FIELD OF THE INVENTION

The invention relates to a method and a device for the recovery of hydrocarbons from an oil reservoir.

BACKGROUND

In the recovery of hydrocarbons from oil fields, various techniques are employed to optimize oil and gas production.

Traditional displacement methods use water to displace oil in a field, effectively pushing the oil to a collector point. Chemicals such as surfactants may be added to alter the flow and mixing properties of the oil/water mixture that is obtained.

An alternative method is to use gas to displace oil. However, in many cases the availability of large volumes of pressurized gas is limited, making the method relatively expensive. It may also be troublesome to maintain the pressure once a major part of the oil is recovered, and collapse of the field structure may also become a problem. In addition, not all gasses are suitable or desirable to have injected into an oil field. Nitrogen gas is fairly inert but is relatively expensive to obtain. Another known method is to re-inject the natural gas (mostly consisting of methane) produced from the oil field.

Combined methods include water-alternating-gas (WAG) which uses intermittent injection of water and gas, and SWAG wherein water and gas are injected simultaneously. Carbon dioxide is a favoured gas to inject in the WAG method. Such methods are believed to yield 5-10% recovery improvement under favourable conditions, compared to continuous water injection. A disadvantage of the known techniques is hat relatively large fractions of oil are left in the field, and that the injected water may leave residual oil fractions harder to recover. Also, residual chemicals in the oil field potentially hamper future attempts for recovering the residual oil. SUMMARY

It is an object of the invention to increase the oil recovery yield. It is another object of the invention to increase the oil recovery yield while minimizing the introduction of additional agents into the oil field that may complicate future attempts to recover the remaining oil.

The invention provides a method for producing hydrocarbons, comprising the steps of providing production water, injecting the production water into an oil reservoir, and collecting hydrocarbons displaced by the production water from the oil reservoir, wherein the salinity of the production water is lowered prior to or during injection into the oil reservoir by admixing low salinity water having a dissolved salt content lower than the production water. It was discovered that lowering the salinity of the water increased the yield in the recovery of oil from an oil reservoir. Also, the addition of low salinity water does not add to the complexity of the compounds present in the oil field. The low salinity water may be added to the production water prior to injection, or the low salinity water may be co-injected, causing mixture after injection. Preferably, the low salinity water may be obtained as a combustion product from hydrocarbons collected from the oil reservoir.

The patent application WO2008029124 describes that aqueous displacement fluids having a total dissolved solids (TDS) content of between 200 pp, and 10.000 ppm are useful in recovering crude oil. Relatively expensive methods such as reverse osmosis or forward osmosis are mentioned as methods to achieve injection water with the desired total dissolved solids.

Preferably, the salinity of the resulting mixture of production water and low salinity water is monitored continuously or at intervals, and optionally the mixing ratio is adjusted to keep the salinity within predetermined limits. Thus, the yield of the retrieved hydrocarbons becomes more predictable, which allows for better planning of the logistic process and further process steps after the retrieval of the hydrocarbons from the oil field. The amount of dissolved salts in water can be determined by well-known chemical analysis techniques. A relatively quick way to monitor relative salinity is to measure the electric conductivity of the water, wherein a high electric conductivity corresponds to a high dissolved salt content. Using such methods the low salinity water may for instance have an electric conductivity below 1000 με/οιτι, preferably below 100 με/ατι, most preferably below 10 με/οιτι. It is preferred if the average dissolved salt content of the production water after admixing the low salinity water is less than 10,000 ppm total dissolved solids (TDS), preferably less than 5,000 ppm TDS, more preferably less than 2,000 ppm TDS. Preferably, the admixed low salinity water has a multivalent cation content lower than 0.02 mol %, preferably less than 0.005 mol %. The lower the diluted salt content of the low salinity water, the smaller the volume of low salinity water that needs to be added for a significant increase in oil recovery. That is to say, Sea Water normally has around 30,000-40,000 ppm TDS and the lower the diluted salt content of the low salinity water, the smaller is the volume of low salinity water that needs to be added to produce a reduced salinity production water which can be used to obtain a significant increase in oil recovery. Low salinity water may be obtained from various sources, such as fresh water sources or collected rain water. However, low salinity water is often not readily available, so active desalination through known processes such as reverse osmosis may be needed. An advantageous form of low salinity water is distilled water, which may be obtained through for instance reduced pressure distillation or solar evaporation.

