CN113087295B - Shale gas fracturing flowback fluid standard discharge treatment process method and system - Google Patents

Shale gas fracturing flowback fluid standard discharge treatment process method and system Download PDF

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CN113087295B
CN113087295B CN202110371241.7A CN202110371241A CN113087295B CN 113087295 B CN113087295 B CN 113087295B CN 202110371241 A CN202110371241 A CN 202110371241A CN 113087295 B CN113087295 B CN 113087295B
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fracturing
fluid
shale gas
flowback
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CN113087295A (en
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杨杰
李静
林冬
向启贵
王兴睿
王越
胡金燕
赵靓
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Petrochina Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F9/00—Multistage treatment of water, waste water or sewage
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/24—Treatment of water, waste water, or sewage by flotation
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/28—Treatment of water, waste water, or sewage by sorption
    • C02F1/281—Treatment of water, waste water, or sewage by sorption using inorganic sorbents
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/40—Devices for separating or removing fatty or oily substances or similar floating material
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
    • C02F1/441—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis by reverse osmosis
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
    • C02F1/444—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis by ultrafiltration or microfiltration
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/50—Treatment of water, waste water, or sewage by addition or application of a germicide or by oligodynamic treatment
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/52—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
    • C02F1/5236—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities using inorganic agents
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/52—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
    • C02F1/54—Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities using organic material
    • C02F1/56—Macromolecular compounds
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2103/00—Nature of the water, waste water, sewage or sludge to be treated
    • C02F2103/10—Nature of the water, waste water, sewage or sludge to be treated from quarries or from mining activities
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F3/00—Biological treatment of water, waste water, or sewage
    • C02F3/34—Biological treatment of water, waste water, or sewage characterised by the microorganisms used
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F5/00—Softening water; Preventing scale; Adding scale preventatives or scale removers to water, e.g. adding sequestering agents

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  • Life Sciences & Earth Sciences (AREA)
  • Hydrology & Water Resources (AREA)
  • Engineering & Computer Science (AREA)
  • Environmental & Geological Engineering (AREA)
  • Water Supply & Treatment (AREA)
  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Separation Using Semi-Permeable Membranes (AREA)
  • Water Treatment By Sorption (AREA)

Abstract

The invention discloses a shale gas fracturing flowback fluid standard discharge treatment process method and a system thereof, relating to the technical field of sewage treatment, and comprising the following units which are connected in sequence: the device comprises an air flotation unit, an effective softening unit, an effective coagulation sedimentation unit, an activated carbon particle adsorption unit, an ultrafiltration unit and a reverse osmosis unit. The system and the process method can effectively reduce turbidity and scaling ions (calcium and the like) in the return liquid, improve the quality of membrane inlet water, and process membrane strong brine into crystallized salt meeting the industrial salt standard.

Description

Shale gas fracturing flowback fluid standard discharge treatment process method and system
Technical Field
The invention relates to the technical field of sewage treatment, in particular to a shale gas fracturing flowback fluid standard discharge treatment process method and a system thereof.
Background
With the acceleration of the adjustment steps of energy structures in China, shale gas becomes one of important energy source succeed sources. The exploitation life of the shale gas field can reach 30-50 years, the development and utilization value is high, and the shale gas field becomes an important component in the field of oil and gas resources. The hydraulic fracturing is used as a hydraulic pressurization drilling technology widely applied to shale gas exploitation, and the main principle is that fracturing fluid is injected into a well under a pressurization condition to break a tight structure of a rock stratum to form cracks, so that shale gas is released from the rock cracks. During shale gas fracturing operation, a large amount of fracturing flowback fluid is generated, which contains a large amount of complex components such as sludge, suspended matters, oil, organic matters, soluble salts and the like, and also contains various complex additives. The number of drilled wells of shale gas wells planned by the oil and gas field company in southwest of China in 2019-2025 is about 2180, fracturing flowback fluid generated by each shale gas well is about 1-1.5 ten thousand, and the total amount of the produced flowback fluid is estimated to be 2180-3270 ten thousand.
At present, the shale gas fracturing flowback fluid of oil and gas field company in southwest of China petroleum is recycled after being treated, the recycling rate of the flowback fluid in Chongqing area is above 90%, the rest of the flowback fluid reaches the standard and is discharged outside, the recycling rate of the flowback fluid in Sichuan area is above 85%, and the rest of the flowback fluid is used for reinjection and is discharged after reaching the standard after being treated. The reinjection well is generally deviated, the transportation distance is long, the reinjection cost is high, and the reinjection layer and the reinjection well are selected more seriously, so the site selection of the reinjection well is difficult; after the shale gas well enters a gas production stage, the demand of the fracturing fluid is obviously reduced, the recycling amount of the flowback fluid after treatment is sharply reduced, and the requirements of fracturing fluid companies on the recycling water quality are higher and higher, so that the flowback fluid is reinjected and the recycling after treatment is limited, and the standard reaching discharge of the flowback fluid after treatment is a new trend of future development.
