Recycling and resource utilization system and method for gas field produced water

Abstract

The invention discloses a system and a method for treating gas field produced water to facilitate resource utilization, the gas field produced water passes through a pretreatment device, a carbon dioxide softening device and a special anti-blocking ultrafiltration device, and the water quality of the special anti-blocking ultrafiltration device is treated to meet the water quality requirement of a required recycling way to be recycled; calcium carbonate generated in the softening process of the carbon dioxide device is recycled and directly used as a drilling fluid additive. The invention provides the full-automatic treatment system and the full-automatic treatment process method which are short in flow; according to the method disclosed by the invention, the energy consumption, the agent dosage and the sludge yield are greatly reduced, and the whole-process recycling of the gas field produced water, the sludge and the process precipitate (calcium carbonate) is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of waste liquid treatment, and more particularly to a system and method for recycling and utilizing produced water from a gas field. Background Art

[0002] In the process of gas field development, a large amount of produced water is produced with the production of natural gas. The composition of gas field produced water is complex, which brings many problems to the environment and resource utilization. First, the produced water of gas fields contains a large amount of suspended matter, such as silt, mineral particles, etc. These suspended matter not only affect the water quality, but also may be deposited in the transmission pipelines and processing equipment, causing blockage and wear, increasing maintenance costs and operating risks. Secondly, petroleum is also a common component in gas field produced water. Its existence forms include floating oil, dispersed oil, emulsified oil, etc., which cannot be discharged directly. At present, it is mainly deep-layer reinjection. In addition, there is a certain amount of dissolved salt in the produced water, and there may also be heavy metal ions, such as manganese, copper, zinc, etc. These heavy metal ions are toxic. Once they enter the environment, they will accumulate in the food chain, posing a potential threat to biological and human health.

[0003] At present, the treatment of gas field produced water mainly adopts physical, chemical and biological methods for treatment and then reinjection. Although physical treatment methods such as natural sedimentation, flotation and filtration can remove some suspended matter and oil substances, the treatment effect is limited, and it is difficult to effectively remove fine particles and dissolved pollutants. Chemical treatment methods include coagulation, precipitation and oxidation. Although they can remove some pollutants, they need to add a large amount of chemical agents, which is easy to cause secondary pollution and has high treatment costs. Biological treatment methods have a certain removal effect on organic matter, but gas field produced water usually contains high concentrations of salt and heavy metal ions, which will inhibit the growth and activity of microorganisms, resulting in unstable treatment effects. At the same time, biological treatment methods occupy a large area, take a long time to treat, and are greatly affected by the season and water quality and quantity, so they are not suitable for gas field produced water treatment.

[0004] With the increasing shortage of water resources and the continuous improvement of environmental protection requirements, it is particularly urgent to recycle and recycle gas field produced water, and some areas have already prohibited the reinjection of gas field water. On the one hand, the reinjection of gas field water will cause potential pollution to the environment and damage the ecological environment. On the other hand, produced water contains a large amount of recyclable resources. For example, water resources themselves can be treated and reused in water injection during oil field exploitation, drilling and fracturing in oil and gas field development, etc., to achieve the recycling of water resources and reduce the cost of gas field exploitation. At the same time, oil substances and ions in produced water can be recycled or converted for the production of fuel oil or other chemical products and additives, improving resource utilization efficiency. Summary of the invention

[0005] The first object of the present invention is to provide a system which has a simple connection structure and realizes resource recovery and utilization of gas field produced water.

[0006] The second object of the present invention is to provide a method for directly and effectively recycling calcium carbonate filter cake without further drying treatment, which greatly reduces energy consumption and environmental pollution caused by process by-products compared with conventional gas field produced water resource recycling processes.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] A system for recycling and utilizing produced water from a gas field, comprising: a pretreatment device, a carbon dioxide softening device, a special anti-blocking ultrafiltration device, a sludge dehydration device, a calcium carbonate dehydration recovery device, and an ultrafiltration product water tank;

[0009] Wherein, the pretreatment device, the carbon dioxide softening device, the special anti-blocking ultrafiltration device and the ultrafiltration product water tank are connected in sequence;

[0010] The pretreatment device is cyclically connected to the sludge dewatering device;

[0011] The pretreatment device is cyclically connected to the carbon dioxide softening device;

[0012] The special anti-blocking ultrafiltration device is cyclically connected to the carbon dioxide softening device;

[0013] The calcium carbonate dehydration recovery device is cyclically connected with the carbon dioxide softening device.

