Full-quantitative treatment process for shale gas fracturing flow-back fluid

Abstract

The invention relates to the technical field of petroleum industry, and particularly provides a shale gas fracturing flow-back fluid full-quantification treatment process. The main process or process comprises pretreatment, electrocatalytic oxidation, resin, ultrafiltration and reverse osmosis, and is provided with a membrane concentrated solution treatment system (membrane distillation treatment and MVR treatment). The process not only solves the problem of environmental pollution caused by non-standard discharge of the fracturing flow-back fluid, but also recycles salt in the fracturing flow-back fluid, and the whole process flow is low in operation cost, stable in operation and high in automation degree.

Description

Technical Field

[0001] The invention relates to the technical field of petroleum industry, and specifically provides a fully quantitative treatment process for shale gas fracturing flowback fluid. Background Art

[0002] China's recoverable shale gas resources rank among the top in the world, but because shale gas exists in dense reservoirs, it is difficult to extract it using conventional technology. Large-scale fracturing is required to obtain a relatively considerable output. After the fracturing fluid enters the reservoir, it fractures the rock, forming artificial cracks that allow the fracturing fluid to penetrate into the cracks. After the fracturing construction is completed, the gas well enters the drainage stage, and the shale gas escapes along the cracks and pushes the fracturing fluid back.

[0003] Hydraulic fracturing has led to a rapid increase in shale gas production, but it has also brought about the problem of fracturing flowback fluid disposal. The water quality of fracturing flowback fluid is characterized by high concentration of suspended particulate matter (SS), high salt content, high organic content, high oil content and a wide range of water quality changes. Due to its large aquatic ecotoxicity, the proper disposal of flowback fluid is not only a requirement for environmental protection in the development area, but also a major technical content for the clean and sustainable development of shale gas.

[0004] The disposal methods of shale gas fracturing flowback fluid mainly include reuse preparation, deep well reinjection and treatment to meet the standards for external discharge. Reuse preparation depends on the difference between the water quality requirements of the flowback fluid and the preparation water quality requirements. However, when the fracturing construction of the entire shale gas block is reduced and the demand for reuse of the flowback fluid is reduced, the water produced by the gas wells in the block still needs to find a reliable disposal method. Deep well reinjection is limited by regional geological conditions and external environmental conditions. Some development areas are far away from the reinjection wells, resulting in high disposal costs for the flowback fluid. Treatment to meet the standards for external discharge has become a realistic choice for the disposal of flowback fluid by shale gas development companies in the Sichuan Basin.

[0005] Considering the complex and changeable water quality of shale gas fracturing flowback fluid, it is unrealistic to expect to achieve harmless treatment through a single technical unit. Its treatment and discharge must be a treatment process that integrates multiple technical units. At present, the conventional treatment process for fracturing flowback fluid is designed with coagulation and sedimentation, softening, oxidation, desalination and other modules as the main modules. Among them, coagulation and sedimentation are relatively mature, and desalination mainly adopts reverse osmosis (RO), mechanical vapor recompression evaporation (MVR) and other technologies. However, due to the high oil content, high organic content and complex composition of fracturing flowback fluid, there are still problems such as oil substances not being removed in pretreatment, affecting subsequent processes, low salt purity, and high operating costs. Therefore, it is still necessary to design reasonable pretreatment technology, salt separation technology and low operating cost technology to ensure the stable operation of the desalination unit and the standard discharge and reuse, and to ensure the clean and sustainable development of shale gas.

[0006] In view of this, the present invention is proposed. Summary of the invention

[0007] The object of the present invention is to provide a fully quantitative treatment process for shale gas fracturing flowback fluid.

[0008] In order to achieve the above object, the present invention particularly adopts the following technical solutions:

[0009] A fully quantitative treatment process for shale gas fracturing flowback fluid comprises the following steps:

[0010] (1) Pretreatment: including flotation treatment, softening treatment and sedimentation treatment in sequence, wherein the flotation treatment is used to remove suspended matter and oil substances in the shale gas fracturing flowback fluid, the softening treatment is used to precipitate calcium and magnesium ions in the shale gas fracturing flowback fluid, and the sedimentation treatment is used to remove solid matter in the shale gas fracturing flowback fluid.

[0011] (2) Electrocatalytic oxidation treatment: Oxidation and reduction of organic matter in shale gas fracturing flowback fluid into inorganic matter.

[0012] (3) Resin treatment: Use high-salt chelating resin and ammonia removal resin to remove calcium, magnesium ions and ammonia nitrogen in shale gas fracturing return fluid.

[0013] (4) Ultrafiltration treatment: MBR flat membrane is used to remove solid matter and some organic matter in shale gas fracturing return fluid.

[0014] (5) Reverse osmosis treatment: Reel reverse osmosis is used to remove salt, organic matter and ammonia nitrogen from shale gas fracturing flowback fluid, so that the effluent meets the discharge standards.

[0015] (6) Membrane distillation treatment: A hydrophobic membrane is used as the filter medium. Under heating conditions, the water molecules in the raw material solution evaporate and become water vapor. The water vapor pressure difference on both sides of the hydrophobic membrane is used as the driving force for mass transfer. Water vapor passes through the hydrophobic membrane, but the raw material solution cannot. As the water molecules continue to evaporate, the raw material solution becomes concentrated. The raw material solution is sequentially subjected to at least one membrane distillation to achieve the concentration of the raw material solution.

[0016] (7) MVR treatment: Evaporate the concentrated solution obtained in step (6) to obtain a crystalline salt.