At present, the standard discharge process after shale gas fracturing flowback fluid treatment mainly comprises pretreatment and membrane method desalination, wherein the pretreatment mainly comprises medicament softening and coagulation, namely, water hardness is reduced by adding softening medicament (lime and soda ash), the softening medicament adding amount is generally determined by the indoor experimental result of an individual sample in the earlier stage, then suspended matters, large-particle substances, macromolecular organic matters, inorganic scales and the like are removed by adding medicament (coagulation and flocculating agent) precipitation, the coagulating medicament adding amount is generally determined by the indoor experimental result of the individual sample in the earlier stage, finally soluble and small-molecular organic matters and inorganic ions are removed by membrane treatment, and finally, the effluent is discharged out in the standard manner.
The existing process has many problems, for example, the dosage of softening and coagulating chemicals is generally determined according to the results of individual sample indoor experiments in the early stage, which causes inaccurate dosage, because the quality of shale gas fracturing flowback liquid changes greatly with time, the optimal dosage of softening chemicals with different water qualities is different, if the addition amount of lime is too much or too little, the residual hardness is too high, if the addition amount of sodium carbonate is too little, the hardness of effluent is too high, if the addition amount of sodium carbonate is too much, the alkalinity of effluent is too high, which are both loads of back-end membrane desalination, such as the generation of inorganic scale on the membrane concentrated water side, membrane pollution is generated, if the addition amount of coagulating-flocculating agent is too little, the effluent turbidity is increased, if the addition amount is too much, the chemicals are wasted, and even the effluent turbidity is increased, these risks of back-end membrane pollution are increased, and even the effluent water does not reach the effluent standard, in addition, after the unsuitable pretreatment effluent enters the membrane treatment unit, the generated strong brine has complex components, generally contains a large amount of scaling ions (the content of calcium and salt is generally more than 70000mg/L, and the strong brine contains magnesium, barium, strontium and carbonate), organic matters and other impurities, and is difficult to be placed.
Disclosure of Invention
The invention aims to: the invention provides a standard discharge treatment process method and a system for shale gas fracturing flowback fluid, aiming at the defects of the existing standard discharge treatment technology for shale gas fracturing flowback fluid, especially aiming at the defects of the pretreatment technology, such as the problems that the turbidity, scale forming ions (calcium and the like) of the shale gas fracturing flowback fluid with different water quality cannot be stably and effectively reduced due to the inaccurate dosage of pretreatment agents of the fracturing flowback fluid with different water quality, and membrane strong brine with better water quality and single component (mainly containing sodium chloride) cannot be generated.
The technical scheme adopted by the invention is as follows:
a shale gas fracturing flowback fluid standard discharge treatment process method comprises the following steps:
step 1, enabling the fracturing flow-back fluid to enter an air flotation unit, wherein air flotation is to introduce air into the flow-back fluid, and formed bubbles can bring petroleum types with density lower than that of the flow-back fluid to the position above the liquid level, and divalent iron ions (such as ferrous sulfide) in the flow-back fluid are oxidized into ferric insoluble substances (such as ferric hydroxide) and are brought to the position above the liquid level of the flow-back fluid through the bubbles, so that scum is finally formed and scraped off through a mud scraper, and then bactericide (NaClO) with concentration of 2-8mg/L is added into the air flotation unit to remove Sulfate Reducing Bacteria (SRB), saprophytic bacteria (TGB) or iron bacteria (FB) and other bacteria in the fracturing flow-back fluid;
step 2, enabling the supernatant in the air floatation unit to enter into the effective partA softening unit, wherein NaOH is added into the effective softening unit and mechanical stirring is carried out, and the speed gradient of the mechanical stirring is 80-12 s ～1 The hydraulic retention time is 10-15 min to adjust the pH value of the fracturing flow-back fluid to 10-11.5, so that magnesium ions in the flow-back fluid form Mg (OH) 2 Precipitating; then adding the calcium hardness (as CaCO) in the fracturing flowback fluid into the effective softening unit 3 In terms of mg/L) 0.9-1.1 times of Na 2 CO 3 (mg/L in mg/L) and mechanically stirring at a speed gradient of 80-120 s ～1 The hydraulic retention time is 10-15 min, so that calcium ions form CaCO 3 Settling;