[0014] Preferably, the pretreatment device includes a regulating water tank, a pH regulating sedimentation tank, an air flotation oil removal device and a dosing device;

[0015] The regulating water tank, pH regulating sedimentation tank, flotation oil removal equipment and carbon dioxide softening device are connected in sequence; the dosing device is connected to the pH regulating sedimentation tank; the pH regulating sedimentation tank is connected to the sludge dehydration device; the regulating water tank is connected to the sludge dehydration device; the flotation oil removal equipment is cyclically connected to the carbon dioxide softening device;

[0016] The air flotation oil removal equipment is a shallow air flotation device, the gas source is carbon dioxide, and the gas is supplied from a carbon dioxide storage tank via a vaporizer.

[0017] Preferably, the carbon dioxide softening device comprises a primary softener, a secondary softener, a deep softener, a clarifier, a carbon dioxide storage tank, a vaporizer and a wide-range hardness online monitor;

[0018] The wide-range hardness online monitor includes a wide-range hardness online monitor 1 and a wide-range hardness online monitor 2;

[0019] The flotation oil removal equipment, the primary softener, the secondary softener, the deep softener, the clarifier and the special anti-blocking ultrafiltration device are connected in sequence; the clarifier is cyclically connected to the calcium carbonate dehydration recovery device; the carbon dioxide storage tank is connected to the vaporizer; the vaporizer is respectively connected to the primary softener, the secondary softener and the flotation oil removal equipment; the special anti-blocking ultrafiltration device is cyclically connected to the clarifier;

[0020] The wide-range hardness online monitor 1 is connected to the pipeline between the flotation oil removal equipment and the primary softener in the pretreatment device; the wide-range hardness online monitor 2 is connected to the pipeline between the clarifier and the special anti-blocking ultrafiltration device;

[0021] Preferably, the special anti-blocking ultrafiltration device is an inorganic membrane with a filtration accuracy of 100-200nm.

[0022] Preferably, the calcium carbonate dehydration and recovery device is a high-pressure diaphragm plate and frame filter press.

[0023] Preferably, the sludge dewatering device is a high-pressure diaphragm filter press.

[0024] A method for recycling produced water from a gas field, comprising the following steps:

[0025] Step 1: Add the produced water from the gas field into the regulating water tank in the pretreatment device, and enter the pH regulating sedimentation tank after homogenization, and then add the pH regulator and oxidant into the pH regulating sedimentation tank to obtain effluent and sludge;

[0026] Step 2: The effluent from step 1 is sent to an air flotation oil removal device, and the upper floating oil layer enters a shallow air flotation device for oil-water separation to obtain floating oil with a high oil content and deoiled effluent. The recovered floating oil can be reprocessed or simply blended for use as fuel, etc.;

[0027] Step 3: Send the deoiled water in step 2 to a primary softener, then introduce carbon dioxide gas and add a pH regulator, react and then enter a deep softener and a clarifier to obtain softened water and calcium carbonate precipitate; the primary softener, the secondary softener and the deep softener are reaction equipment for gradually reducing the hardness of the water, the clarifier can remove most of the suspended matter in the water, the carbon dioxide storage tank is for storing and providing the liquid carbon dioxide and flotation gas source required for the entire process, the vaporizer is for converting carbon dioxide from liquid to gas and adding it to the primary softener, the secondary softener and the flotation oil removal equipment in the pretreatment device, the calcium carbonate dehydration recovery device is for dehydrating and recovering the calcium carbonate precipitate at the bottom of the clarifier, and two wide-range hardness online monitors are used to monitor the hardness of the water inlet and outlet of the carbon dioxide softening device;