[0017] Furthermore, in step (1), the flotation treatment adopts dissolved air flotation method.

[0018] Preferably, the surface load of the flotation tank in the dissolved air flotation method is 2 to 3 m 3 / (m 2 h), preferably 2.5 m 3 / (m 2h); the dissolved air water inlet pressure of the flotation tank is 0.55-0.75MPa, preferably 0.65MPa; the circulation rate of the flotation tank outlet water is 10%-30%, preferably 20%; the PAC configuration concentration of the flotation tank is 5%-15%, preferably 10%; the PAM of the flotation tank 阳 The concentration is configured to be 0.2‰ to 0.8‰, preferably 0.5‰; the diameter of the bubbles released from the flotation tank is 10 to 50 μm.

[0019] Furthermore, in step (1), the softening treatment adopts a double alkali softening method.

[0020] Preferably, the concentration of the sodium carbonate solution in the softening tank in the double alkali softening method is 15% to 25%, preferably 20%; the concentration of the sodium hydroxide solution is 5% to 10%; the concentration of PAC is 5% to 15%, preferably 10%; 阴 The configuration concentration is 0.2‰~0.8‰, preferably 0.5‰.

[0021] Preferably, the material of the softened water tank is carbon steel rubber lining, SS304 or SS316L, preferably SS304 or SS316L; double alkali is added to control the pH of the shale gas fracturing return fluid in the water tank to 10-12, preferably pH 10.

[0022] Furthermore, in step (1), the sedimentation treatment adopts an inclined tube sedimentation tank, the diameter of the inclined tube is 50mm-100mm, the thickness is not less than 1mm, the inclination angle is 45°-60°, the sedimentation time is 0.5-1.5h, preferably 1h, and the material of the inclined tube is PP material.

[0023] Furthermore, in step (2), the electrode for electrocatalytic oxidation is made of a composite metal oxide porous film electrode material and is used in combination with a modified titanium dioxide photocatalyst.

[0024] Preferably, the reaction temperature of the electrocatalytic oxidation is 25-35° C., the pH is 3-5, and the standard redox electromotive force can reach up to +2.8V.

[0025] Furthermore, in step (3), the particle sizes of the high-salt chelating resin and the ammonia removal resin are independently 0.3 to 1.2 mm, and the maximum exchange capacity can reach 2.0 meq / ml.

[0026] Furthermore, in step (4), the filtration pore size of the ultrafiltration membrane is 0.05 μm, and the operating flux of the membrane is >10 L / (m 2 ·h), operating pressure is -0.01~-0.05MPa, average aeration volume is 3~5L / (min·m 2 ).

[0027] Preferably, a bag filter is arranged after the ultrafiltration water inlet pump, and the filter pore size of the bag filter is 0.5-1 μm, preferably 1 μm.

[0028] Furthermore, in step (5), the filtration pore size of the reverse osmosis membrane is 0.56-0.7 nm, and the operating flux of the membrane is >25 L / (m 2 ·h), the operating pressure is 0.6~1MPa, and the maximum transmembrane pressure difference is 2.5bar.

[0029] Preferably, a bag filter and a core filter are arranged after the reverse osmosis water inlet pump, the bag filter has a filtration pore size of 5 μm, and the core filter has a filtration pore size of 1 μm.

[0030] Furthermore, in step (6), each effect of membrane distillation is composed of a membrane assembly, one side of the membrane assembly is provided with the hydrophobic membrane for separating salt and water in the raw material solution, and the other side is provided with a condenser for condensing water vapor transmitted by the hydrophobic membrane, and the raw material solution is located on the side of the hydrophobic membrane away from the condenser.

[0031] Preferably, the hydrophobic membrane is a polytetrafluoroethylene membrane, and the double-sided surfaces of the polytetrafluoroethylene membrane are hydrophobized to prepare a superhydrophobic nanowire coating, maintaining a contact angle θ in the range of 70°<θ<150°, and a sliding angle less than 10°, a membrane pore size of 0.2-0.4 μm, and a thickness of 0.06-0.25 mm.

[0032] Preferably, the condensing sheet is an aluminum plate coated with an acid-resistant coating on the surface, the thickness of the aluminum plate is 0.06 to 0.25 mm, and the acid-resistant coating is a SiO2-TiO2-PDMS hybrid acid-resistant coating, wherein the mass ratio of SiO2-TiO2 is (0.05 to 0.25): 1, and the mass fraction of PDMS is 15%.

[0033] Preferably, the water vapor contact surface of the condenser sheet is provided with a super-hydrophobic fluorinated coating.

[0034] Preferably, the vertical distance between the hydrophobic membrane and the condensing sheet which are arranged in parallel is 0.2-0.4 mm.

[0035] Preferably, the sealing material of the membrane assembly is a corrosion-resistant rubber product.

[0036] Furthermore, in step (7), the pressure of the start-up heat source steam of the MVR is 0.2 MPa and the temperature is 120°C. The secondary steam coming out of the evaporator is compressed, the pressure and temperature are increased, the thermal enthalpy is increased, and then it is sent to the first effect evaporator of the evaporation crystallization unit as the heat source of the first effect. The secondary steam evaporated from the first effect evaporator is used as the heat source of the second effect. The heat is transferred to the fourth effect in sequence through the secondary steam, and the heating steam itself is condensed into water. The main materials of each effect evaporation chamber are S31603 and Q345R, the working medium is water vapor and salt water, the temperatures of the first effect, the second effect, the third effect, and the fourth effect are 130°C, 110°C, 90°C, and 70°C respectively, the operating pressure of the first effect is -0.1 to 0.1 MPa, and the operating pressures of the second effect, the third effect, and the fourth effect are all independently -0.1 MPa.