and 3, enabling the supernatant (with the turbidity X) in the effective softening unit to enter an effective coagulation and sedimentation unit to reduce the turbidity of the fracturing flowback fluid, firstly adding a coagulant (polyaluminium chloride (PAC)) into the effective coagulation and sedimentation unit to enable small particulate matters in the flowback fluid to form large particulate matters through bridging and electrical neutralization, adding the coagulant and then mechanically stirring, wherein the speed gradient of mechanical stirring is 250-400 s ～1 The hydraulic retention time is 10-15 min, and the adding amount Y (mg/L) of coagulant polyaluminium chloride (PAC) and the turbidity X (NTU) of the fracturing flow-back fluid meet the following conditions: y is 0.58X + 94.7;
then adding a flocculating agent (anionic polyacrylamide PAM with the molecular weight of 800-1500 ten thousand) with the molecular weight of 800-1200 ten thousand into the effective coagulation sedimentation unit to ensure that large granular substances form large flocs through electrical neutralization and bridging, adding the flocculating agent, and then mechanically stirring at the speed gradient of 30-50 s ～1 The hydraulic retention time is 10-15 min; the dosage Z (mg/L) of flocculant anionic Polyacrylamide (PAM) and the turbidity X (NTU) of the fracturing flow-back fluid satisfy the following conditions: z is 0.034X-0.017;
then standing and precipitating the flocculated fracturing flow-back fluid for 20-30 min;
step 4, enabling the supernatant in the effective coagulation sedimentation unit to enter an activated carbon particle adsorption unit, adding activated carbon with the concentration of 200-500mg/L into the activated carbon particle adsorption unit, and mechanically stirring, wherein the speed gradient of mechanical stirring isIs 50 to 100s ～1 Stirring for 5-8 min, settling for 10-20 min after stirring, and further removing suspended matters and small particles in the fracturing flow-back fluid through activated carbon adsorption;
step 5, enabling the supernatant in the activated carbon particle adsorption unit to enter an ultrafiltration unit, and filtering the fracturing flow-back fluid through an ultrafiltration membrane in the ultrafiltration unit; then adding HCl into the water discharged from the ultrafiltration membrane to adjust the pH value of the fracturing flow-back fluid to 6.5-7.5; then adding sodium bisulfite into the effluent of the ultrafiltration membrane to adjust the ORP (oxidation reduction potential) of the effluent of the ultrafiltration membrane to between 100mv and +100 mv; then adding 20-40mg/L of phosphorus-containing scale inhibitor into the effluent of the ultrafiltration membrane;
step 6, enabling effluent of the ultrafiltration unit to enter a reverse osmosis unit, and performing desalination treatment on the fracturing flow-back liquid through a reverse osmosis membrane in the reverse osmosis unit to obtain reverse osmosis fresh water and reverse osmosis concentrated water; and (3) feeding the reverse osmosis concentrated water into a Mechanical Vapor Recompression (MVR) evaporator for evaporation and crystallization, and performing centrifugal separation on crystallized salt by adopting a centrifugal machine, wherein the rotating speed of the centrifugal machine is 2000 rpm.
Preferably, in the step 1, the air capacity in the air flotation unit is 2-5% of the water inflow of the fracturing flow-back fluid, and the hydraulic retention time is 8-15 min; the bactericide is NaClO.
Preferably, in the step 5, the average pore diameter of the activated carbon particles is 2-3 nm, and the iodine value is 700-1100 mg/g.
Preferably, the coagulant in step 3 is polyaluminium chloride (PAC); the flocculating agent is anionic Polyacrylamide (PAM) and has the molecular weight of 800-1500 ten thousand.
Preferably, in the step 5, the ultrafiltration membrane is hollow fiber type dead-end filtration, and the filtration pore diameter is 0.01-0.03 μm.
Preferably, in step 5, the phosphorus-containing scale inhibitor comprises an inorganic phosphorus-containing scale inhibitor and/or an organic phosphorus-containing scale inhibitor.
Preferably, the inorganic phosphorus-containing scale inhibitor comprises sodium tripolyphosphate or sodium hexametaphosphate; the organic phosphorus-containing scale inhibitor comprises hydroxyethylidene diphosphonic acid or polyamino polyether methylene phosphonate.
