A carbon dioxide flooding produced water pretreatment process

Abstract

The present application relates to the technical field of oil produced water treatment, and particularly relates to a carbon dioxide flooding produced water pretreatment process, the pretreatment process comprising the following steps: (1) air floatation treatment: carbon dioxide flooding produced water enters an air floatation unit, and carbon dioxide and oil are preliminarily removed to obtain air floatation unit effluent; (2) electrochemical flocculation: the air floatation unit effluent enters an electrochemical unit, the pH is adjusted to be acidic, a flocculating agent is added, carbon dioxide is recovered, and suspended solids are removed to obtain electrochemical unit effluent; (3) filtration: the electrochemical unit effluent enters a filtration unit, and suspended solids are further removed to obtain filtration unit effluent. The present application has the advantages of low operation cost, low unit investment cost, convenient operation and management, and high removal efficiency.

Description

Technical Field

[0001] This invention relates to the field of oilfield produced water treatment technology, specifically to a carbon dioxide flooded produced water pretreatment process. Background Technology

[0002] In the later stages of oilfield development, the water content of crude oil increases significantly, with some oilfields producing fluids containing 60%–90% water, and some even reaching 97%. Improving oil recovery has become a critical technical issue in crude oil extraction. Gas injection flooding is one of the main technologies for enhancing oil recovery. Carbon dioxide flooding involves injecting carbon dioxide into the reservoir, which reduces crude oil viscosity, improves the crude oil-to-water mobility ratio, and causes a significant expansion of the crude oil volume. This expansion facilitates the separation of the remaining oil from formation water and the rock surface, thereby increasing the oil recovery rate.

[0003] Carbon dioxide flooding (CFD) shows promising application prospects in the development of low-permeability and high-water-cut oil reservoirs, and is of great significance for mitigating the greenhouse effect and protecting ambient air quality. It is the most promising oil recovery technology and is widely used in oilfield development. However, produced water from CFD-enhanced oilfields not only contains pollutants found in conventional produced water, such as oil, suspended impurities, various anions and cations, and surfactants, but also contains large amounts of dissolved carbon dioxide and other gases. While improving oil recovery, this is accompanied by severe corrosion and scaling problems.

[0004] The injection of large amounts of carbon dioxide can lead to corrosion and scaling in near-wellbore formations, perforation holes, downhole tubing, pumps, surface equipment, and pipelines, causing formation and pipeline blockages and affecting normal oilfield production and development. In addition, the presence of large amounts of carbon dioxide will increase the concentration of suspended solids in the water, affecting the oil-water separation effect of the water injection station, making it difficult for the reinjected water to meet the reinjection standards for oil and particulate matter, and will also have adverse effects on the surrounding soil environment.

[0005] Currently, the treatment of produced water in carbon dioxide flooding oilfields mainly involves adding corrosion inhibitors and other chemicals. This method only alleviates the corrosion and scaling problems of the injection facilities superficially, without removing the carbon dioxide from the produced water. The large dosage of chemicals results in high treatment costs and can easily cause secondary pollution of the reinjected water. In addition, vacuum methods and high-gravity processes are under research. Vacuum methods have a certain effect on removing carbon dioxide from water, but they require sophisticated equipment, including vacuum pumps and negative pressure operation, leading to high investment costs. High-gravity processes are still immature, have high operating costs, and have not yet been applied in actual industrial settings.

[0006] For example, Chinese patent application CN106609371A discloses a corrosion inhibitor, its preparation method, and its application. This corrosion inhibitor contains: 25-35% by weight of an amphoteric surfactant, 20-28% by weight of sulfur-containing organic matter, 15-25% by weight of nitrogen-containing organic matter, 1-10% by weight of an oxygen scavenger, and 2-39% by weight of water. This corrosion inhibitor possesses corrosion inhibition, scale inhibition, and bactericidal properties, effectively solving the corrosion problem of crude oil gathering and transportation pipelines. It can inhibit the corrosion of the metal itself and eliminate under-scale corrosion caused by scaling and corrosion caused by microorganisms, making it suitable for treating produced water from carbon dioxide-driven oilfields. However, this corrosion inhibitor only alleviates the corrosion problem of oilfield equipment on the surface and cannot completely remove carbon dioxide from the produced water. The reagent consumption cost is high, and it is prone to causing secondary water pollution.