[0037] Compared with the prior art, the technical effects of the present invention are:

[0038] The present invention provides a fully quantitative treatment process for shale gas fracturing flowback fluid. The entire pretreatment is complete and reasonable. In the pretreatment stage, the removal rate of oil-containing substances, calcium and magnesium ions, and SS (suspended solids) is above 90%, which prevents impurities from causing a series of problems such as clogging, scaling, and foaming in subsequent units, thereby ensuring that subsequent treatment units can operate stably.

[0039] The present invention has a stable, reliable and applicable effect on the removal of calcium and magnesium ions. First, more than 90% of calcium and magnesium are removed by a double alkali (sodium carbonate and sodium hydroxide) softening method in the pretreatment stage, and then the remaining calcium and magnesium ions and other divalent ions are selectively removed by a high-salt chelating resin, and the maximum exchange capacity can reach 2.0 meq / ml. Since most of the divalent ions are removed in the pretreatment and resin treatment stages, the crystal salt produced by MVR has a high purity and can be exported as a product salt.

[0040] The processes adopted by the present invention are all energy-saving technologies among existing water treatment technologies, especially membrane concentrate treatment technology. Membrane distillation can use the steam generated by the previous effect as the heat source of the next effect for further evaporation and concentration. The number of effects can usually reach 2 to 6. The power consumption is 2 to 10 kWh and the steam consumption is 0 to 0.4 t for evaporating one ton of concentrated liquid. MVR uses steam mechanical recompression technology to increase the temperature and pressure of the secondary steam generated by itself, and further uses it as a heat source for evaporation. In addition, the four effects are used in combination to evaporate and crystallize one ton of membrane distillation concentrated water, and the power consumption is 20 to 50 kWh.

[0041] The present invention has a high degree of automation, a small amount of dosing, and a low operating cost. The electrocatalytic oxidation control system controls and changes the dosing system, flow rate, current density, electrocatalytic oxidation system residence time, aeration, etc. by monitoring indicators such as COD (chemical oxygen demand), pH, conductivity, and flow rate, so that the effluent can meet the standards while minimizing the operating cost. In addition, the entire process flow of the present invention is reasonably designed, and it is only necessary to adjust the alkali in the softening stage and the acid in the catalytic oxidation stage. There is no need to repeatedly adjust the pH, and the amount of dosing is small. The cost of dosing per ton of water treated by the entire process is 5 to 10 yuan. Finally, the distilled water from membrane distillation and the condensed water from MVR can be recycled, which not only saves water resource consumption, but also creates economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0043] Figure 1 This is the main process flow chart of shale gas fracturing flowback fluid treatment in the present invention. DETAILED DESCRIPTION

[0044] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than 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.

[0045] The present invention provides a fully quantitative treatment process for shale gas fracturing flowback fluid, the process flow chart of which is as follows: Figure 1 As shown, the main processes or procedures include "pretreatment + electrocatalytic oxidation + resin + ultrafiltration + reverse osmosis" and a membrane concentrate treatment system (membrane distillation treatment and MVR treatment).

[0046] Preprocessing

[0047] It includes flotation, softening and precipitation. First, a micro-bubble flotation device is used to remove the oil content in the fracturing return fluid and reduce the SS in the return fluid. Then, a double alkali method is used to precipitate the calcium and magnesium ions in the return fluid, and finally it is removed from the system through precipitation.

[0048] The flotation tank adopts the form of microbubbles (microbubble flotation device), which consists of a tubular reactor, an inclined plate dissolved air flotation, a dissolved air flotation machine, an upper scraper, a lower scraper, a dissolved air tank, a circulating pump, etc. After the feed liquid passes through the tubular reactor, it enters the dissolved air flotation machine and mixes with the dissolved air water. The feed liquid is in the inclined plate dissolved air flotation box, separated by the inclined plate, and flows out of the flotation box through a special flow channel. The flocs are attached to the small bubbles, separated from the water by the inclined plate set in the flotation machine cavity, and then float to the surface of the liquid and are scraped away by the upper scraper. The sediment at the bottom of the flotation machine is scraped by the lower scraper to the drain valve and discharged. Part of the water discharged from the flotation machine is recirculated through the circulating pump, and the circulating water is injected into the inclined dissolved air tank, and quickly mixed with the compressed air in it to become saturated dissolved air water.

[0049] The main technical parameters of the flotation tank are shown in Table 1. Table 1 Main technical parameters of flotation tank

[0050] The softening tank includes a drug configuration and dosing device, a softened water tank, and an agitator.