The system based on the shale gas fracturing flowback fluid standard discharge treatment process method comprises the following units which are connected in sequence: the device comprises an air flotation unit, an effective softening unit, an effective coagulation sedimentation unit, an activated carbon particle adsorption unit, an ultrafiltration unit and a reverse osmosis unit;
an air flotation unit: the method is used for reducing petroleum substances in the fracturing flow-back fluid and oxidizing ferrous ions in the fracturing flow-back fluid into ferric ions to form precipitates for removal;
an effective softening unit: the device is used for softening the fracturing flow-back fluid treated by the air floatation unit, removing calcium ions in the fracturing flow-back fluid and reducing the hardness of the fracturing flow-back fluid;
the effective coagulation sedimentation unit: for reducing turbidity of the supernatant from the effective softening unit;
activated carbon particle adsorption unit: the device is used for removing suspended matters in the supernatant from the effective coagulation sedimentation unit by adsorption;
an ultrafiltration unit: the device is used for filtering the supernatant from the activated carbon particle adsorption unit and further removing impurities in the fracturing flow-back fluid;
a reverse osmosis unit: used for desalting the effluent from the ultrafiltration unit.
Preferably, the effective softening unit is provided with a pH monitor.
Preferably, the effective softening unit is provided with a hardness meter and an online turbidity meter.
Compared with the prior art, the invention has the beneficial effects that:
1) compared with the existing technology that the softening medicine adding amount is not determined according to the actually changed water quality and the medicine adding is not accurate due to the large influence of human factors, the softening medicine adding amount of the invention is determined according to the actual fracturing flowback liquid water quality and changes along with the actual water quality, namely NaOH and Na of an effective softening unit of which the effluent water quality is influenced 2 CO 3 The chemical dosing amount is accurate, the chemical dosing device can adapt to the fracturing flow-back fluid with changed water quality, and can accurately and obviously reduce scale forming ions in the flow-back fluid with changed water quality without bringing other inorganic pollutants, so that the risk of inorganic scale scaling pollution of the rear-end membrane concentrated solution is reduced;
2) compared with the existing process that the dosage of coagulation-flocculation dosing is not determined according to the changed water quality, so that dosing is not accurate, and the water quality of effluent is affected, the invention masters the water quality change rule of shale gas fracturing flowback fluid in a certain block of south China in the early stage, carries out a large number of experiments, establishes a coagulation dosing base database, and the optimal dosage of coagulation-flocculation dosing in the process comes from the database, and the dosage changes along with the change of the actual water quality, and the dosage of PAC and PAM of an effective coagulation sedimentation unit is accurate, so that the process can adapt to fracturing flowback fluid with changed water quality, and the turbidity of the flowback fluid can be accurately and obviously reduced, thereby reducing the filtration burden of a rear-end medium and reducing the risk of membrane pollution;
3) the dosing of the effective softening unit and the effective coagulation sedimentation unit has important guiding significance for automatic dosing in the industrial application process of the standard-reaching discharge treatment device, after the dosing is used together with an online turbidity meter, a pH monitor and a hardness meter, the logical relation between online monitoring data and the dosing amount of the process can be associated and recorded in a PLC system, and for fracturing return liquid with changed water quality, the PLC control system can automatically match the appropriate dosing amount through set conditions, so that the equipment can achieve the purpose of automatic dosing, the device can achieve standardized production, operators only need to carry out 'fool' type, the existing fracturing return liquid standard-reaching discharge treatment device can hardly achieve standard-reaching standardized dosing, the dosing amount has high requirements on professional technology of the operators, and the operation process is complex;
4) the activated carbon adsorption unit is different from the traditional activated carbon filtration, and the activated carbon particle adsorption unit can better improve the water quality of the flowback fluid by screening out parameters such as a characteristic adsorbent, the adding amount of the characteristic adsorbent, the hydraulic retention time and the like, and activated carbon particles are added into the flowback fluid for adsorption instead of the conventional activated carbon rod for filtering the flowback fluid;
5) compared with the existing standard discharge technology which does not consider salt discharge, or the treated concentrated water has poor quality and contains a large amount of scaling ions and other impurities, so that the crystallized salt is complex, the method can accurately reduce the hardness, colloid, suspended matters, turbidity and the like of the returning fluid with changed water quality, and when the membrane is treated by a reverse osmosis membrane at the rear end, the membrane concentrated water has good quality and single component (mainly containing sodium chloride), so that the produced crystallized salt has good quality, can reach the secondary standard of industrial salt (GB/T5462-2015), and lays a technical foundation for zero discharge of the shale gas fracturing returning fluid and resource utilization of byproduct salt thereof.
Drawings
FIG. 1 is a schematic diagram of the system of the present invention;
FIG. 2 is a schematic diagram of the process flow steps of the present invention.
