the first feed supply of water to provide a first supply of treated water of low salinity; means for providing a second feed supply of water; means for treating the second feed supply of water to provide a second supply of treated water having a reduced concentration of divalent ions in comparison to the second feed supply and a higher salinity than the first supply of treated water; and mixing means for mixing the first supply of treated water and the second supply of treated water to provide a supply of mixed water having a desired salinity suitable for injection into an oil bearing reservoir.

Embodiments of methods and water treatment systems in accordance with the invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of a process of the invention;

Figure 2 is a schematic diagram of a reverse osmosis process;

Figure 3 is a schematic diagram of a two stage osmosis process; and Figure 4 is a schematic diagram of a nanofiltration process.

Figure 1 shows a process of the invention in which a chlorinated and strained feed water is supplied at 10 under a pressure which may not be suitable for the operation of the process downstream. A pump system 11 is utilised so as to increase the supply pressure to a desired value. The raised pressure feed water 12 then flows to a filtration system 13 where the water is filtered to remove particulate matter down to a desired level, typically to a level commensurate with a Silt Density Index (SDI 15 minutes) of less than 5 and preferably less than 3. The SDI reduction can be achieved using a variety of well understood methods including microfiltration, ultrafiltration, media filter systems and cartridge filtration. The filtrate 14 is then dosed with chlorine scavenger 15 downstream of the filtration system 13 to remove any residual free chlorine that could otherwise damage the membrane materials disposed downstream and described below. At desired times, the feed water is dosed with a biocide stream 16 to control biological activity which might otherwise occur in the system. Downstream of the point at which biocide is dosed intermittently, the filtered and conditioned feed water is split into first 17 and second 18 feed streams. Scale inhibitor is dosed at 19 into the first feed stream 17 so as to minimise scaling on the downstream membrane surface (described below). Scale inhibitor is dosed at 20 into the second feed stream 18 for the same purposes.

The first supply stream 17 is used to supply a reverse osmosis system. To enable reverse osmosis to occur, the water pressure is increased via a high- pressure booster pump system 21 and the increased pressure water supply is fed to a reverse osmosis membrane system 22, where upon a permeate stream 23 and reject stream 24 are produced.

The second feed stream 18 is fed to a nanofiltration system. To enable nanofiltration to occur, the feed pressure is increased by a high-pressure booster pump system 25, and the resultant high pressure supply is fed to a nanofiltration membrane system 26, where upon a permeate stream 27 and a reject stream 28 are produced.

The salinity of the reverse osmosis permeate stream 23 is extremely low, and in fact is somewhat lower than the optimal range of salinities for oil recovery applications in which the water is injected into an oil bearing reservoir. The salinity of the nanofiltration permeate stream 27 is considerably higher than the salinity of the reverse osmosis permeate stream 23; however, the concentration of certain, undesirable divalent cations, principally Ca²⁺, is greatly reduced by the nanofiltration process. The reverse osmosis permeate stream 23 and nanofiltration permeate stream 27 are mixed at 29 at a desired mixing ratio in order to produce a mixed water supply 30 which may be injected into an oil bearing reservoir for oil recovery purposes. By controlling the mixing ratio of the reverse osmosis permeate stream 23 and nanofiltration permeate stream 27, it is possible to produce a mixed water supply that has the optimal salinity for oil recovery in the oil bearing reservoir that the system is intended to be used with. A further advantage relates to the concentration of divalent ions in the mixed water supply 30. The present inventors have recognised that if the concentration of divalent ions in the water supplied to an oil bearing reservoir are above a certain level, then there is a risk that damage may be caused, for example by precipitation of generated surfactants. The reduction in the divalent ion concentration of the nanofiltration permeate stream 27 produced by the nanofiltration process leads to a mixed water supply that has relatively low concentrations of divalent ions.

