Seawater treatment to obtain low hardness, high salinity water for enhanced oil recovery.

Abstract

Highly saline feedwater, such as seawater, is treated to produce a reverse osmosis (RO) concentrate and an RO filtered product. Optionally, some or all of the RO concentrate can be filtered to produce a nanofiltration (NF) filtered product. Optionally, some feedwater can be filtered to produce an NF filtered product without first being concentrated by RO processing. The NF filtered product, or a mixture of RO and NF filtered products, can be used to produce produced water for injection into oil-bearing reservoirs to enhance crude oil recovery. Optionally, the produced water can have a higher salinity than the feedwater, or at least 30 g / L. The produced water can have a hardness of less than 20 mg / L.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of Indian Patent Application No. 201911018156, filed on May 7, 2019, which is incorporated by reference.

[0002] This specification relates to the treatment of seawater to produce water for injection into oil-bearing reservoirs. [Background technology]

[0003] WO 2019 / 053092 A1, "Method of Controlling Salinity of a Low Salinity Injection Water," describes a method that includes producing two mixtures of low salinity water for injection into two injection wells in different regions of an oil-bearing reservoir. The mixtures include varying amounts of nanofiltration and reverse osmosis filtered products produced by filtering a high salinity feedwater, such as seawater. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 053092 Summary of the Invention

[0005] In the methods described herein, a highly saline feedwater, such as seawater, is treated to produce a reverse osmosis (RO) concentrate and an RO filtered product. The RO concentrate (alternatively called waste) is filtered to produce a nanofiltration (NF) filtered product. Optionally, some feedwater can be filtered to produce an NF filtered product without first being concentrated by RO processing. The NF filtered product, or a mixture of the RO and NF filtered products, can be used to produce produced water for injection into an oil-bearing reservoir. The produced water can have a salinity of 30 g / L or more, for example, in the range of 30 g / L to 50 g / L. Optionally, the produced water can have a higher salinity than the feedwater, for example, higher than seawater (typically greater than about 35 g / L TDS or 40 g / L TDS). The produced water can have a hardness of less than 20 mg / L. The salinity and hardness of the product water may optionally be varied over time or may be kept approximately constant despite changes in the feed water, for example, by varying the proportion of the feed water processed by NF processing that is RO concentrate, and / or by varying the mix ratio of RO filtered product to NF filtered product in the product water.

[0006] The system described herein includes an RO system and an NF system. An inlet to the RO system is connected to a feedwater source. An inlet to the NF system is connected to a feedwater source. A concentrate outlet of the RO system is connected to a feed inlet of the NF system. A filtered product outlet from the NF system is connected to an injection system, e.g., via a holding tank. A filtered product outlet from the RO system is connected to an injection system, e.g., via a holding tank. The injection system is adapted to inject water containing the RO filtered product and / or the NF filtered product into an oil-bearing reservoir, where the NF filtered product can include the treated RO concentrate at least some of the time. The system has a suitable arrangement of pipes and valves (or other flow control devices) such that various flow ratios within the system can be varied by the controller, optionally automatically or optionally over time. For example, the controller can operate the flow controls or other devices to vary one or more of: (a) the relative amount of feedwater flowing to the inlet of the RO unit relative to the NF unit; (b) the amount of RO concentrate, if any, flowing to the NF unit; and (c) the relative amounts of RO filtered product and NF filtered product in the product water. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram of a water treatment system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Water is a key component in many Enhanced Oil Recovery (EOR) and Chemical Enhanced Oil Recovery (CEOR) technologies, which may be employed onshore or offshore. Important parameters such as salinity and hardness are preferably maintained within acceptable ranges to produce useful fluids for injection. Different oil-bearing reservoirs and different flood stages within the same reservoir may require injection water with different salinities. The water treatment system should ideally be able to provide produced water with different salinities with minimal or no manual intervention. The feed water is most often seawater. Seawater typically has a salinity of 35 g / L or more (measured as total dissolved solids, TDS) as recovered, but if pretreated to remove suspended solids, the salinity may be 40 mg / L or more. Seawater hardness (total hardness as CaCO3) may be 6500 mg / L or more, with little, if any, being removed in pretreatment.

[0009] In some examples, the system described herein can be used to produce treated water of various salinities. The system can be used to treat feed water of various salinity levels, for example, 35 g / l to 50 g / l or 40 g / l to 50 g / l TDS, to produce a product fluid that can have a hardness (total hardness as CaCO3) of less than 20 mg / l. Optionally, the system can maintain a generally constant effluent salinity over a period of time in which the feed water salinity varies. The salinity of the product water can be between 30 g / l TDS to 50 g / l TDS, which is desirable for use in at least some oil-bearing reservoirs around the world. In some examples, the system has the ability to produce product water of different salinities at different times. Optionally, the system can be modularized to allow for easier movement from location to location or for scaling up and down in size. The system can include an RO system and an NF system along with appropriate auxiliary equipment, such as pipes, pumps, tanks, valves and other controls, sensors, and one or more controllers.

[0010] In the methods described herein, water is treated with RO and / or NF membranes to produce different outlet streams. The different outlet streams can be mixed in different proportions to produce product water with selected characteristics. The feed water may be pretreated in one or more solid-liquid separation units, for example to protect the membranes.

[0011] The produced water may be treated with polymers. The produced water may be produced according to the requirements of the particular oil-bearing reservoir and polymer mixture. The system optionally includes digital controls to track fluid properties or adjust the mixing of different streams.

[0012] The system and method feeds a selected amount of RO concentrate to the NF system. This serves to allow for varying the TDS of the product along with any variable mix of RO and NF filtered products. Using the RO concentrate increases hardness as well as TDS. However, the NF unit (multiple passes, i.e., 3 passes) removes hardness (and typically other possible scaling ions such as sulfate) while maintaining most of the desired monovalent salinity (i.e., NaCl).