[0028] Step 4: The softened produced water in step 3 is quickly sent to a special anti-blocking ultrafiltration device for filtration to remove calcium carbonate in the water and prevent the generated calcium carbonate from dissolving back, thereby obtaining concentrated water and produced water with SS ≤ 1 mg / L; the produced water is recovered to the ultrafiltration product water tank for recycling; the concentrated water is sent to the clarification tank in the carbon dioxide softening device;

[0029] Step 5: Send the sludge in the pH adjustment sedimentation tank in step 1 to a sludge dehydration device for filter pressing and dehydration to obtain a mud cake, the moisture content of which is less than 55%, and is used as raw material for downstream brick factories and other enterprises; the generated filtrate returns to the regulating water tank in the pretreatment device;

[0030] Step 6: Send the calcium carbonate precipitate in the carbon dioxide softening device in step 3 to the calcium carbonate dehydration recovery device for filter pressing and dehydration to obtain a filter cake of calcium carbonate, which is directly converted into a drilling fluid additive according to the water content without further drying treatment; the resulting filtrate is sent to the clarification tank in the carbon dioxide softening device.

[0031] Preferably, the pH adjuster in step 1 includes sodium hydroxide, and the pH of the gas field produced water is adjusted to 10-12; the oxidant includes sodium hypochlorite, and the concentration of the oxidant in the gas field produced water is 50-150 mg / L.

[0032] In step 1, the reaction equation of sodium hypochlorite and ferrous iron is:

[0033] 2Fe 2+ +ClO - +4OH - +H2O=2Fe(OH)3+Cl -

[0034] The generated Fe(OH)3 acts as a flocculant to further assist in removing suspended matter and colloids in the produced water of the gas field.

[0035] Preferably, in step 3, two wide-range hardness online monitors are used to monitor the hardness of the water entering and exiting the carbon dioxide softening device. The wide-range hardness online monitor has a monitoring range of 20-50000 mg / L (in terms of calcium carbonate). The amount of carbon dioxide added is automatically calculated and controlled by monitoring the hardness value and the carbon dioxide conversion rate. The pH regulator includes sodium hydroxide, and the pH of the gas field produced water is adjusted to 10-12.

[0036] In step 3, carbon dioxide reacts with Ca in the effluent 2+ Mg 2+ The reaction equation is:

[0037] Ca 2+ +2OH - =Ca(OH)2↓

[0038] Mg 2+ +2OH - =Mg(OH)2↓

[0039]

[0040] Preferably, the pressure at the feed inlet of the sludge dehydration device in step 5 and the calcium carbonate dehydration and recovery device in step 6 is ≥0.8Mpa.

[0041] It can be seen from the above technical solution that compared with the prior art, the present invention has the following beneficial effects:

[0042] 1. The present invention provides a system and a process method for recycling produced water from a gas field. After the produced water from the gas field is treated by the method of the present invention, all the produced water is recycled, and the recycled solid calcium carbonate is directly used as a drilling fluid additive, eliminating the risk of secondary environmental pollution caused by discharge, reinjection or drying;

[0043] 2. The present invention provides a system with a simple connection structure; the equipment occupies a small area, operates fully automatically, is easy to operate, and has low cost;

[0044] 3. The carbon dioxide softening device in the present invention can effectively remove the hardness in the effluent, with low processing cost and can also achieve the "dual carbon" goal;

[0045] 4. The method of the present invention adopts a special anti-clogging ultrafiltration device, which has strong anti-pollution ability, large membrane flux, small footprint, good water quality, and effectively prevents calcium ions from dissolving back. The produced water is directly recycled as qualified reuse water, and the concentrated water is sent to the clarification tank in the carbon dioxide softening device for sedimentation, and the supernatant enters the special anti-clogging ultrafiltration device. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0047] Figure 1 This is the process flow chart of the recovery and resource utilization system of gas field produced water;

[0048] Figure 2 Comparative photos of the appearance of water samples before and after pretreatment in step 1 of Example 1 of the present invention;

[0049] Figure 3 This is the particle size distribution diagram of calcium carbonate recovered in step 3 of Example 1 of the present invention;

[0050] Figure 4 This is an appearance diagram of the drilling fluid prepared for recovering calcium carbonate in step 3 of Example 1 of the present invention;