[0007] Chinese patent application CN104961287A discloses a method and system for zero-discharge treatment of oily wastewater. This method involves flocculating and settling oilfield produced water, followed by three stages of filtration: resin deoiling, nanofiltration membrane desalination and hardening treatment, and reverse osmosis membrane desalination. The resulting primary concentrate undergoes chemical hardening removal, filtration, and reverse osmosis desalination. The secondary concentrate is treated by vacuum deoxygenation and carbon dioxide removal before entering an MVR distillation system, where the resulting brine crystallizes into salt. The desalinated water undergoes further hardening or desalination and deoxygenation treatment before entering a steam injection boiler. This method uses vacuum deoxygenation and carbon dioxide removal, requiring sophisticated equipment and incurring high investment costs.

[0008] Carbon dioxide flooded produced water is highly corrosive. If it enters an existing produced water treatment system, it will cause severe corrosion of the produced water treatment equipment and pipelines. Consequently, the quality of the treated purified water will not consistently meet the standards, and the maintenance and repair costs of the produced water treatment equipment and pipelines will increase, affecting the normal operation of the produced water treatment system.

[0009] Therefore, providing a pretreatment process for carbon dioxide flooded produced water, after which the produced water can be mixed into a conventional produced water treatment system, has become an urgent technical problem to be solved. Summary of the Invention

[0010] The purpose of this invention is to overcome the shortcomings of the prior art and provide a carbon dioxide flooded produced water pretreatment process. The pretreated produced water meets the following requirements: carbon dioxide content ≤20mg / L, oil content ≤20mg / L, suspended solids ≤20mg / L. The effluent can be used to prepare fracturing fluid or mixed into a conventional produced water treatment system for further treatment, thereby enabling the normal operation of the conventional produced water system.

[0011] Compared with existing processes, this treatment process has the advantages of high removal efficiency, low investment cost (compared to the investment cost of membranes), low operating cost, convenient operation and management, and reduced fracturing fluid preparation cost. It is also lower in cost than membrane degassing units.

[0012] This invention is achieved through the following technical solutions:

[0013] A carbon dioxide flooded produced water pretreatment process includes the following steps:

[0014] (1) Air flotation treatment: Carbon dioxide-driven produced water enters the air flotation unit to initially remove carbon dioxide and oil, and obtains the air flotation unit effluent;

[0015] (2) Electrochemical flocculation: The effluent from the air flotation unit enters the electrochemical unit, the pH is adjusted to acidic, flocculants are added, carbon dioxide is recovered and suspended solids are removed to obtain the effluent from the electrochemical unit;

[0016] (3) Filtration: The water from the electrochemical unit enters the filtration unit to further remove suspended solids and obtain the water from the filtration unit.

[0017] Preferably, in step (1), the carbon dioxide-driven produced water enters the air flotation unit and stays for 10-30 minutes, with the pressure of the vortex jet device being 0.2-0.5 MPa.

[0018] In step (1), the air flotation unit utilizes the adhesion effect of microbubbles generated by the bursting of bubbles during the swirling pressurization process to initially remove free carbon dioxide and oil from the extracted water.

[0019] Preferably, in step (2), the pH is adjusted to 4.1-5.2.

[0020] In step (2), the carbon in the form of carbonate and bicarbonate in the extracted water is released by adjusting the pH to form free CO2. The separated gaseous CO2 is recovered and injected into the injection zone for recycling, and the separated electrochemical unit effluent enters the next unit.

[0021] Preferably, the flocculant in step (2) is a mixture of polyacrylamide and polyethyleneimine in a mass ratio of 2:1.