Detailed Description
The present invention will be described in further detail in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The invention provides a shale gas fracturing flowback fluid standard discharge treatment process method and a system thereof (shown in figures 1 and 2), aiming at the defects of the existing shale gas fracturing flowback fluid standard discharge treatment technology, especially the defects of the pretreatment technology, such as inaccurate dosage of pretreatment agents of fracturing flowback fluids with different water qualities, the problems of incapability of stably and effectively reducing turbidity and scale forming ions (calcium, magnesium, barium, strontium and carbonate) of shale gas fracturing flowback fluid with changed water quality and incapability of generating membrane strong brine with better water quality and single component (mainly containing sodium chloride), and comprising the following units which are sequentially connected:
an air flotation unit: the method is used for reducing petroleum substances in the fracturing flow-back fluid and oxidizing ferrous ions in the fracturing flow-back fluid into ferric ions to form precipitates for removal.
An effective softening unit: the device is used for softening the fracturing flow-back fluid treated by the air floatation unit, removing calcium ions and the like in the fracturing flow-back fluid and reducing the hardness of the fracturing flow-back fluid.
The effective coagulation sedimentation unit: for reducing the turbidity of the supernatant from the effective softening unit.
Activated carbon particle adsorption unit: used for removing suspended matters in the supernatant from the effective coagulation sedimentation unit by adsorption.
An ultrafiltration unit: the filter is used for filtering the supernatant from the activated carbon particle adsorption unit and further removing impurities in the fracturing flow-back fluid.
A reverse osmosis unit: used for desalting the effluent from the ultrafiltration unit.
The invention also discloses a shale gas fracturing flowback fluid standard discharge treatment process method, which is realized based on the treatment system and comprises the following steps:
step 1, enabling the fracturing flow-back liquid to enter an air flotation unit, wherein air flotation is to introduce air into the flow-back liquid, enabling formed bubbles to bring petroleum types with density lower than that of the flow-back liquid to the position above the liquid level, oxidizing ferrous ions (such as ferrous sulfide) in the flow-back liquid into ferric insoluble substances (such as ferric hydroxide), bringing the ferric insoluble substances to the position above the liquid level of the flow-back liquid through the bubbles, finally forming scum, scraping and removing the scum through a mud scraper, and then adding a bactericide (NaClO) with concentration of 2-8mg/L into the air flotation unit to remove Sulfate Reducing Bacteria (SRB), saprophytic bacteria (TGB), iron bacteria (FB) and other bacteria in the fracturing flow-back liquid;
furthermore, the air capacity in the air flotation unit is 2% -5% of the inflow of the fracturing flow-back fluid, and the hydraulic retention time is 8-15 min.
Step 2, enabling the supernatant in the air floatation unit to enter an effective softening unit, adding NaOH into the effective softening unit, and mechanically stirring at the speed gradient of 80s -1 ～120s -1 The hydraulic retention time is 10-15 min to adjust the pH value of the fracturing flow-back fluid to 10-11.5, so that magnesium ions in the flow-back fluid form Mg (OH) 2 Precipitating; then adding the calcium hardness (as CaCO) in the fracturing flowback fluid into the effective softening unit 3 In terms of mg/L) 0.9-1.1 times of Na 2 CO 3 (mg/L) and mechanically stirring at a speed gradient of 80s -1 ～120s -1 The hydraulic retention time is 10-15 min, so that calcium ions form CaCO 3 Settling;
step 3, the supernatant (turbidity is X) in the effective softening unit enters an effective coagulation sedimentation unit to reduceThe turbidity of the fracturing flowback fluid is prepared by adding coagulant (polyaluminium chloride PAC) into an effective coagulating sedimentation unit to make small particulate matters in the flowback fluid form large particulate matters through bridging, electric neutralization and the like, adding the coagulant, and then mechanically stirring at a speed gradient of 250 -1 ～400s -1 The hydraulic retention time is 10-15 min, and the adding amount Y (mg/L) of the polyaluminium chloride (PAC) and the turbidity X (NTU) of the fracturing flow-back fluid meet the following conditions: y is 0.58X + 94.7;
then adding a flocculating agent (anionic polyacrylamide PAM with the molecular weight of 800-1500 ten thousand) with the molecular weight of 800-1200 ten thousand into the effective coagulation sedimentation unit to ensure that large granular substances form large flocs through electrical neutralization, bridging and the like, adding the flocculating agent, and then mechanically stirring at the speed gradient of 30 DEG -1 ～50s -1 The hydraulic retention time is 10-15 min; the dosage Z (mg/L) of the anionic Polyacrylamide (PAM) and the turbidity X (NTU) of the fracturing flow-back fluid satisfy the following conditions: z is 0.034X-0.017;
then standing and precipitating the flocculated fracturing flowback fluid for 20-30 min;