[0051] Figure 5 This is a comparison diagram of the appearance of softened water (left) and ultrafiltration water (right) in step 4 of Example 1 of the present invention;

[0052] Figure 6 Appearance of guar gum fracturing fluid prepared with tap water (left) and ultrafiltration produced water (right) in step 4 of Example 1 of the present invention;

[0053] Figure 7 Appearance of guar gum fracturing fluid prepared with tap water (left) and ultrafiltration produced water (right) in step 4 of Example 1 of the present invention;

[0054] Figure 8 This is a comparison diagram of the appearance of water samples before and after pretreatment in step 1 of Example 2 of the present invention;

[0055] Fig. 9 This is the calcium carbonate particle size distribution diagram of step 3 of Example 2 of the present invention;

[0056] Fig.10 This is a comparison diagram of the appearance of softened water produced in step 4 of Example 2 of the present invention (left) and ultrafiltration water produced (right);

[0057] Fig.11 A comparison diagram of polymer fracturing fluids prepared with tap water (left) and ultrafiltration produced water (right) in step 4 of Example 2 of the present invention;

[0058] Fig.12 Appearance of the degelling liquid for preparing the polymer fracturing fluid using tap water (left) and ultrafiltration produced water (right) in step 4 of Example 2 of the present invention;

[0059] Fig.13 This is a structural diagram of the pretreatment device of the present invention;

[0060] Fig.14 This is a structural diagram of the carbon dioxide softening device of the present invention. DETAILED DESCRIPTION

[0061] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0062] Example 1

[0063] This embodiment provides a system for recycling and utilizing produced water from a gas field, including: a pretreatment device, a carbon dioxide softening device, a special anti-blocking ultrafiltration device, a sludge dehydration device, a calcium carbonate dehydration recovery device, and an ultrafiltration product water tank;

[0064] Among them, the pretreatment device, the carbon dioxide softening device, the special anti-blocking ultrafiltration device, and the ultrafiltration product water tank are connected in sequence;

[0065] The pretreatment device is cyclically connected with the sludge dewatering device;

[0066] The pretreatment device is cyclically connected with the carbon dioxide softening device;

[0067] The carbon dioxide softening device is cyclically connected with the calcium carbonate dehydration recovery device;

[0068] The special anti-blocking ultrafiltration device is cyclically connected with the carbon dioxide softening device;

[0069] In this embodiment, the pretreatment device includes a water adjustment tank, a pH adjustment sedimentation tank, an air flotation oil removal device, and a dosing device:

[0070] The regulating water tank, the pH regulating sedimentation tank, the flotation oil removal equipment and the carbon dioxide softening device are connected in sequence; the dosing device is connected to the pH regulating sedimentation tank; the pH regulating sedimentation tank is connected to the sludge dewatering device; the regulating water tank is also connected to the sludge dewatering device; the flotation oil removal equipment and the carbon dioxide softening device are circulated;

[0071] The dosing equipment is used to add pH regulator and oxidant to the pH regulating sedimentation tank;

[0072] The flotation oil removal equipment is a shallow flotation device whose gas source is carbon dioxide, which is supplied by a carbon dioxide storage tank through a vaporizer to remove and recover oil in the water.

[0073] In this embodiment, the carbon dioxide softening device includes a primary softener, a secondary softener, a deep softener, a clarifier, a carbon dioxide storage tank, a vaporizer and a wide-range hardness online monitor:

[0074] The flotation oil removal equipment, the primary softener, the secondary softener, the deep softener, the clarifier and the special anti-blocking ultrafiltration device are connected in sequence; the clarifier is circularly connected to the calcium carbonate dehydration recovery device; the carbon dioxide storage tank is connected to the vaporizer; the vaporizer is respectively connected to the primary softener, the secondary softener and the flotation oil removal equipment; the wide-range hardness online monitor 1 is connected to the pipeline between the flotation oil removal equipment and the primary softener; the wide-range hardness online monitor 2 is connected to the pipeline between the clarifier and the special anti-blocking ultrafiltration device; the special anti-blocking ultrafiltration device is circularly connected to the clarifier.

[0075] In this embodiment, the special anti-clogging ultrafiltration device is an inorganic membrane with a filtration accuracy of 100-200nm.