The mixing of the reverse osmosis permeate stream and the nanofiltration permeate stream, in particular their relative proportions, can be controlled manually or automatically using a suitable flow control system. Such flow control systems are well known to those skilled in the art. It is particularly preferred that an automatic flow control system is utilised that controls the mixing in accordance with a measured variable. For example, conductivity readings of the mixed water supply can be taken as a measure of the total dissolved solids content, and used to control the mixing of the reverse osmosis permeate stream and nanofiltration permeate stream. Suitable control systems, which might incorporate a microprocessor, would readily suggest themselves to the skilled reader. The flow rate of the mixed water supply might be controlled in a similar manner. Furthermore, it is possible to make other measurements, such as measurements of calcium ion concentration, and to use these measurements to control the water treatment system. It may be necessary to augment these readings with laboratory testing in order to ensure that the system is performing according to expectation. In further embodiments, data relating to in-situ measurements made on the water treatment system are conveyed to a central monitoring location by suitable means, such as by telemetry or over the Internet.

Figure 2 shows a reverse osmosis system in more detail. The first feed stream, having had scaling inhibitor added thereto, enters a high-pressure booster pump system 34 and is fed to a one stage reverse osmosis membrane system 36. Permeate water 37 from the reverse osmosis membrane system 36 flows to a tank 38. The tank also provides a reservoir of clean water to allow natural backflow (due to the natural osmosis effect) through the system in the event of an emergency shutdown. The tank 38 may also have other uses, for example it may be used as the cleaning-in-place (CIP) tank feeding a CIP pump for membrane cleaning. The reject stream 39 may contain sufficient energy to justify the inclusion of an energy recovery device 40, such as a Pelton Wheel device. The reject stream then exits the energy recovery device 40 at a much reduced pressure. The low salinity reverse osmosis permeate 37 exits the tank 38 and subsequently is pressurised with a pumping system 41 and transferred downstream for blending with the nanofiltration permeate stream.

From time to time, the reverse osmosis membrane will require cleaning under a variety of conditions, for example, an acid solution to remove mineral scale from the membrane surface. The permeate contained in the CIP tank is ideal for creating the cleaning solution, by mixing a quantity of liquid or powder cleaning chemical with the low salinity water to create a cleaning solution with the desired properties, e.g. pH.

It is possible to utilise other reverse osmosis systems. Figure 3 depicts a two-stage reverse osmosis system in which a feed water supply 50 enters a first membrane stage 51 which produces a first permeate stream 52 and a first reject stream 53. The first reject stream 53 feeds a second membrane stage 54 and thus produces a second permeate stream 55 and a second reject stream 56. The first and second permeate streams 52, 55 can then be combined into a single product stream 57.

Figure 4 shows a nanofiltration system in greater detail. A second feed stream 60, having undergone scale inhibition, is fed into a high-pressure booster pump system 62 and then into a first nanofiltration membrane 64, thereby producing a first permeate stream 65 and a first reject stream 66. The first reject stream 66 feeds a second nanofiltration membrane 70 which produces a second permeate stream 67 and a second reject stream 68. The first and second permeate streams 65, 67 are then combined into a single stream 69. The skilled reader will appreciate that further equipment, such as a CIP system, might be provided, and that other embodiments of nanofiltration systems, such as a single nanofiltration membrane system, might be utilised.

TABLE 1 :

Table of projected reverse osmosis (RO) and nanofiltration (NF) treated water salinities and individual ionic species

TABLE 2:

Table of blended water compositions: RO permeate - NF permeate

ro

TABLE 3:

Table of blended water compositions: RO permeate - feed

ω

Feed water may come from a variety of sources, such as seawater, aquifer water, river water, and water produced from an oil bearing reservoir. Table 1 provides typical, exemplar water analyses for a seawater feed source and the reverse osmosis and nanofiltration permeates produced therefrom. It can be seen that the reverse osmosis permeate is of very low salinity, whereas the nanofiltration permeate produces only a 26% reduction in total dissolved solids (and a 10% reduction in Na⁺ concentration). However, nanofiltration produces very substantial reductions in divalent ion concentrations. As noted above, in order to produce low salinity water with desired levels of divalent ions, it is necessary to combine the nanofiltration permeate with the reverse osmosis permeate in certain proportions. Table 2 provides typical water analyses for blended water over the range 100% reverse osmosis permeate: 0% nanofiltration permeates to 80% reverse osmosis permeate: 20% nano-filtration permeate. It should be noted that the salinity as measured by TDS rises towards 5500 mgl^'1 with divalent calcium ion concentration increasing from 1 mgl^'1 to 8 mgl^"1 over the same range. In comparison, Table 3 gives typical water analyses for blended water over the range 100% reverse osmosis permeate: 0% seawater feed to 80% reverse osmosis permeate: 20% seawater feed. It should be noted that the TDS concentration rises towards 7400 mgl^'1, with the divalent calcium ion concentration increasing from 1 mgl^'1 to 97 mgl^"1 over the same range. Thus, the present invention enables a blended water stream having relatively low concentrations of divalent ions to be produced.