[0051] The drug configuration and dosing device includes a Na2CO3 configuration and dosing device, a NaOH configuration and dosing device, a PAC (polyaluminium chloride) configuration and dosing device, and a PAM (polyacrylamide) configuration and dosing device. The Na2CO3 configuration and dosing device includes a conical bucket, a fan, a storage tank, a screw conveying device, a solution configuration and dosing device. First, the sodium carbonate is forked to the working platform by a forklift, and then the sodium carbonate is poured into the conical bucket. The sodium carbonate is blown into the sodium carbonate storage tank by the wind force of the fan, and then the sodium carbonate is transferred to the solution configuration and dosing device by the screw conveying device. The solution configuration and dosing device includes a triple box, a stirrer, a metering pump and an automatic control system. The triple boxes are a dissolving box, a maturing box and a storage box. When sodium carbonate enters the dissolving box through the dispensing port, water required for the dispensing is automatically added. Under the action of the stirrer, the sodium carbonate is fully stirred and dissolved. With the continuous addition of sodium carbonate and water, the liquid level of the dissolving box gradually rises until it reaches the overflow port and overflows into the maturing box. The sodium carbonate solution is matured in the maturing box. As the liquid level of the maturing agent rises, it will overflow into the storage box, and then the liquid level of the storage box rises, and the metering pump starts to add the sodium carbonate solution. The configuration and dosing device of NaOH, PAC and PAM are the same as those of sodium carbonate solution, and all adopt the form of triple tanks. The configuration concentration of sodium carbonate solution is 15% to 25%, the configuration concentration of sodium hydroxide solution is 5% to 10%, the configuration concentration of PAC is 5% to 15%, and the configuration concentration of PAM is 5% to 15%. 阴The configuration concentration is 0.2‰~0.8‰. The softening water tank is made of corrosion-resistant materials, and can be selected but not limited to one of carbon steel lining rubber, SS304, and SS316L. Double alkali is added to control the pH of the shale gas fracturing return fluid in the water tank to 10-12. The speed of the agitator is 1430r / min, and it is equipped with a speed regulator.

[0052] The sedimentation tank adopts an inclined tube sedimentation tank, the diameter of the inclined tube is 50mm-100mm, the thickness is not less than 1mm, the inclination angle is 45°-60°, the sedimentation time is 0.5-1.5h, preferably 1h, and the material of the inclined tube is PP material.

[0053] Electrocatalytic oxidation treatment

[0054] The process of causing pollutants to undergo direct electrochemical reactions on electrodes, or using strong oxidizing active substances produced on the electrode surface to cause pollutants to undergo redox reactions to produce harmless substances.

[0055] Direct electrochemical reaction can convert organic pollutants and some inorganic pollutants into harmless substances through anodic oxidation, and the cathode can also remove nitrate ions and heavy metal ions from water. Indirect electrochemical reaction can use the redox agent produced by electrochemical reaction to convert pollutants into harmless substances. The redox agent produced at this time is an intermediary for the exchange of electrons between pollutants and electrodes. This intermediary can be a catalyst or a short-lived intermediate produced by electrochemistry. In addition, O2 can also be reduced to H2O2 at the cathode, and then OH is generated. Due to the strong oxidation ability of OH, the standard redox electromotive force can reach up to +2.8V, and it reacts with organic compounds such as addition, substitution, electron transfer, and bond breaking, and can mineralize almost any organic pollutant. The electrocatalytic oxidation electrode adopts a composite metal oxide porous film electrode material and is organically combined with a modified titanium dioxide photocatalyst, so that the efficiency of generating active oxygen species in the reaction system is increased by more than 1 times. The reaction temperature of the electrocatalytic oxidation device is 25-35°C, and the reaction pH is 3-5.

[0056] Resin treatment

[0057] It includes high-salt chelating resin and ammonia removal resin. High-salt chelating resin is a high-molecular compound that selectively chelates calcium and magnesium ions in the form of ionic bonds or coordination bonds, thereby removing calcium and magnesium ions from wastewater. The acidic group of sulfonic acid (-SO3H) contained in ammonia removal resin is easily ionized into H+ ions in water. The NH4+ ions contained in the water react with the H+ ions ionized by the ammonia removal resin. + Ion exchange is performed so that the cation NH 4+ is transferred to the resin, and the H + Exchange into water to reduce ammonia nitrogen in wastewater.

[0058] High-salt chelating resin is a macroporous weak-acid cation exchange resin, the main structure of which is macroporous cross-linked polystyrene, and the ammonia removal resin is yellow opaque spherical particles. The particle size of the two resins is 0.3-1.2 mm, and the maximum exchange capacity can reach 2.0 meq / ml. After the resin is saturated with adsorption, it is first analyzed with H2SO4 or HCl with a mass percentage concentration of 5%-10%, the flow rate is 4-5 BV / H, the time is 30-45 minutes, and the dosage is 2.5-4 BV; then it is backwashed with pure water or soft water, the backwash flow rate is 5-10 BV / H, and the backwash time is 30-45 minutes; then 2-3 BV of NaOH solution with a mass percentage concentration of 5%-10% is used for transformation, the transformation flow rate is 4-5 BV / H, and the time is 30-45 minutes.

[0059] Ultrafiltration

[0060] Using MBR (Membrane Bio-Reactor) flat membrane, under the action of pressure or external suction force, water flows into the filter plate of the flat membrane, and then flows out to the water production pool through the water intake of the flat membrane, thereby achieving the effect of solid-liquid separation.

[0061] The ultrafiltration uses MBR flat membrane, and the bottom is aerated to stir the water in the ultrafiltration tank to prevent the remaining SS or sticky substances from clogging the flat membrane. The pore size of the ultrafiltration membrane is 0.05μm, and the operating flux of the membrane is >10L / (m 2 ·h), operating pressure is -0.01~-0.05MPa, average aeration volume is 3~5L / (min·m 2 ). A bag filter is installed after the ultrafiltration water inlet pump. The filter pore size of the bag filter is 0.5-1μm, preferably 1μm. The bag filter is used to remove particles, colloids and macromolecular substances with a particle size greater than 1μm to protect the ultrafiltration system from being blocked.