[0076] In this embodiment, the sludge dewatering device is a high-pressure diaphragm plate and frame filter press;

[0077] In this embodiment, the calcium carbonate dehydration and recovery device is a high-pressure diaphragm plate and frame filter press.

[0078] Example 2

[0079] This embodiment provides a method for recycling produced water from a gas field, which is as follows:

[0080] Produced water from a natural gas processing plant, Q = 8m 3 / h, mineralization: 37803mg / L, Ca 2+ :9028mg / L,Mg 2+ :620mg / L,SO4 2- :16mg / L,SS:175mg / L,Fe 2+ :21mg / L,Fe 3+ :134mg / L, Oil:58mg / L.

[0081] A method for recycling produced water from a gas field, comprising the following steps:

[0082] Step 1: Add the produced water from the gas field to the regulating water tank in the pretreatment device, send it to the pH adjustment reaction sedimentation tank after homogenization, add pH regulator to adjust the pH of the produced water from the gas field to 10-12; then add oxidant 2.6Kg / m 3 ; Make the gas field produced water undergo a chemical reaction to remove iron, suspended matter and colloids in the gas field produced water to obtain effluent and sludge; then send the effluent from the adjustment reaction sedimentation tank to the flotation oil removal equipment, and the upper floating oil layer enters the shallow flotation device for oil-water separation to obtain recovered floating oil and deoiled effluent; the specific removal effect is shown in Table 1:

[0083] Table 1 Pretreatment inlet and outlet water quality analysis table

[0084]

[0085] Sludge production is 0.98m 3 / h, the sludge is sent to the sludge dewatering device for filter pressing and dehydration (no conditioning is required); the pressure at the feed inlet of the sludge dewatering device is 0.8MPa, and after pressing for 1h, a mud cake with a moisture content of 47.2% is obtained, which is transported to downstream brick factories and other enterprises for use as raw materials; the filtrate from the high-pressure diaphragm plate and frame filter press is sent to the regulating water tank of the pretreatment device.

[0086] Step 2: The deoiled water in step 1 is sent to the primary softener in the carbon dioxide softening device, and then carbon dioxide gas is introduced into the primary softener and the secondary softener in the carbon dioxide softening device and a pH regulator is added to adjust the pH of the gas field produced water to 10-12, and after the reaction, the produced water enters the deep softener and the clarifier to remove the hardness in the water and obtain softened produced water and calcium carbonate; the specific removal effect is shown in Table 2;

[0087] In the carbon dioxide softening device (the following embodiments are the same and will not be repeated), the primary softener, the secondary softener and the deep softener are reaction equipment for gradually reducing the hardness of water, the clarifier is for removing part of the suspended matter in the water, the carbon dioxide storage tank is for storing and providing the liquid carbon dioxide and the flotation gas source required for the entire process, the vaporizer is for converting carbon dioxide from liquid to gas and adding it to the primary softener, the secondary softener and the flotation oil removal equipment in the pretreatment device, the calcium carbonate dehydration recovery device is for dehydrating and recovering the calcium carbonate precipitate at the bottom of the clarifier, and two wide-range hardness online monitors are for online real-time monitoring of the inlet and outlet hardness of the carbon dioxide softening device;

[0088] Among them, the wide-range hardness online monitor has a monitoring range of 20-50000 mg / L (calculated as calcium carbonate), and automatically calculates and controls the amount of carbon dioxide added by monitoring the hardness value and carbon dioxide conversion rate; the wide-range hardness online monitor 1 monitors the hardness of the water entering the primary softener, and the wide-range hardness online monitor 2 monitors the hardness of the effluent from the clarifier;

[0089] Table 2 Carbon dioxide softening water quality analysis table

[0090]

[0091] Calcium carbonate production is 0.78m 3 / h, and sent to the calcium carbonate recovery device for filter pressing and dehydration; the pressure at the feed inlet of the calcium carbonate recovery device was 0.8MPa, and the pressing was carried out for 1h to obtain calcium carbonate with a water content of 16.12%. The particle size distribution of the recovered calcium carbonate was shown in Figure 3 , calcium carbonate is bagged and utilized as a drilling fluid additive; the filtrate from the high-pressure diaphragm plate and frame filter press is sent to the clarification tank in the carbon dioxide softening device.