[0022] More preferably, in step (2), concentrated hydrochloric acid is added to adjust the pH to 4.1-5.2.

[0023] Preferably, the electrochemical unit in step (2) includes a pre-oxidation module and an electrocoagulation module, wherein the pH is adjusted to acidic in the pre-oxidation module and a flocculant is added in the electrocoagulation module.

[0024] More preferably, in the pre-oxidation module, the anode is made of stainless steel, the cathode is made of graphite, the oxidation time is 2-5 minutes, the electrode spacing is 5-10 cm, and the current intensity is 5-10 A.

[0025] More preferably, in the electrocoagulation module, both the cathode and anode are made of aluminum plates, the reaction time is 10-20 minutes, the plate spacing is 20-30 cm, and the current intensity is 10-20 A. The cathode and anode plates are distributed in multiple sets at intervals within the electrochemical unit.

[0026] Preferably, the filtration unit in step (3) is a dual-media filtration unit.

[0027] More preferably, the dual-filter unit in step (3) is a mixture of anthracite and quartz sand, with a mass ratio of 1:1-1.5.

[0028] Preferably, the carbon dioxide content, oil content and suspended solids content of the effluent from the filtration unit in step (3) are ≤20mg / L.

[0029] In step (3), the effluent from the electrochemical reaction unit is pumped to the dual-filter unit to further remove suspended solids from the extracted water.

[0030] More preferably, the carbon dioxide flooded produced water pretreatment process includes an air flotation unit, an electrochemical unit, and a filtration unit, and mainly includes the following steps:

[0031] (1) Air flotation unit: Carbon dioxide drives the produced water into the air flotation unit. In the air flotation unit, after the water and gas are mixed, under the pressure (0.2-0.5MPa) of the vortex jet device, micro bubbles with a diameter of about 20μm-60μm are generated. These bubbles rise faster. By utilizing the adsorption effect of micro bubbles on small oil droplets and suspended solid particles (<1μm) in the produced water, the oil droplets and suspended particles in the produced water can be quickly carried to the water surface. The particles that rise slower are separated on the inclined plate in the air flotation unit, further achieving the purpose of removing suspended solids. The residence time of the produced water in the air flotation unit is 10min-30min.

[0032] (2) Electrochemical unit: The effluent from the air flotation unit is lifted into the electrochemical unit. The electrochemical unit consists of a pre-oxidation module and an electrocoagulation module. In the pre-oxidation module, concentrated hydrochloric acid is added to adjust the pH of the produced water to 4.1-5.2 to release carbon in the form of carbonate and bicarbonate in the produced water, thereby achieving the purpose of removing free CO2. In the electrocoagulation module, flocculant is added. Under the combined action of ionization and flocculant, suspended solids in the produced water are removed. The effluent meets the requirements of carbon dioxide content <20mg / L and suspended solids <30mg / L. The separated gas phase is sent to the CO2 recovery injection area for circulation reinjection. The separated produced water enters the next unit.

[0033] (3) Filtration unit: The water from the electrochemical reaction unit is pumped to the dual-filter unit by a booster pump to further remove suspended solids from the extracted water.

[0034] When the produced water treatment equipment is operating normally, the carbon dioxide-driven produced water first enters the flotation unit. Under the combined action of various forces such as interfacial tension, bubble buoyancy, and hydrostatic pressure difference, oil and suspended solids in the produced water are removed. Subsequently, the produced water enters the electrochemical unit, where the pH value of the produced water is adjusted and flocculants are added to release carbon in the form of carbonate and bicarbonate ions, forming free CO2. At the same time, some suspended solids in the produced water are removed. The effluent enters a dual-media filter, and the filtered water enters a conventional produced water treatment system for further treatment. The treated produced water is then used for water injection in oil fields.

[0035] The beneficial effects of this invention are:

[0036] The produced water treated by this invention can be used to formulate fracturing fluids and to be incorporated into conventional produced water treatment systems. When used to formulate fracturing fluids, it can reduce the discharge of produced water, alleviate environmental pressure, and improve the utilization rate of produced water. When incorporated into conventional produced water treatment systems, this invention does not generate secondary pollution during the treatment process and the treated water can be used for oilfield reinjection.