It is advantageous if the low salinity water is obtained as a combustion product from hydrocarbons. Water may be collected from exhaust fumes using a cooler, which is essentially a distillation process. The water has a relatively low salinity comparable to distilled water and is often readily available from combustion of hydrocarbons already occurring at or near oil production sites. Energy obtained from the same combustion process may also be used in the method, for instance for the transport, admixing, injection and/or hydrocarbon collection steps.

Preferably, the low salinity water may be obtained as a combustion product from hydrocarbons collected from the oil reservoir and/or a neighboring oil reservoir. Thus, at least part of the low salinity water needed may be obtained from the oil production itself, resulting in an essentially self-sustainable process.

More preferably, the low salinity water may be obtained as a combustion product from hydrocarbons mostly derived from the combustion of natural gas. Gas, primarily methane gas, obtained during the retrieval of oil is readily available, hence the method allows for easy integration into existing methods.

It may be preferred if also a non-aqueous injectant, preferably consisting of e.g. carbon dioxide, nitrogen gas or mixtures thereof, is injected into the oil reservoir simultaneously or intermittently with the production water. Thus, the amount of retrieved hydrocarbons may be even further enhanced. Improvements relating to existing water-alternating-gas injection (WAG) are enhanced using the method described herein. Preferably, the non-aqueous injectant essentially consists of carbon dioxide, wherein the carbon dioxide is a combustion product obtained from the combustion of hydrocarbons collected from the oil reservoir. Using carbon-dioxide makes the system at least partly self-sustainable. More preferably, both the low salinity water and the carbon dioxide injected into the oil reservoir are combustion products obtained from the combustion of hydrocarbons collected from the oil reservoir

In addition to the measures described above, the yield may be further improved if the production water comprises at least one mobility control agent, preferably a polymer mobility control agent. A mobility control agent may be used to increase the viscosity of the production water.

The mobility control agent may be selected from the group consisting of alkyl sulfonates, olefin sulfonate, aryl sulfonates, polysaccharides, polyacrylamides, polyethyleneoxides, polypropyleneoxides.

The invention further provides a device for the recovery of hydrocarbons from an oil reservoir, comprising

- a source of production water

- a source of low salinity water having a dissolved salt content lower than the production water,

- at least one injector coupled to the source of production water and the source of low salinity water, arranged for injecting a mixture of the production water and the low salinity water into the oil reservoir.

Such a device can be used to perform the method as described herein.

It is preferred if the injector is coupled to the source of production water and the source of low salinity water through a mixer for mixing the production water and low salinity water. This construction allows for the mixing of the production water and the low salinity water to the desired salinity. The mixer may be provided with a monitor, such as an electrical conductivity meter, to monitor the resulting salinity. This information may be used to alter the ratio of production water and low salinity water.

Preferably, the source of low salinity water comprises a combustor for hydrocarbons, supplied with an exhaust water collector. Thus, water obtained from combustion of hydrocarbons may be collected for use in lowering the salinity of the production water. If the amount of low salinity water obtained by combustion is insufficient, also low salinity water from other sources may be admixed simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the invention are illustrated by way of non-limiting examples in which : Figure 1 shows a production method for hydrocarbons.

Figure 2 shows a device for performing the method as described herein.

DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 schematically shows a device 1 as described herein. In particular, Figure 1 illustrates a device for production of hydrocarbons according to aspects of the claimed method. Further in particular, Figure 1 illustrates a production method for producing hydrocarbons according to the invention and the relative position of functional units of a device used with the method. The device 1 may comprise an air separator unit 2. The device 1 , wherein an air separator unit 2 separates air 3 into nitrogen gas 4 and oxygen

5. That is to say, in the device 1 , an air separator unit 2 separates air 3 into nitrogen gas 4 and oxygen 5. The oxygen is used in a combustion unit 6 to combust hydrocarbons 7, yielding energy 8, water 9 and carbon dioxide 1 0 as main products. The water 9 from the combustion unit 6, collected by a distillation cooler, has a low amount of dissolved salts, leading to a low salinity comparable to distilled water. The low salinity water 9 is lead to a first injector unit 1 1 . In the injector unit 1 1 , the water is brought to the desired temperature and pressure. The low salinity water 9 is mixed with regular production water 1 2 having a higher salinity than the water 9 retrieved from the combustor 6. Optionally, other liquids and/or flow-affecting compounds such as surfactants may also be added prior to injection. The mixed water 1 3 has a lower salinity than the original production water 1 2, and is injected into an oil field 14. The relatively low sal inity water displaces hydrocarbons present in the oil field, that are collected at one or more production wells 1 5 through a collector unit 1 6. Part 1 7 of the collected hydrocarbons is transported away for further processing, whereas another part of the hydrocarbons is lead to the combustor 6 as fuel. The part used for fuel is preferably the volatile fraction of the produced hydrocarbons, in particular methane gas.