[0092] The calcium carbonate generated by carbon dioxide softening is directly used as an additive for compound drilling fluid. The dynamic-plastic ratio of the compound drilling fluid is tested by adding the required concentration of recycled calcium carbonate according to the water content. The experimental results are as follows:

[0093] Table 3 Comparison of conventional properties of drilling fluids between the experimental group and the control group

[0094]

[0095] It can be seen that the drilling fluid compounded with calcium carbonate has excellent filtration performance, and the dynamic plastic ratio meets the requirement of 0.36-0.48Pa / (mPa·s). The recovered calcium carbonate can be directly used as a drilling fluid additive.

[0096] Step 3: The softened water produced in step 2 is quickly sent to a special anti-clogging ultrafiltration device for filtration to remove suspended solids in the water, mainly calcium carbonate. The produced water is sent to the ultrafiltration product water tank for recycling, and the concentrated water is sent to the clarification tank in the carbon dioxide softening device. The specific removal effect is shown in Table 4:

[0097] Table 4 Anti-blocking ultrafiltration inlet and outlet water quality analysis table

[0098]

[0099] The guar fracturing fluid was prepared by adding 0.4% (w / w) guar gum to the ultrafiltration water and 0.8% (w / w) cross-linking agent. -1 The viscosity of the shearing fluid is not less than 95 mPa·s after 90 minutes of shearing. The fluid can be completely broken within 60 minutes with an addition of 0.3‰ (w / w) of ammonium persulfate. The residue content of the broken fluid is 310.6 mg / L, the surface tension is 31.7 mN / m, and the anti-swelling rate is 65.2%, which meets the requirements of SY / T7627-2021 "Technical Requirements for Water-Based Fracturing Fluids".

[0100] The experimental results of the base fluid viscosity of the fracturing fluid prepared with tap water and ultrafiltration water, the viscosity of the supernatant after gel breaking, the surface tension, the residue content, and the anti-swelling rate are shown in Table 5:

[0101] Table 5 Comparison of fracturing fluid performance prepared with tap water and ultrafiltration water

[0102]

[0103]

[0104] It can be seen from the above table that the fracturing fluid prepared with ultrafiltration water meets the requirements of plant jelly fracturing fluid residue content <600mg / L, surface tension <32mN / m, and anti-swelling rate >60%.

[0105] Example 3

[0106] This embodiment provides a method for recycling and utilizing produced water from a gas field to treat produced water from a gas field, as follows:

[0107] Produced water from a natural gas processing plant, Q: 8m 3 / h, mineralization: 84276mg / L, Ca 2+ :10871mg / L,Mg 2+ :427mg / L,SO42- :65mg / L,SS:160mg / L,Fe 2+ :25mg / L,Fe 3+ :20mg / L,Oil:170mg / L.

[0108] Step 1: Add the produced water from the gas field to the regulating water tank in the pretreatment device, homogenize it and then send it to the pH adjustment reaction sedimentation tank, add pH regulator to adjust the pH of the produced water from the gas field to 10-12; oxidant 0.75kg / m 3 ; Make the gas field produced water undergo a chemical reaction to remove iron, suspended matter and colloids in the gas field produced water to obtain effluent and sludge; then send the effluent from the pH adjustment reaction sedimentation tank to the flotation oil removal equipment, and the upper floating oil layer enters the shallow flotation device for oil-water separation to obtain recovered floating oil and deoiled effluent; the specific removal effect is shown in Table 6:

[0109] Table 6 Pretreatment inlet and outlet water quality analysis table

[0110]

[0111] Sludge production is 1.06m 3 / h, the sludge is sent to the sludge dewatering device for filter pressing and dehydration (no conditioning is required); the pressure at the feed inlet of the sludge dewatering device is 0.8MPa, and after pressing for 1h, a mud cake with a moisture content of 47.6% is obtained, which is transported to downstream brick factories and other enterprises for raw material utilization. The filtrate from the high-pressure diaphragm plate and frame filter press is sent to the regulating water tank of the pretreatment device.