[0013] The product water can have one or more of: (a) total hardness less than 20 mg / L or less than 10 mg / L as CaCO3, and total hardness less than 10 mg / L as sulfate, along with (b) a TDS greater than 30,000 mg / L or greater than 35,000 mg / L. This system produces a high TDS product water with low hardness without the need to add salt to the product water.

[0014] FIG. 1 shows an example of a water treatment system. In the top line, a portion of the feedwater flows through line 3 to a seawater RO (SWRO) unit. The feedwater in this example is filtered and seawater is used, but other feedwater sources and other pretreatments may be used. The filtered product from the seawater RO unit (SWRO filtered product) is further processed in a brackish water RO (BWRO) unit. This produces a brackish water RO filtered product (BWRO filtered product). The use of the BWRO unit is optional and may be omitted. In another option, a bypass line around the BWRO unit may optionally include one or more controllable valves such that a selected portion of the SWRO filtered product bypasses the BWRO unit. Generally speaking, the SWRO unit can produce a filtered product with approximately 1000 mg / L TDS or salinity and little hardness. The BWRO unit can produce a filtered product with little salt and little hardness (i.e., the filtered product is drinkable). The presence or absence of a BWRO unit, or the degree to which it is bypassed, can be used to vary the quality of the RO filtration product. Optionally, the SWRO and BWRO units may have one or more stages.

[0015] A portion of the feed water optionally flows through line 5 to one or more NF units. The split of the feed water between line 3 and line 5 is preferably controllable, e.g., can vary between 0-100% in line 3 or line 5. In some examples, there may be no flow through line 3 or no flow through line 5 at some time or all of the time. In some examples, there is always at least some flow through line 3. The NF unit can also selectively receive SWRO waste from line 21. In another option not shown, the NF unit can also selectively receive BWRO concentrate. The amount of RO concentrate, if any, sent to the NF unit can be varied. In some examples, at least some RO concentrate, e.g., SWRO concentrate, is sent to the NF unit at some time or all of the time.

[0016] The NF system can have one or more stages. In the illustrated example, a triple-pass RO system may be used. Some or all of the NF filtered product (in line 9) can be mixed with some or all of the RO filtered product (in lines 16 or 25 or a combination thereof) to create the final product water. The final product water can be injected into the oil-bearing composition, optionally after being mixed with one or more chemicals to enhance oil recovery.

[0017] Generally speaking, any of the elements and / or controllable elements of the system of FIG. 1 can be used to create one or more qualities in the final product water. It is typically desirable for the final product water to have low hardness, e.g., less than 20 mg / L hardness as CaCO3. However, the salinity (or TDS) requirement of the final product water may vary over time or location and may be higher or lower than the salinity of the feed water. The system shown in FIG. 1 can typically produce a final product water with a hardness of less than 20 mg / L when fed with seawater that is typically pretreated (i.e., seawater that has been treated by one or more of a clarifier, dissolved air filtration unit, media filter, and microfiltration or ultrafiltration membranes or similar processing units). The system shown in FIG. 1 can also receive feedwater with a salinity of up to 50 g / L and produce a final product water with a salinity higher, lower, or the same as the feed.

Claims

1. 1. A method for treating water, comprising: treating a portion of the feedwater by reverse osmosis to produce an RO filtrate product and an RO concentrate; treating a portion of the RO concentrate by nanofiltration to produce an NF filtered product; using the NF filtered product or a mixture of the RO filtered product and the NF filtered product as produced water for injection into an oil-containing composition; Including, the product water has a salinity greater than the salinity of the feed water, where the salinity (g / L) is measured as total dissolved solids (TDS); method.

2. 10. The method of claim 1, further comprising treating a portion of the feedwater by nanofiltration to produce an additional NF filtration product.

3. 3. The method of claim 1 or 2, wherein the feed water has a salinity of 35 g / L or more of TDS.

4. The method described in claim 1 or 2, wherein the supply water comprises seawater.

5. The method of any one of claims 1 to 3, wherein the produced water has a salinity of 40 g / L TDS or greater.

6. The feed water comprises seawater and the product water comprises CaCO ３ The method according to any one of claims 1 to 4, wherein the total hardness is less than 20 mg / L.

7. 1. A water treatment system comprising: RO system, NF System Including, an inlet to the RO system is connected to a feed water source and a concentrate outlet of the RO system is connected to a feed inlet of the NF system, the NF system and the RO system being configured to produce a product water having a salinity greater than a salinity of the feed water, where the salinity (g / L) is measured as total dissolved solids (TDS); system.

8. The system of claim 7 , wherein an inlet to the NF system is connected to the supply water source.

9. 9. The system of claim 7 or 8, wherein a filtered product outlet from the NF system is connected to an injection system and a filtered product outlet from the RO system is connected to the injection system.

10. 10. The system of claim 9, wherein the injection system is adapted to inject the produced water, including an RO filtered product and / or an NF filtered product, into an oil-bearing reservoir, and the NF filtered product may include treated RO concentrate at least at some points.

11. 11. The system of any one of claims 7 to 10, comprising pipes and valves or other flow control devices disposed between the feed water source, the RO system, the NF system, and the product water, and configured such that various flow ratios within the system can be varied over time, either automatically or via a controller.

12. The system of any one of claims 7 to 11, wherein a controller can operate flow controls or other devices to vary one or more of: (a) the relative amount of feed water flowing to the inlet of the RO unit versus the feed water flowing to the inlet of the NF unit; (b) the amount of RO concentrate, if any, flowing to the NF unit; and (c) the relative amounts of RO filtered product and NF filtered product in the final volume of product water.

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