[0062] Reverse osmosis treatment

[0063] The spiral reverse osmosis system applies pressure to the membrane on the feed liquid side, allowing water to pass through the membrane and other substances to be retained, thereby achieving the effect of solid-liquid separation. The reverse osmosis water enters the water production pool, and the concentrate enters the membrane concentrate treatment system.

[0064] The reverse osmosis adopts spiral reverse osmosis, the pore size of the reverse osmosis membrane is 0.56~0.7nm, and the operating flux of the membrane is >25L / (m 2·h), the operating pressure is 0.6~1MPa, and the maximum transmembrane pressure difference is 2.5bar. A bag filter and a core filter are installed after the reverse osmosis water inlet pump. The filter aperture of the bag filter is 5μm, and the filter aperture of the core filter is 1μm. The bag filter is used to remove particles, colloids and macromolecular substances with a particle size greater than 5μm, and the core filter is used to remove particles, colloids and macromolecular substances with a particle size greater than 1μm, to protect the reverse osmosis system from being blocked or damaged.

[0065] Membrane distillation

[0066] The hydrophobic membrane is used as the filtration medium, and the steam pressure difference on both sides of the membrane is used as the driving force for mass transfer. Under heating conditions, the water molecules in the raw liquid evaporate and become water vapor that passes through the membrane, while the solution cannot pass through the membrane. As the water molecules continue to evaporate, the raw material becomes concentrated, achieving the purpose of liquid concentration.

[0067] Each effect of membrane distillation is composed of a membrane assembly. A hydrophobic membrane is provided on one side of the membrane assembly to separate salt and water from the raw material solution, and a condenser is provided on the other side to condense the water vapor transmitted by the hydrophobic membrane. The raw material solution is located on the side of the hydrophobic membrane away from the condenser.

[0068] The hydrophobic membrane is selected from polytetrafluoroethylene film but is not limited to this material, and the double-sided surface of the polytetrafluoroethylene film is hydrophobized to prepare a super-hydrophobic nano-wire coating, and the contact angle θ is kept in the range of 70°<θ<150°, and the sliding angle is less than 10°, the membrane pore size is 0.2~0.4μm, and the thickness is 0.06~0.25mm. The cooling plate is an aluminum plate with an acid-resistant coating on the surface, and the thickness of the aluminum plate is 0.06~0.25mm, and the acid-resistant coating is a SiO2-TiO2-PDMS (polydimethylsiloxane) organic-inorganic hybrid acid-resistant coating, wherein the mass ratio of SiO2-TiO2 is (0.05~0.25): 1, and the mass ratio of PDMS is 15%, and the aluminum plate is hydrophobized near the steam side surface to prepare a super-hydrophobic fluorinated coating. The vertical distance between the outer walls of the hydrophobic film and the cooling plate facing each other is 0.2-0.4 mm, and the sealing materials of the hydrophobic film and the cooling plate are corrosion-resistant rubber products.

[0069] For the membrane distillation link, membrane distillation contains a multi-effect distillation process. For example, the raw material enters the first effect, and after being heated by the heat source, the water molecules in the raw material evaporate into water vapor. A water vapor pressure difference is generated in this space and the cavity formed by the diaphragm (i.e., hydrophobic membrane) and the condenser, so that the water vapor enters the cavity through the diaphragm, while the raw material solution cannot pass through the membrane, thereby achieving the purpose of raw material concentration. Then, the raw material concentrated in the first effect can enter the second effect, and the water vapor in the cavity of the first effect can transfer heat to the second effect through the condenser, serving as a heat source for the evaporation of the raw material water molecules into water vapor in the second effect, thereby realizing the recycling of waste heat. At the same time, the water vapor in the cavity of the first effect releases heat and becomes distilled water for recycling. In the second effect, the raw material is further concentrated according to a process similar to the first effect. By analogy, until the temperature of the raw material after the multi-effect is about 50°C, the membrane distillation link ends, the final concentration of the raw material is achieved, the water vapor of each effect is recycled in the form of distilled water, and the waste heat of each effect is also utilized.

[0070] In one embodiment, when the temperature of the raw material in any effect cannot reach the boiling point of the standard gas pressure of water molecules, in order to achieve evaporation of water molecules into water vapor, the gas pressure value in the membrane component can be reduced so that the water molecules in the raw material at the temperature of the effect are converted into water vapor.

[0071] MVR Processing

[0072] Through steam mechanical recompression technology, the secondary steam generated by itself is heated and pressurized, and then used as a heat source for evaporation. The condensate of MVR enters the condensate pool, and the crystallized salt produced can be exported as product salt due to its high purity.