[0112] Step 2: The effluent from the oil removal device in step 1 is sent to the primary softener in the carbon dioxide softening device, and then carbon dioxide gas and pH regulator are introduced into the primary softener and the secondary softener in the carbon dioxide softening device. After the reaction, the effluent enters the deep softener and the clarification tank to remove the hardness in the water, and softened water and calcium carbonate precipitation are obtained; the specific removal effect is shown in Table 7:

[0113] Table 7 Carbon dioxide softening water quality analysis table

[0114]

[0115] The calcium carbonate precipitation yield is 0.84m 3 / h, and sent to the calcium carbonate recovery device for filter pressing and dehydration; the pressure at the feed inlet of the calcium carbonate recovery device is 0.8MPa, and after pressing for 1h, calcium carbonate with a water content of 16.42% is obtained, and the calcium carbonate is utilized as a drilling fluid additive; the filtrate of the high-pressure diaphragm plate and frame filter press returns to the clarification tank in the carbon dioxide softening device.

[0116] Step 3: The softened water produced in step 2 is sent to a special anti-clogging ultrafiltration device for rapid filtration to remove suspended solids in the water, mainly calcium carbonate. The produced water is sent to the ultrafiltration product water tank for recycling, and the concentrated water is sent to the clarification tank in the carbon dioxide softening device; the specific removal effect is shown in Table 8:

[0117] Table 8 Anti-blocking ultrafiltration device inlet and outlet water quality analysis table

[0118]

[0119]

[0120] The produced water from the gas field was treated and prepared into polymer fracturing fluid with 1% (w / w) polymer thickener and 0.2% (w / w) crosslinking agent. -1 The viscosity is not less than 80mPa·s after shearing for 90 minutes. The gel can be completely broken within 120 minutes at an addition of 0.3‰ of ammonium persulfate. The residue content of the broken gel liquid is 86mg / L, the surface tension is 30.4mN / m, and the anti-swelling rate is 73.3%. It is compatible with formation water and meets the requirements of SY / T7627-2021 "Technical Requirements for Water-Based Fracturing Fluids".

[0121] The experimental results of base fluid viscosity of fracturing fluid prepared with tap water and ultrafiltration water, viscosity of supernatant after gel breaking, surface tension, residue content, and anti-swelling rate are shown in the following table:

[0122] Table 9 Viscosity of fracturing fluid base fluid and performance of degelling fluid prepared with tap water and ultrafiltration produced water

[0123]

[0124] It can be seen from the above table that the polymer fracturing fluid degelling liquid prepared by ultrafiltration produced water meets the requirements of polymer fracturing fluid residue content <200 mg / L, surface tension <32 mN / m, and anti-swelling rate >60%.

[0125] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0126] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A system for recycling produced water from a gas field, characterized in that: include: Pretreatment device, carbon dioxide softening device, special anti-blocking ultrafiltration device, sludge dehydration device, calcium carbonate dehydration recovery device, ultrafiltration product water tank; Wherein, the pretreatment device, the carbon dioxide softening device, the special anti-blocking ultrafiltration device and the ultrafiltration product water tank are connected in sequence; The pretreatment device is cyclically connected to the sludge dewatering device; The pretreatment device is cyclically connected to the carbon dioxide softening device; The calcium carbonate dehydration recovery device is cyclically connected to the carbon dioxide softening device; The special anti-blocking ultrafiltration device is cyclically connected with the carbon dioxide softening device.

2. A system for recycling and utilizing produced water from a gas field according to claim 1, characterized in that: The pretreatment device includes a water adjustment tank, a pH adjustment sedimentation tank, an air flotation oil removal device and a dosing device; The regulating water tank, pH regulating sedimentation tank, flotation oil removal equipment and carbon dioxide softening device are connected in sequence; the dosing device is connected to the pH regulating sedimentation tank; the pH regulating sedimentation tank is connected to the sludge dehydration device; the regulating water tank is connected to the sludge dehydration device; the flotation oil removal equipment and the carbon dioxide softening device are cyclically connected; The air flotation oil removal equipment is a shallow air flotation device, the gas source is carbon dioxide, and the gas is supplied from a carbon dioxide storage tank via a vaporizer.