step 4, enabling the supernatant in the effective coagulation sedimentation unit to enter an activated carbon particle adsorption unit, adding activated carbon with the concentration of 200-500mg/L into the activated carbon particle adsorption unit, and mechanically stirring at the speed gradient of 50 -1 ～100s -1 Stirring for 5-8 min, settling for 10-20 min after stirring, and further removing suspended matters and small particles in the fracturing flow-back fluid through activated carbon adsorption;
further, the average pore diameter of the activated carbon is 2-3 nm, and the iodine value is 700-1100 mg/g;
step 5, enabling the supernatant in the activated carbon particle adsorption unit to enter an ultrafiltration unit, and filtering the fracturing flow-back fluid through an ultrafiltration membrane in the ultrafiltration unit, wherein the ultrafiltration membrane is hollow fiber type dead-end filtration, and the filtration pore diameter is 0.01-0.03 mu m; then adding HCl into the effluent of the ultrafiltration membrane to adjust the pH value of the fracturing flow-back fluid to 6.5-7.5; then adding sodium bisulfite into the effluent of the ultrafiltration membrane to adjust the ORP of the effluent of the ultrafiltration membrane to-100 mv to +100 mv; adding 20-40mg/L of phosphorus-containing scale inhibitor (such as inorganic phosphorus-containing scale inhibitor, such as sodium tripolyphosphate and sodium hexametaphosphate), and organic phosphorus-containing scale inhibitor (such as hydroxyethylidene diphosphonic acid and polyamino polyether methylene phosphonate);
step 6, enabling the effluent of the ultrafiltration unit to enter a reverse osmosis unit, and performing desalination treatment on the fracturing flow-back liquid through a reverse osmosis membrane in the reverse osmosis unit to obtain reverse osmosis fresh water and reverse osmosis concentrated water; and (3) feeding the reverse osmosis concentrated water into a Mechanical Vapor Recompression (MVR) evaporator for evaporation and crystallization, and performing centrifugal separation on crystallized salt by adopting a centrifugal machine, wherein the rotating speed of the centrifugal machine is 2000 rpm.
The shale gas fracturing flowback fluid in south china is taken as an example for detailed description.
A softening and medicine adding process flow is designed according to a single water sample experiment by using a pilot plant for the shale gas fracturing flowback liquid in south China, namely, sodium hydroxide is added firstly, the adding amount is 300ppm, sodium carbonate is added, the adding amount is 1500ppm, the optimized softening process scheme is provided, namely, the adding amount of NaOH is used for increasing the pH value of inlet water to 10, namely, the adding end point is obtained, Na is used for adding medicine, and the softening process flow is characterized in that 2 CO 3 The dosage needs to be measured by a water quality analysis method for quickly measuring the calcium hardness and Na content of the flow-back fluid 2 CO 3 The dosage (mg/L) is calcium hardness (as CaCO) 3 mg/L), and the results of the designed and optimized protocol softening tests on 6 samples of a well are shown in table 1 below.
TABLE 1 comparative analysis of quality of softened effluent of shale gas well fracturing flowback fluid in south China
Figure GDA0003723094790000091
The water quality analysis result in table 1 shows that the effluent quality is good and stable according to the optimization scheme, specifically, the hardness of the optimized softened effluent is lower than 260mg/L, the fluctuation of the effluent hardness is large according to the design scheme and ranges from 50 mg/L to 580mg/L, the optimized alkalinity is stable and ranges from 470 mg/L to 600mg/L, the alkalinity of the effluent is generally high and reaches 2000mg/L at most, the sodium carbonate is generally added excessively, the pH value of the optimized effluent fluctuates from 8.7 to 11.65, the sodium hydroxide addition is inaccurate, and the pH value of the effluent of the design scheme is always stable at about 10. The test results show that the water outlet effect is unstable and the water quality fluctuation is large when the chemicals are added according to the design method, the optimized softening scheme is good, and the hardness, the pH value and the alkalinity of the softened water outlet of the shale gas fracturing flowback fluid can be stably controlled.
An effective coagulation sedimentation unit is arranged behind the effective softening unit, a coagulation-flocculation dosing process flow is designed according to a single sample experiment, the dosing amount of a coagulant is 200mg/L, the dosing amount of a flocculating agent is 3mg/L, and an optimized process scheme is provided by a large number of previous theoretical calculations and indoor experimental researches, namely the relation between the optimal dosing amount (Y, mg/L) of the coagulant (PAC) and the turbidity (X, NTU) of return drainage is Y-0.58X +94.7, the relation between the optimal dosing amount (Y, mg/L) of an anion type and the turbidity (X, NTU) of return drainage is Y-0.034X-0.017, and the results of coagulation-flocculation tests of 6 samples after the well is softened according to the design scheme and the optimized scheme are shown in the following table 2.