[0073] The startup heat source of MVR uses high-grade steam generated by the boiler, with a pressure of 0.2MPa and a temperature of 120℃. The secondary steam coming out of the evaporator is compressed by the steam compressor, and the pressure, temperature and enthalpy increase. Then it is sent to the evaporator of the evaporation crystallization unit, which is used as the heat source of the first effect. The secondary steam evaporated from the first effect evaporator is used as the heat source of the second effect. The heat is transferred to the fourth effect through the secondary steam in sequence, and the heated steam itself is condensed into water. The concentrated water of membrane distillation is pumped into the mixed condensate plate preheater through the raw material pump to exchange heat with the mixed condensate discharged from the evaporation system. After heating, it enters the fourth effect evaporation tank. The solution entering the evaporation tank is mixed with the original circulating liquid in the evaporation tank and enters the heating chamber. After heating, it enters the evaporation chamber again. Since the circulating liquid coming out of the heating chamber is higher than its boiling point, the pressure decreases and boils after entering the evaporation chamber, generating secondary steam. When the evaporation is concentrated to a certain concentration, it is pumped into the third effect evaporation tank by the fourth effect transfer pump. After evaporation and concentration in the third effect evaporation tank, the liquid is pumped into the first and second effect evaporation tanks by the third effect transfer pump for flash crystallization. The liquid in the evaporation tanks of effect I and II flashes and crystallizes, and the crystals are precipitated and settle into the salt slurry. The salt slurry is discharged from the evaporation tank of effect I to the evaporation tank of effect II by gravity. The crystals crystallized and precipitated in the evaporation tank of effect II are cooled and discharged into the salt slurry barrel by gravity. The crystals crystallized and precipitated in the evaporation tank of effect III are discharged into the salt slurry barrel by gravity. The salt slurry pump is pumped into the thickener for thickening and centrifugal dehydration. The dehydrated salt slurry is bagged and transported out, and the centrifugal mother liquor returns to the evaporation system. When the impurities in the mother liquor reach a certain concentration, the mother liquor pump is pumped out of the evaporation crystallization system to the buffer tank of the mother liquor treatment system, and the buffer tank flows into the rake dryer for drying. The dried impurities are transported out for disposal. The non-condensable gas separates water vapor and non-condensable gas through the surface condenser, and then the non-condensable gas is extracted from the system by the water ring vacuum pump and discharged to the atmosphere. The main materials of each effect evaporation chamber are S31603 and Q345R, the working medium is water vapor and salt-containing brine, and other technical parameters are shown in Table 2.

[0074] Table 2 Technical parameters of each effect

[0075] The invention discloses a fully quantitative treatment process for shale gas fracturing flowback fluid, which not only solves the environmental pollution problem caused by irregular discharge of fracturing flowback fluid, but also recovers the salt therein, and the entire process has low operating cost, stable operation and high degree of automation.

[0076] The present invention also provides a treatment system capable of realizing the above-mentioned shale gas fracturing flowback fluid full-scale treatment process, which includes a pretreatment system, an electrocatalytic oxidation system, a resin system, an ultrafiltration system, a reverse osmosis system, a membrane distillation system and an MVR system which are sequentially arranged and connected via pipelines. The internal equipment of each system in the treatment system is as described in the above-mentioned treatment process.

[0077] The present invention is further described below by way of examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or directly purchased from the market.

[0078] Example 1

[0079] The inlet water quality of the shale gas fracturing flowback fluid treatment system in a well area in Sichuan is shown in Table 3.

[0080] Table 3 Example 1 Influent Water Quality Serial number name Numeric unit Remark 1 Water intake 62.5 <![CDATA[m 3 / h]]> average value 2 pH 6～8 3 TOC 90～300 mg / L 4 Conductivity 30～40 ms / cm 5 SS 100～300 mg / L 6 TDS 30000～40000 mg / L 7 Chloride ion 15000～20000 mg / L 8 Sulfate 150～250 mg / L 9 Ammonia nitrogen 40～80 mg / L 10 hardness 1200～1600 mg / L 11 Calcium 400～600 mg / L 12 Magnesium ion 50～150 mg / L 13 Silicon dioxide 50～150 mg / L 14 Sodium ion 10000～15000 mg / L

[0081] It can be seen from Table 3 that the shale gas fracturing flowback fluid of Example 1 has a high salt content, high hardness, and contains a certain amount of TOC (total organic carbon), ammonia nitrogen and oil substances. It is difficult to treat it to meet the standards using conventional physical, chemical and biological methods. By using this process method, the effluent can meet the surface water Class III standard in Table 1 of the "Surface Water Environmental Quality Standard" (GB3838-2002), and the system is stable. The technical parameters of the main process sections are as follows.

[0082] (1) Pretreatment: Pretreatment mainly removes hardness and oil. After pretreatment, the pH is between 10 and 12, the total hardness is between 400 and 600 mg / L, the calcium and magnesium hardness is reduced to below 20 mg / L, and the oil content is reduced to below 1 mg / L.

[0083] (2) Electrocatalytic oxidation: The main function of electrocatalytic oxidation is to remove organic matter and oil substances in the fracturing return fluid. After electrocatalytic oxidation, TOC is reduced to below 50 mg / L and oil substances are reduced to below 0.1 mg / L.

[0084] (3) Resin treatment: After treatment with high-salt chelating resin and ammonia removal resin, further hardness and ammonia removal are carried out. After resin treatment, the calcium and magnesium hardness is reduced to below 2 mg / L, and the ammonia nitrogen is reduced to 4-8 mg / L.

[0085] (4) Ultrafiltration: The main function of ultrafiltration is to remove SS and a small amount of organic matter. The SS of ultrafiltration water is almost 0, and TOC is reduced to below 35 mg / L.

[0086] (5) Reverse osmosis: The function of reverse osmosis is to remove salt, organic matter, ammonia nitrogen, etc., which can ensure that the effluent meets the discharge standards. If the chloride ion of the first-stage reverse osmosis effluent cannot meet the surface water Class III standard in Table 1 of the "Surface Water Environmental Quality Standard" (GB3838-2002), the second-stage reverse osmosis can be used to ensure that the effluent meets the discharge standards. 3 / h, conductivity is about 1.2×10 5 us / cm.

[0087] (6) Membrane distillation: The membrane concentrate of reverse osmosis is first concentrated by membrane distillation to produce 9.5m3 / h of distilled water can be reused as pure water or clean water, producing 9.5m 3 / h of concentrated liquid enters the MVR evaporator.