Numerous variations to the techniques explained above might be contemplated. For example, instead of nanofiltration, filtration by a RO type membrane, particularly a loose RO membrane, or an ion exchange membrane might be performed. The choice of technique to employ may be influenced by the feed sources utilised, in particular the salinity of the feed sources.

Claims

1. A method of providing a supply of water of controlled salinity suitable for injection into an oil bearing reservoir including the steps of: substantially desalinating a first feed supply of water to provide a first supply of treated water of low salinity; treating a second feed supply of water to provide a second supply of treated water having a reduced concentration of divalent ions in comparison to the second feed supply and a higher salinity than the first supply of treated water; and mixing the first supply of treated water and the second supply of treated water to provide a supply of mixed water having a desired salinity suitable for injection into an oil bearing reservoir.

2. A method according to claim 1 in which the step of treating a second feed supply of water is performed by nanofiltration.

3. A method according to claim 1 or claim 2 in which the first feed supply is substantially desalinated by a reverse osmosis process.

4. A method according to claim 1 or claim 2 in which the first feed supply is substantially desalinated by an ion exchange process.

5. A method according to any previous claim in which the salinity of the supply of mixed water is in the range 100 to 2500 mg I^"1 .

6. A method according to any previous claim in which the salinity of the first supply of treated water is less than 100 mg I^"1.

7. A method according to any previous claim in which the first supply of treated water and the second supply of treated water are mixed in a volume ratio (volume of first supply to volume of second supply) of 9:1 or greater.

8. A method according to any previous claim in which the first feed supply of water and/or the second feed supply of water is derived from seawater, aquifer water, river water, brackish water, water obtained from an oil bearing reservoir, synthetic saline water, waste water produced by the desalination or waste water produced by the treatment of the second feed supply of water.

9. A method according to any previous claim in which a source of water is split to provide both the first feed supply of water and the second feed supply of water.

10. A method according to any previous claim in which the water in the first feed supply and/or the second feed supply has undergone at least one of: filtration to remove particulate matter; chlorine scavenging; dosing with a biocide; and scale inhibition.

11. A method according to any previous claim in which the mixing of the first supply of treated water and the second supply of treated water is controlled in accordance with a measured variable.

12 A method according to claim 11 in which the measured variable is a property of the supply of mixed water.

13. A method according to claim 12 in which the measured variable relates to the salinity of the supply of mixed water, and preferably is the conductivity of the supply of mixed water.

14. A method of enhancing oil recovery from an oil bearing reservoir including the steps of: substantially desalinating a first feed supply of water to provide a first supply of treated water of low salinity; treating a second feed supply of water to provide a second supply of treated water having a reduced concentration of divalent ions in comparison to the second feed supply and a higher salinity than the first supply of treated water; mixing the first supply of treated water and the second supply of treated water to provide a supply of mixed water having a desired salinity suitable for injection into an oil bearing reservoir; and injecting the supply of water into the oil bearing reservoir.

15. A water treatment system for providing a supply of water of controlled salinity suitable for injection in an oil bearing reservoir including: means for providing a first feed supply of water; means for substantially desalinating the first feed supply of water to provide a first supply of treated water of low salinity; means for providing a second feed supply of water; means for treating the second feed supply of water filtration to provide a second supply of treated water having a reduced concentration of divalent ions in comparison to the second feed supply and a higher salinity than the first supply of treated water; and mixing means for mixing the first supply of treated water and the second supply of treated water to provide a supply of mixed water having a desired salinity suitable for injection into an oil bearing reservoir.