TABLE 2 comparative analysis of the quality of the coagulation-flocculation effluent of the fracturing flowback fluid of certain shale gas well in south of Sichuan
Figure GDA0003723094790000101
The water quality analysis results in table 2 show that the effluent quality is better and stable according to the indoor optimized coagulation-flocculation scheme, overall, the turbidity of the effluent after the coagulation-flocculation optimization is lower than 4NTU and can reach 1.1NTU at least, and the turbidity of the effluent is higher as a whole according to the design scheme and can reach more than 20NTU at most, and the fluctuation of the turbidity of the effluent is larger.
The effective coagulation sedimentation unit is followed by an activated carbon adsorption unit, the conventional design is that the activated carbon filter stick filters the flowback fluid, the optimized scheme of the invention is that activated carbon particles are added into the flowback fluid in an adsorption way so that the flowback fluid can fully adsorb pollutants, and the comparison analysis of the effluent quality of the two schemes is shown in the following table 3.
TABLE 3 comparative analysis of water quality of activated carbon adsorption effluent of shale gas well fracturing flowback fluid in south of Sichuan
Figure GDA0003723094790000102
The water quality analysis results in table 3 show that the effluent quality of the optimized adsorption scheme is good and stable, the effluent turbidity of the optimized scheme is lower than 0.5NTU, the overall effluent turbidity of the filtering scheme according to the conventional design is high and reaches more than 4.7NTU at most, and the effluent turbidity fluctuation is large.
The active carbon adsorption effluent enters an ultrafiltration-reverse osmosis membrane system for desalination, and the quality of the reverse osmosis membrane fresh water and the quality of the raw water are shown in the following table 4. The table lists 12 items in raw water which exceed the primary standard of Integrated wastewater discharge Standard (GB 51/190-1996) and Water saving pollutant discharge Standard (DB 51/190-93) and then carries out standard comparison on 12 items of fresh water treated by the process and the standard.
TABLE 4 analysis of the quality of the shale gas well fracturing flowback raw water and reverse osmosis membrane effluent
Figure GDA0003723094790000111
The water quality analysis results in the table 4 show that the originally overproof items of the shale gas fracturing flowback fluid all reach the primary standards of Integrated wastewater discharge Standard (GB 51/190-1996) and Water saving pollutant discharge Standard (DB 51/190-93) in Sichuan after being treated by the process.
The reverse osmosis membrane concentrated water enters the evaporation unit, and the evaporated salt is analyzed, and the result is shown in table 5.
TABLE 5 analysis of the product composition of the evaporative crystalline salts
Figure GDA0003723094790000112
Figure GDA0003723094790000121
The analysis of the components of the crystallized salt shows that the content of the water insoluble substances in the crystallized salt in the design scheme does not reach the standard, and the crystallized salt product in the process is purer and has single component, mainly contains sodium chloride and reaches the secondary standard of refined industrial dry salt.
The above-mentioned embodiments only express the specific embodiments of the present application, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present application. It should be noted that, for those skilled in the art, without departing from the technical idea of the present application, several changes and modifications can be made, which are all within the protection scope of the present application.