[0088] (7) MVR: 8.2m 3 / h of MVR condensate meets the discharge requirements after reverse osmosis treatment, and the reverse osmosis concentrated water enters the membrane distillation system. 2.4t / h of crystallized salt is produced, and the water content of the crystal is about 15%. Since the purity of the produced sodium chloride crystals exceeds 97%, it can be exported as product salt.

[0089] Example 2

[0090] The main influent water quality of a shale gas fracturing flowback fluid treatment system in Sichuan is shown in Table 4.

[0091] Table 4 Example 2 Influent Water Quality Serial number name Numeric unit Remark 1 Water intake 35 <![CDATA[m 3 / h]]> average value 2 pH 7～9 3 COD 300～500 mg / L 4 Chloride ion 8000～12000 mg / L 5 hardness 600～800 mg / L

[0092] It can be seen from Table 4 that the shale gas fracturing flowback fluid of Example 2 has a high salt content, high hardness, and contains certain organic matter. It is difficult to treat it to meet the standards using conventional physical, chemical and biological methods. By using this process method, the effluent can meet the surface water Class III standard in Table 1 of the "Surface Water Environmental Quality Standard" (GB3838-2002), and the system is stable. The technical parameters of the main process sections are as follows.

[0093] (1) Pretreatment: Pretreatment mainly removes hardness. After pretreatment, the pH is between 10 and 12, and the total hardness is reduced to 200 to 300 mg / L.

[0094] (2) Electrocatalytic oxidation: The main function of electrocatalytic oxidation is to remove organic matter and oil substances in the fracturing return fluid. After electrocatalytic oxidation, COD is reduced to below 150 mg / L.

[0095] (3) Resin: After treatment with high-salt chelating resin, the hardness is further removed. After resin treatment, the total hardness is reduced to below 100 mg / L.

[0096] (4) Ultrafiltration: The main function of ultrafiltration is to remove SS and a small amount of organic matter. The SS of ultrafiltration water is almost 0 and the COD is reduced to below 100 mg / L.

[0097] (5) Reverse osmosis: The function of reverse osmosis is to remove salt and organic matter, etc., which can ensure that the effluent meets the discharge standards. If the chloride ion of the first-level reverse osmosis effluent cannot meet the "Surface Water Environmental Quality Standards", (GB3838-2002) Table 1 surface water Class III standard, can be carried out by secondary reverse osmosis to ensure that the effluent meets the discharge standards. Reverse osmosis membrane concentrate 11.4m 3 / h, TDS about 5.04×104 mg / L.

[0098] (6) Membrane distillation: The membrane concentrate of reverse osmosis is first concentrated by membrane distillation to produce 5.7m 3 / h of distilled water is reused as pure water or clean water, generating 5.7m 3 / h of concentrated liquid enters the MVR evaporator.

[0099] (7) MVR: 5.3m 3 / h of MVR condensate meets the discharge requirements after reverse osmosis treatment, and the reverse osmosis concentrated water enters the membrane distillation system. 0.66t / h of crystallized salt is produced, and the water content of the crystal is about 15%. Since the purity of the produced sodium chloride crystals exceeds 97%, it can be exported as product salt.

[0100] Note that the above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may also include more other equivalent embodiments without departing from the technical concept of the present invention, all of which belong to the protection scope of the present invention.

Claims

1. A fully quantitative treatment process for shale gas fracturing flowback fluid, characterized in that: The following steps are involved: (1) Pretreatment: comprising flotation treatment, softening treatment and precipitation treatment in sequence, wherein the flotation treatment is used to remove suspended matter and oil substances in the shale gas fracturing flowback fluid, the softening treatment is used to precipitate calcium and magnesium ions in the shale gas fracturing flowback fluid, and the precipitation treatment is used to remove solid matter in the shale gas fracturing flowback fluid; (2) electrocatalytic oxidation treatment: oxidizing and reducing organic matter in the shale gas fracturing flowback fluid into inorganic matter; (3) Resin treatment: using high-salt chelating resin and ammonia removal resin to remove calcium, magnesium ions and ammonia nitrogen in the shale gas fracturing flowback fluid; (4) Ultrafiltration treatment: using MBR flat membrane to remove solid matter and some organic matter in the shale gas fracturing return fluid; (5) Reverse osmosis treatment: using spiral reverse osmosis to remove salt, organic matter and ammonia nitrogen in the shale gas fracturing flowback fluid, so that the effluent meets the discharge standards; (6) Membrane distillation: Using a hydrophobic membrane as the filter medium, when heated, the water molecules in the raw material solution evaporate and become water vapor. The water vapor pressure difference on both sides of the hydrophobic membrane is used as the driving force for mass transfer. Water vapor passes through the hydrophobic membrane, but the raw material solution cannot. As the water molecules continue to evaporate, the raw material solution becomes concentrated. The raw material solution is sequentially subjected to at least one membrane distillation effect to achieve the concentration of the raw material solution. (7) MVR treatment: Evaporate the concentrated solution obtained in step (6) to obtain a crystalline salt.