3. A system for recycling and utilizing produced water from a gas field according to claim 2, characterized in that: The carbon dioxide softening device comprises a primary softener, a secondary softener, a deep softener, a clarifier, a carbon dioxide storage tank, a vaporizer and a wide-range hardness online monitor; The wide-range hardness online monitor includes a wide-range hardness online monitor 1 and a wide-range hardness online monitor 2; The flotation oil removal equipment, the primary softener, the secondary softener, the deep softener, the clarifier and the special anti-blocking ultrafiltration device are connected in sequence; the clarifier is cyclically connected to the calcium carbonate dehydration recovery device; the carbon dioxide storage tank is connected to the vaporizer; the vaporizer is respectively connected to the primary softener, the secondary softener and the flotation oil removal equipment; the special anti-blocking ultrafiltration device is cyclically connected to the clarifier; The wide-range hardness online monitor 1 is connected to the pipeline between the flotation oil removal equipment and the primary softener; the wide-range hardness online monitor 2 is connected to the pipeline between the clarifier and the special anti-blocking ultrafiltration device.

4. The system for recycling and utilizing produced water from a gas field according to claim 3 is characterized in that: The special anti-blocking ultrafiltration device is a special anti-blocking inorganic membrane with a filtration accuracy of 100-200nm; The calcium carbonate dehydration and recovery device is a high-pressure diaphragm plate and frame filter press; The sludge dewatering device is a high-pressure diaphragm plate and frame filter press.

5. A method for recycling produced water from a gas field, characterized in that: The system of claim 4 comprises the following steps: Step 1: Add the produced water from the gas field into the regulating water tank, and after homogenization, enter the pH regulating sedimentation tank, and then add the pH regulator and oxidant into the pH regulating sedimentation tank to obtain effluent and sludge; Step 2: The effluent in step 1 is sent to the flotation oil removal equipment, and the upper floating oil layer enters the shallow flotation device for oil-water separation to obtain floating oil and deoiled effluent; Step 3: sending the deoiled water in step 2 to a primary softener, then introducing carbon dioxide gas and adding a pH regulator into the primary softener and the secondary softener, and then entering a deep softener and a clarifier after reaction to obtain softened water and calcium carbonate precipitation; Step 4: The softened produced water in step 3 is quickly sent to a special anti-clogging ultrafiltration device for filtration to obtain produced water and concentrated water; the produced water is sent to an ultrafiltration product water tank for recycling, and the concentrated water is sent to a clarification tank; Step 5: sending the sludge in the pH adjustment sedimentation tank in step 1 to a sludge dewatering device for filter pressing and dehydration to obtain a mud cake, and the generated filtrate is sent to a regulating water tank; Step 6: Send the calcium carbonate precipitate in step 3 to a calcium carbonate dehydration recovery device for filter pressing and dehydration to obtain a filter cake of calcium carbonate, which is directly converted into a drilling fluid additive according to its water content without further drying treatment, and the resulting filtrate is sent to a clarification tank.

6. The method for recycling gas field produced water according to claim 5, characterized in that: In step 1, the pH regulator includes sodium hydroxide, which adjusts the pH of the gas field produced water to 10-12; the oxidant includes sodium hypochlorite, and the concentration of the oxidant in the gas field produced water is 50-150 mg / L.

7. The method for recycling produced water from a gas field according to claim 5, characterized in that: In step 3, two wide-range hardness online monitors are used to monitor the hardness of the water entering and exiting the carbon dioxide softening device. The wide-range hardness online monitor has a monitoring range of 20-50000 mg / L in terms of calcium carbonate. The amount of carbon dioxide added is automatically calculated and controlled by monitoring the hardness value and the carbon dioxide conversion rate. The pH adjuster includes sodium hydroxide, which is used to adjust the pH of gas field produced water to 10-12.

8. The method for recycling produced water from a gas field according to claim 5, characterized in that: The pressure at the feed inlet of the sludge dehydration device in step 5 and the calcium carbonate dehydration recovery device in step 6 is ≥0.8MPa.

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