[0037] This invention also features low investment cost, low operating cost, and simple operation, while exhibiting good removal effects on carbon dioxide, suspended solids, and oil. Detailed Implementation

[0038] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as a result. However, these embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0039] Example 1

[0040] A method for pretreatment of produced water by carbon dioxide flooding includes the following steps:

[0041] (1) Air flotation: The produced water (containing 928.7 mg / L of oil, 497.2 mg / L of suspended solids, and 249.8 mg / L of carbon dioxide) driven by carbon dioxide enters the air flotation unit for oil removal. The reaction time is controlled at 10 min and the swirl jet velocity is 5 m / s. 3 / h, the effluent reaches an oil content of 302.6 mg / L and a carbon dioxide content of 149.9 mg / L;

[0042] (2) Electrochemical unit: The effluent from the air flotation unit first enters the pre-oxidation module (with concentrated hydrochloric acid added), with an oxidation time of 2 min, an electrode spacing of 6 cm, and a current intensity of 5 A; then it enters the electrocoagulation module (with flocculant added), with a reaction time of 10 min, an electrode spacing of 20 cm, a current intensity of 10 A, and a hydraulic retention time of 50 min. The effluent meets the requirements of carbon dioxide content of 18.7 mg / L and suspended solids of 29.8 mg / L.

[0043] The electrochemical unit is an integrated unit, mainly composed of a pre-oxidation module and an electrocoagulation module;

[0044] The concentration of hydrochloric acid is 10 mg / L (pH 5.1); the flocculant is prepared by mixing polyacrylamide and polyethyleneimine at a mass ratio of 2:1, and the dosage of the flocculant is 6 mg / L.

[0045] (3) Filtration: The effluent from the electrochemical unit enters a filtration unit composed of anthracite and quartz sand (mass ratio 1:1) to remove suspended solids for the second time. The effluent reaches a carbon dioxide content of 17.9 mg / L, an oil content of 18.6 mg / L, and a suspended solids content of 18.8 mg / L.

[0046] Example 2

[0047] A method for pretreatment of produced water by carbon dioxide flooding includes the following steps:

[0048] (1) Air flotation: The produced water (containing 946.8 mg / L of oil, 467.5 mg / L of suspended solids, and 241.6 mg / L of carbon dioxide) driven by carbon dioxide enters the air flotation unit for oil removal. The reaction time is controlled at 30 min and the swirl jet velocity is 6 m / s. 3 / h, the effluent contains 213.6 mg / L of oil and 129.9 mg / L of carbon dioxide;

[0049] (2) Electrochemical unit: The effluent from the air flotation unit first enters the pre-oxidation module (with concentrated hydrochloric acid added), with an oxidation time of 5 min, an electrode spacing of 10 cm, and a current intensity of 10 A; then it enters the electrocoagulation module (with flocculant added), with a reaction time of 20 min, an electrode spacing of 30 cm, a current intensity of 20 A, and a hydraulic retention time of 60 min. The effluent meets the requirements of a carbon dioxide content of 11.5 mg / L and a suspended solids content of 21.7 mg / L.

[0050] The electrochemical unit is an integrated unit, mainly composed of a pre-oxidation module and an electrocoagulation module;

[0051] The concentration of hydrochloric acid added is 10 mg / L (pH 4.3); the flocculant is prepared by mixing polyacrylamide and polyethyleneimine at a mass ratio of 2:1, and the dosage of the flocculant is 6 mg / L.

[0052] (3) Filtration: The effluent from the electrochemical unit enters a filtration unit composed of anthracite and quartz sand (mass ratio 1:1.5) to remove suspended solids for the second time. The effluent has a carbon dioxide content of 9.9 mg / L, an oil content of 12.6 mg / L, and a suspended solids content of 11.6 mg / L.