Claims (7)

1. The standard-reaching discharge treatment process method of the shale gas fracturing flowback fluid is characterized by comprising the following steps of:
step 1, enabling fracturing flowback liquid to enter an air flotation unit, introducing air into the flowback liquid, enabling formed bubbles to bring petroleum with density lower than that of the flowback liquid above a liquid level, oxidizing ferrous ions in the flowback liquid into ferric insoluble substances, bringing the ferric insoluble substances above the liquid level of the flowback liquid through the bubbles, finally forming floating slag, scraping and removing the floating slag through a mud scraper, and then adding a bactericide with concentration of 2-8mg/L into the air flotation unit to remove sulfate reducing bacteria, saprophytic bacteria or iron bacteria in the fracturing flowback liquid;
step 2, enabling the supernatant in the air floatation unit to enter an effective softening unit, adding NaOH into the effective softening unit, and mechanically stirring at a speed gradient of 80-120 s - 1 The hydraulic retention time is 10-15 min to adjust the pH value of the fracturing flow-back fluid to 10-11.5, so that magnesium ions in the flow-back fluid form Mg (OH) 2 Precipitating; then adding Na with the content of 0.9-1.1 times of the hardness of calcium in the fracturing flow-back fluid into the effective softening unit 2 CO 3 And carrying out mechanical stirring with a speed gradient of 80-120 s - 1 The hydraulic retention time is 10-15 min, so that calcium ions form CaCO 3 Settling;
step 3, enabling the supernatant in the effective softening unit to enter an effective coagulation sedimentation unit to reduce the turbidity of the fracturing flow-back fluid, and firstly adding the supernatant into the effective coagulation sedimentation unitThe coagulant is used for enabling small particulate matters in the flowback liquid to form large particulate matters through bridging and electric neutralization, mechanical stirring is carried out after the coagulant is added, and the speed gradient of the mechanical stirring is 250-400 s - 1 The hydraulic retention time is 10-15 min, and the addition amount of a coagulant Ymg/L and the turbidity X NTU of the fracturing flow-back fluid meet the following conditions: y is 0.58X + 94.7;
then adding a flocculating agent with the molecular weight of 800-1200 ten thousand into the effective coagulation sedimentation unit to enable large granular substances to form large flocs through electric neutralization and bridging, adding the flocculating agent, and then mechanically stirring at the speed gradient of 30-50 s - 1 The hydraulic retention time is 10-15 min; the addition amount of the flocculant Zmg/L and the turbidity X NTU of the fracturing flowback fluid meet the following conditions: z is 0.034X-0.017;
then standing and precipitating the flocculated fracturing flow-back fluid for 20-30 min;
step 4, enabling the supernatant in the effective coagulation sedimentation unit to enter an activated carbon particle adsorption unit, adding activated carbon with the concentration of 200-500mg/L into the activated carbon particle adsorption unit, and mechanically stirring at the speed gradient of 50-100 s - 1 Stirring for 5-8 min, settling for 10-20 min after stirring, and further removing suspended matters and small particles in the fracturing flow-back fluid through activated carbon adsorption;
step 5, enabling the supernatant in the activated carbon particle adsorption unit to enter an ultrafiltration unit, and filtering the fracturing flow-back fluid through an ultrafiltration membrane in the ultrafiltration unit; then adding HCl into the water discharged from the ultrafiltration membrane to adjust the pH value of the fracturing flow-back fluid to 6.5-7.5; then adding sodium bisulfite into the effluent of the ultrafiltration membrane to adjust the ORP of the effluent of the ultrafiltration membrane to-100 mv to +100 mv; then adding 20-40mg/L of phosphorus-containing scale inhibitor into the effluent of the ultrafiltration membrane;
step 6, enabling effluent of the ultrafiltration unit to enter a reverse osmosis unit, and performing desalination treatment on the fracturing flow-back liquid through a reverse osmosis membrane in the reverse osmosis unit to obtain reverse osmosis fresh water and reverse osmosis concentrated water; and (3) feeding the reverse osmosis concentrated water into a mechanical steam recompression evaporator for evaporation and crystallization, and performing centrifugal separation on crystallized salt by adopting a centrifugal machine, wherein the rotating speed of the centrifugal machine is 2000 rpm.
2. The method for treating the shale gas fracturing flow-back fluid to reach the standard for discharge according to claim 1, wherein in the step 1, the air capacity in the air flotation unit is 2-5% of the water inflow of the fracturing flow-back fluid, and the hydraulic retention time is 8-15 min; the bactericide is NaClO.
3. The shale gas fracturing flowback fluid standard discharge treatment process method according to claim 1, wherein in the step 5, the average pore diameter of the activated carbon particles is 2-3 nm, and the iodine value is 700-1100 mg/g.
4. The shale gas fracturing flow-back fluid standard discharge treatment process method according to claim 1, wherein the coagulant in step 3 is polyaluminium chloride; the flocculating agent is anionic polyacrylamide, and the molecular weight of the flocculating agent is 800-1500 ten thousand.
5. The process method for standard discharge treatment of the shale gas fracturing flowback fluid according to claim 1, wherein in the step 5, the ultrafiltration membrane is hollow fiber type dead-end filtration, and the filtration pore size is 0.01-0.03 μm.
6. The shale gas fracturing flowback fluid standard discharge treatment process method of claim 1, wherein in step 5, the phosphorus-containing scale inhibitor comprises an inorganic phosphorus-containing scale inhibitor and/or an organic phosphorus-containing scale inhibitor.
7. The shale gas fracturing flowback fluid standard discharge treatment process method of claim 6, wherein the inorganic phosphorus-containing scale inhibitor comprises sodium tripolyphosphate or sodium hexametaphosphate; the organic phosphorus-containing scale inhibitor comprises hydroxyethylidene diphosphonic acid or polyamino polyether methylene phosphonate.
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