Optionally, carbon dioxide 1 0 from the combustor 6 and/or nitrogen gas 4 from the air separation unit 2 are transported to a second injector unit 1 8. In the injector unit 43, the gaseous injectants are mixed and brought under the desired temperature and pressure. Optionally, additional gases 1 9 are ad mixed, i.e. added to the mixture, such as additional carbon dioxide and/or nitrogen gas from other sources. The gaseous injectants 20 may also be injected into the oil field 14, either simultaneously with the water 13, or intermittently in a water-alternating-gas method.

Figure 2 schematically shows a device 30 that can be integrated in the system in figure 1 . The device 30 comprises a reservoir of production water 31 and a reservoir of low salinity water 32 that has a lower dissolved salt content than the production water 31 . The production water 31 may for instance be supplied from re-used water from the production water, and is optionally supplied with additional mobility control agents as described herein. The low salinity water 32 may for instance be supplied from a hydrocarbon combustion unit. The production water 31 and low salinity water 32 are combined in a mixing unit 33, in a predetermined ratio that can be adjusted to obtain the preferred salinity/dissolved salt content. Alternatively, the production water 31 and low salinity water 32 may be combined in a mixing unit 33, where the two types of water are mixed in a ratio that can be adjusted to obtain the preferred salinity/dissolved salt content. The mixed water 34 is led through a monitoring unit 35, that measures the salinity, for instance through electric conductivity measurements. Optionally, the salinity of the mixed water 34 may then be monitored as the mixed water 34 is led through the said monitoring unit 35. If the salinity as measured is outside predetermined limits, the mixing ratio in the mixer unit 33 may be adjusted accordingly. Subsequently, the mixed water is lead to the actual injector 36 for injection into the oil well.

It may be a further object of the invention to provide an in situ production of low salinity production water to be used to increase the oil recovery yield from an oil reservoir. Accordingly, in other aspects, the invention may relate to a method for in-situ producing a low salinity production water to be used in a process for enhanced recovery of oil, and a device for using with this method for enhanced recovery of hydrocarbons from an oil reservoir. Therefore, in another aspect, the invention may further provide a device for providing a low salinity production water for recovering of hydrocarbons from an reservoir.

Accordingly, in a further optional embodiment, the invention may be defined as a method for producing hydrocarbons, comprising the steps of

a) providing a continous flow of production water

b) injecting the production water into an oil reservoir, and

c) Collecting hydrocarbons displaced by the production water from the oil reservoir, wherein the salinity of the continous flow of production water is adjusted prior to or during injection into the oil reservoir by mixing a continous flow of available high salinity water such as sea water with a continous flow of low salinity water having a dissolved salt content which is so much lower than the dissolved salt content of the available high salinity water that it is possible to obtain a continues flow of production water having a dissolved salt content less than 10,000 ppm total dissolved solids (TDS).

Accordingly, optionally: the average dissolved salt content of the production water after admixing the low salinity water may be less than 5,000 ppm TDS, or even less than 2,000 ppm TDS. Still further optionally, the salinity of both the resulting mixture of production water and of the low salinity water may be monitored continuously or at intervals, and optionally the mixing ratio may be adjusted by adjusting the flow of low salinity water to keep the salinity within predetermined limits. Still further optionally, if the low salinity water is obtained as a combustion product from hydrocarbons collected from the oil reservoir and/or a neighboring oil reservoir, then the combustion may be in situ.

In a further optional embodiment, the invention may be defined as a device for the recovery of hydrocarbons from an oil reservoir, comprising

- a source of available high salinity water,

- a source of low salinity water having a dissolved salt content lower than the available high salinity water,

- a mixer having an inlet for available high salinity water and an inlet for low salinity water and an outlet for mixed water;

- at least one injector coupled to the outlet of the mixer, arranged for injecting the mixed production water into the oil reservoir.

Accordingly, optionally: the source of low salinity water may comprise a combusting unit cumbusting hydrocarbons during production thereby providing a source of carbon dioxide and separate source of low salinity water. Still further optionally, the amount of low salinity water may be controlled by a feed forward control based on a measurement of the salinity of the available high salinity water and a measurement of the salinity of the low salinity water.