[0053] Comparative Example 1

[0054] A method for pretreatment of produced water by carbon dioxide flooding includes the following steps:

[0055] (1) Air flotation: The produced water (containing 972.3 mg / L of oil, 458.9 mg / L of suspended solids, and 244.2 mg / L of carbon dioxide) driven by carbon dioxide enters the air flotation unit for oil removal. The reaction time is controlled at 5 min and the swirl jet velocity is 2 m. 3 / h, the effluent contains 412.8 mg / L of oil and 201.7 mg / L of carbon dioxide;

[0056] (2) Electrochemical unit: The effluent from the air flotation unit first enters the pre-oxidation module (with concentrated hydrochloric acid added), with an oxidation time of 1 min, an electrode spacing of 3 cm, and a current intensity of 4 A; then it enters the electrocoagulation module (with flocculant added), with a reaction time of 8 min, an electrode spacing of 15 cm, a current intensity of 5 A, and a hydraulic retention time of 20 min. The effluent meets the requirements of a carbon dioxide content of 102.6 mg / L and a suspended solids content of 99.8 mg / L.

[0057] The electrochemical unit is an integrated unit, mainly composed of a pre-oxidation module and an electrocoagulation module;

[0058] The concentration of hydrochloric acid added is 10 mg / L (pH 4.3); the flocculant is prepared by mixing polyacrylamide and polyethyleneimine at a mass ratio of 2:1, and the dosage of the flocculant is 6 mg / L.

[0059] (3) Filtration: The effluent from the electrochemical unit enters a filtration unit composed of anthracite and quartz sand (mass ratio 1:1.5) to remove suspended solids for the second time. The effluent reaches a carbon dioxide content of 43.6 mg / L, an oil content of 40.8 mg / L, and a suspended solids content of 32.5 mg / L.

[0060] Calculations show that the carbon dioxide removal rate of Comparative Example 1 was 82.1%.

[0061] Comparative Example 2

[0062] A method for pretreatment of produced water by carbon dioxide flooding includes the following steps:

[0063] (1) Air flotation: The produced water (containing 928.7 mg / L of oil, 497.2 mg / L of suspended solids, and 249.8 mg / L of carbon dioxide) driven by carbon dioxide enters the air flotation unit for oil removal. The reaction time is controlled at 10 min and the swirl jet velocity is 5 m / s. 3 / h, the effluent reaches an oil content of 302.6 mg / L and a carbon dioxide content of 149.9 mg / L;

[0064] (2) The effluent from the air flotation unit first enters the pre-oxidation module (with concentrated hydrochloric acid added), with an oxidation time of 2 min, an electrode spacing of 6 cm, and a current intensity of 5 A; then it enters the electrocoagulation module (with flocculant added), with a reaction time of 30 min, an electrode spacing of 50 cm, a current intensity of 25 A, and a hydraulic retention time of 60 min. The effluent meets the requirements of a carbon dioxide content of 33.4 mg / L and a suspended solids content of 48.9 mg / L.

[0065] The concentration of hydrochloric acid is 10 mg / L (pH 5.1); the flocculant is prepared by mixing polyacrylamide and polyethyleneimine at a mass ratio of 2:1, and the dosage of the flocculant is 6 mg / L.

[0066] (3) Filtration: The effluent from the electrochemical unit enters a filtration unit composed of anthracite and quartz sand (mass ratio 1:1) for secondary removal of suspended solids. The effluent reaches a carbon dioxide content of 32.6 mg / L, an oil content of 25.4 mg / L, and a suspended solids content of 27.5 mg / L.

[0067] Calculations show that the carbon dioxide removal rate of Comparative Example 2 was 86.9%.