2. The treatment process according to claim 1, characterized in that: In step (1), the flotation treatment adopts dissolved air flotation method; Preferably, the surface load of the flotation tank in the dissolved air flotation method is 2 to 3 m 3 / (m 2 h), preferably 2.5 m 3 / (m 2 h); the dissolved air water inlet pressure of the flotation tank is 0.55-0.75MPa, preferably 0.65MPa; the circulation rate of the flotation tank outlet water is 10%-30%, preferably 20%; the PAC configuration concentration of the flotation tank is 5%-15%, preferably 10%; the PAM of the flotation tank 阳 The concentration is configured to be 0.2‰ to 0.8‰, preferably 0.5‰; the diameter of the bubbles released from the flotation tank is 10 to 50 μm.

3. The treatment process according to claim 1, characterized in that: In step (1), the softening treatment adopts a double alkali softening method; preferably, the configuration concentration of the sodium carbonate solution in the softening tank in the double alkali softening method is 15% to 25%, preferably 20%; the configuration concentration of the sodium hydroxide solution is 5% to 10%; the configuration concentration of PAC is 5% to 15%, preferably 10%; the configuration concentration of PAM 阴 The configuration concentration is 0.2‰~0.8‰, preferably 0.5‰; Preferably, the material of the softened water tank is carbon steel rubber lining, SS304 or SS316L, preferably SS304 or SS316L; double alkali is added to control the pH of the shale gas fracturing return fluid in the water tank to 10-12, preferably pH 10.

4. The treatment process according to claim 1, characterized in that: In step (1), the sedimentation treatment adopts an inclined tube sedimentation tank, the diameter of the inclined tube is 50mm-100mm, the thickness is not less than 1mm, the inclination angle is 45°-60°, the sedimentation time is 0.5-1.5h, preferably 1h, and the material of the inclined tube is PP material.

5. The treatment process according to claim 1, characterized in that: In step (2), the electrode for electrocatalytic oxidation is a composite metal oxide porous film electrode material, and is used in combination with a modified titanium dioxide photocatalyst; Preferably, the reaction temperature of the electrocatalytic oxidation is 25-35° C., the pH is 3-5, and the standard redox electromotive force is up to +2.8V.

6. The treatment process according to claim 1, characterized in that: In step (3), the particle sizes of the high-salt chelating resin and the ammonia removal resin are independently 0.3 to 1.2 mm, and the maximum exchange capacity can reach 2.0 meq / ml.

7. The treatment process according to claim 1, characterized in that: In step (4), the filtration pore size of the ultrafiltration membrane is 0.05 μm, and the operating flux of the membrane is >10 L / (m 2 ·h), operating pressure is -0.01~-0.05MPa, average aeration volume is 3~5L / (min·m 2 ); Preferably, a bag filter is arranged after the ultrafiltration water inlet pump, and the filter pore size of the bag filter is 0.5-1 μm, preferably 1 μm.

8. The treatment process according to claim 1, characterized in that: In step (5), the filtration pore size of the reverse osmosis membrane is 0.56-0.7 nm, and the operating flux of the membrane is >25 L / (m 2 h), operating pressure is 0.6~1MPa, and the maximum transmembrane pressure difference is 2.5bar; Preferably, a bag filter and a core filter are arranged after the reverse osmosis water inlet pump, the bag filter has a filtration pore size of 5 μm, and the core filter has a filtration pore size of 1 μm.

9. The treatment process according to claim 1, characterized in that: In step (6), each effect of membrane distillation is composed of a membrane assembly, one side of the membrane assembly is provided with the hydrophobic membrane for separating salt and water from the raw material solution, and the other side is provided with a condenser for condensing the water vapor transmitted by the hydrophobic membrane, and the raw material solution is located on the side of the hydrophobic membrane away from the condenser; Preferably, the hydrophobic membrane is a polytetrafluoroethylene membrane, and the surfaces on both sides of the polytetrafluoroethylene membrane are hydrophobized to prepare a super-hydrophobic nanowire coating, maintaining a contact angle θ in the range of 70°<θ<150°, and a sliding angle of less than 10°, a membrane pore size of 0.2-0.4 μm, and a thickness of 0.06-0.25 mm; Preferably, the condensing sheet is an aluminum plate coated with an acid-resistant coating on the surface, the thickness of the aluminum plate is 0.06-0.25 mm, and the acid-resistant coating is a SiO2-TiO2-PDMS hybrid acid-resistant coating, wherein the mass ratio of SiO2-TiO2 is (0.05-0.25): 1, and the mass fraction of PDMS is 15%; Preferably, the water vapor contact surface of the condensing sheet is provided with a super-hydrophobic fluorinated coating; Preferably, the vertical distance between the hydrophobic membrane and the condensing sheet arranged in parallel is 0.2 to 0.4 mm; Preferably, the sealing material of the membrane assembly is a corrosion-resistant rubber product.

10. The treatment process according to claim 1, characterized in that: In step (7), the pressure of the start-up heat source steam of the MVR is 0.2 MPa and the temperature is 120°C. The secondary steam coming out of the evaporator is compressed, the pressure and temperature are increased, the thermal enthalpy is increased, and then it is sent to the first effect evaporator of the evaporation crystallization unit as the heat source of the first effect. The secondary steam evaporated from the first effect evaporator is used as the heat source of the second effect. The heat is transferred to the fourth effect in sequence through the secondary steam, and the heating steam itself is condensed into water. The main materials of each effect evaporation chamber are S31603 and Q345R, the working medium is water vapor and salt water, the temperatures of the first effect, the second effect, the third effect, and the fourth effect are 130°C, 110°C, 90°C, and 70°C respectively, the operating pressure of the first effect is -0.1 to 0.1 MPa, and the operating pressures of the second effect, the third effect, and the fourth effect are all independently -0.1 MPa.

Images