[0068] Comparative Example 3

[0069] A method for pretreatment of produced water by carbon dioxide flooding includes the following steps:

[0070] (1) Air flotation: The produced water (containing 957.2 mg / L of oil, 488.1 mg / L of suspended solids, and 227.2 mg / L of carbon dioxide) driven by carbon dioxide enters the air flotation unit for oil removal. The reaction time is controlled at 35 min and the swirl jet velocity is 6 m / s. 3 / h, the effluent reaches an oil content of 210.5mg / L and a carbon dioxide content of 126.7mg / L;

[0071] (2) Electrochemical unit: The effluent from the air flotation unit first enters the electrocoagulation module (with flocculant added) for a reaction time of 20 min, with an electrode spacing of 30 cm and a current intensity of 20 A; then it enters the pre-oxidation module (with concentrated hydrochloric acid added), with an oxidation time of 5 min, an electrode spacing of 10 cm, and a current intensity of 10 A; the hydraulic retention time is 60 min, and the effluent meets the requirements of carbon dioxide content of 22.2 mg / L and suspended solids of 25.6 mg / L;

[0072] The electrochemical unit is an integrated unit, mainly composed of a pre-oxidation module and an electrocoagulation module;

[0073] The concentration of hydrochloric acid added is 10 mg / L (pH 4.3); the flocculant is prepared by mixing polyacrylamide and polyethyleneimine at a mass ratio of 2:1, and the dosage of the flocculant is 6 mg / L.

[0074] (3) Filtration: The effluent from the electrochemical unit enters a filtration unit composed of anthracite and quartz sand (mass ratio 1:1.5) to remove suspended solids for the second time. The effluent reaches a carbon dioxide content of 29.4 mg / L, an oil content of 18.4 mg / L, and a suspended solids content of 17.5 mg / L.

[0075] Compared with Comparative Example 3, the carbon dioxide removal rate decreased from 95.9% to 87.1% in Example 2.

[0076] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

Claims

1. A carbon dioxide flooded produced water pretreatment process, characterized in that, Includes the following steps: (1) Air flotation treatment: Carbon dioxide drives the produced water into the air flotation unit to obtain the air flotation unit effluent; (2) Electrochemical flocculation: The effluent from the air flotation unit enters the electrochemical unit, the pH is adjusted to acidic, flocculant is added, carbon dioxide is recovered and suspended solids are removed to obtain the effluent from the electrochemical unit; (3) Filtration: The water from the electrochemical unit enters the filtration unit to further remove suspended solids, resulting in the filtration unit effluent; In step (1), the produced water from the carbon dioxide flooding has an oil content of 928.7-1000 mg / L, a suspended solids content of 467.5-500 mg / L, and a carbon dioxide content of 241.6-250 mg / L. The electrochemical unit in step (2) includes a pre-oxidation module and an electrocoagulation module. Concentrated hydrochloric acid is added to the pre-oxidation module to adjust the pH to 4.1-5.2, and flocculant is added to the electrocoagulation module. In the pre-oxidation module, the anode is made of stainless steel and the cathode is made of graphite. The oxidation time is 2-5 minutes, the electrode spacing is 5-10 cm, and the current intensity is 5-10 A. In the electrocoagulation module, both the anode and cathode are made of aluminum. The reaction time is 10-20 minutes, the electrode spacing is 20-30 cm, and the current intensity is 10-20 A.

2. The pretreatment process according to claim 1, characterized in that, In step (2), the pH is adjusted to 4.1-5.2; the flocculant is a mixture of polyacrylamide and polyethyleneimine in a mass ratio of 2:

1.

3. The pretreatment process according to claim 1, characterized in that, In step (1), the produced water from the carbon dioxide drive enters the air flotation unit and stays for 10-30 minutes. The pressure of the vortex jet device is 0.2-0.5 MPa.

4. The pretreatment process according to claim 1, characterized in that, The filtration unit mentioned in step (3) is a dual-media filtration unit.

5. The pretreatment process according to claim 4, characterized in that, The dual-filter unit mentioned in step (3) is a mixture of anthracite and quartz sand, with a mass ratio of 1:1-1.

5.

6. The pretreatment process according to claim 1, characterized in that, The effluent from the filtration unit in step (3) has a carbon dioxide content ≤20mg / L, an oil content ≤20mg / L, and a suspended solids content ≤20mg / L.