NL2024681B1 - Self-cleaning and filtering method for inorganic ceramic membrane of shale gas fracturing fluid - Google Patents

Abstract

Disclosed is a self—cleaning and filtering method for inorganic ceramic membrane of shale gas fracturing fluid, comprising the following steps: Sl: adding solid cleaning particles to the fracturing flow—back fluid; SZ: filtering the fracturing flow—back fluid, during the process of filtration, solid cleaning particles are used to collide, break, absorb and carry the pollutants adsorbed on the surface of the inorganic ceramic membrane with the flow of the fracturing flow—back fluid, so as to remove the organic and inorganic pollutants on the surface of the ceramic lmembrane. The beneficial effect of the technical scheme proposed in this disclosure is: the solid cleaning particles are used to collide, break, absorb and carry the pollutants adsorbed on the surface of the inorganic ceramic membrane with the flow of the fracturing flow—back fluid, so as to remove the organic and inorganic pollutants on the surface of the ceramic membrane.

Description

P583/NLpd SELF-CLEANING AND FILTERING METHOD FOR INORGANIC CERAMIC MEMBRANE

OF SHALE GAS FRACTURING FLUID

FIELD OF THE DISCLOSURE The disclosure relates to self-cleaning and filtering method for inorganic ceramic membrane of shale gas fracturing fluid.

BACKGROUND In recent years, the shale gas industry has developed rapid- ly. China's proven reserves of shale gas resources have reached the top in the world, and its production has reached the top three in the world. Hydraulic fracturing is a technology widely used in shale gas exploitation. It opens small fractures by hydraulic pressure to allow shale gas to flow to the wellhead. Fracturing flow-back fluid is the mixed fluid that is discharged from the well in the early stage of hydraulic fracturing. The characteris- tics of fracturing flow-back fluid vary due to different geologi- cal conditions. In general, its composition is complex, with high COD, high TSS and high TDS, and it is difficult to handle, which has become a major technical problem facing the oil and gas indus- try. Ceramic membrane separation technology has been widely used in petrochemical, food, biotechnology and pharmaceutical indus- tries, as well as a series of other environment-related indus- tries. Compared with other separation methods (such as adsorption or distillation}, ceramic membrane separation technology is simple with low cost and high efficiency. Therefore, it is widely used in water treatment engineering. The main advantages of ceramic mem- brane filtration process are low energy consumption, easy expan- sion, good coupling with other processes, high strength and con- tinuous automated operation, but its limitation lies in the prob- lem of membrane pollution. For inorganic ceramic membranes, membrane pollution leads to a decrease in flow rate, so membranes must be cleaned regularly. Generally, membrane cleaning methods can be divided into physical methods and chemical methods. The physical method is to first rinse with high flow rate water, and then use a sponge ball to me- chanically clean the pollutants. The chemical method is to use chemical reagents to clean the membrane. These chemical reagents have no damage to the membrane material itself, and has a dissolv- ing effect or a displacing effect on the pollutants.

Organic pollutants in shale gas fracturing fluid mainly in- clude macromolecular hydrocarbons, polyacrylamide, and by-products that are oxidized by gel breaking during fracturing. Inorganic pollutants in shale gas fracturing fluid are mainly metal ions, rock cuttings and clay, among which metal ions mainly include: so- dium, potassium, calcium, magnesium, barium, etc. The total salin- ity of shale gas fracturing fluid can reach 30,000~120,000 mg/L, and the content of solid suspended solids can reach 80~400 mg/L, the COD is about 100~2000 mg/L. Organic matter in shale gas frac- turing fluid has large molecular weight, strong adhesion ability, and easy adsorption on the membrane surface, which is the main factor affecting the performance of ceramic membranes. Over time during the filtration process, the adsorption or accumulation of pollutants in the membrane pores will lead to a decrease in pore size and an increase in membrane resistance, which is difficult to recover, especially for large organic molecules. The effect also greatly exceeds that of clay or other inorganic colloids.

At present, there are related reports on cleaning methods of inorganic ceramic membranes. Chinese patent (Application Number:

201610845396.9) discloses a cleaning agent for inorganic ceramic membrane, which uses sodium tetrapolypropylene benzene sulfonate, magnesium sulfate, corrosion inhibitor, sodium carbonate, sodium chloride, sodium oxychloride, fatty alcohol polymer oxyethylene ether, deionized water, zeolite, and ethylene glycol butyl ether to chemically clean the ceramic membrane. Chinese patent (Applica- tion Number: 201811165720.8) discloses a method for cleaning a flat ceramic membrane. The cleaning steps include: (1) physical cleaning: repeated washing with ultra-pure water at a predeter- mined flow rate and predetermined time; (2) chemical cleaning: re- peatedly cleaning and static immersion with compounded chemicals at a predetermined flow rate and a predetermined time. These tech-

nologies have promoted the promotion of ceramic membrane cleaning technology, but the above-mentioned cleaning technologies have some disadvantages. For example, they have complicated cleaning processes, and many chemicals are needed. In addition, a separate cleaning process can be started only after the ceramic membrane filtration operation is stopped, so that the filtration process cannot be performed continucusly, which reduces the operating ef- ficiency of the inorganic ceramic membrane filtration equipment.

SUMMARY A technical problem to be solved by the disclosure is to solve the membrane pollution problem caused by the adsorption of macromolecular organics and suspended matter in the fracturing flow-back fluid on the membrane surface during the filtration pro- cess of the inorganic ceramic membrane, and also overcome the dis- advantages of the existing ceramics membrane cleaning technology. By adding the preferred solid cleaning particles to the fracturing flow-back fluid to be filtered, and optimizing the process condi- tions of the filtration process, the ceramic membrane filtration process and the cleaning process run synchronously.

A self-cleaning and filtering method for inorganic ceramic membrane of shale gas fracturing fluid, comprising the following steps: Sl: adding solid cleaning particles to the fracturing flow- back fluid; S2: filtering the fracturing flow-back fluid, during the pro- cess of filtration, solid cleaning particles are used to collide, break, absorb and carry the pollutants adsorbed on the surface of the inorganic ceramic membrane with the flow of the fracturing flow-back fluid, so as to remove the organic and inorganic pollu- tants on the surface of the ceramic membrane.

The beneficial effect of the technical scheme proposed in this disclosure is: the solid cleaning particles are used to col- lide, break, absorb and carry the pollutants adsorbed on the sur- face of the inorganic ceramic membrane with the flow of the frac- turing flow-back fluid, so as to remove the organic and inorganic pollutants on the surface of the ceramic membrane. The proposed method has the following advantages: 1) the solid cleaning parti- cles have appropriate density, particle shape and particle size, which enhances the ability to carry organic and inorganic pollu- tants; 2) the solid cleaning particles have suitable surface po- larity and porosity, so they have high efficiency of adsorption and removal of ceramics organic pollutants on the surface of the membrane; 3) the solid cleaning particles have a suitable hard- ness, so they have a high removal efficiency for the inorganic scale on the surface of the ceramic membrane, and also are not easily to be broken and have a high utilization rate; 4) the solid cleaning particles will not cause damage to the ceramic membrane structure.

BRIEF DESCRIPTION OF THE DRAWINGS Accompanying drawings are for providing further understanding of embodiments of the disclosure. The drawings form a part of the disclosure and are for illustrating the principle of the embodi- ments of the disclosure along with the literal description. Ap- parently, the drawings in the description below are merely some embodiments of the disclosure, a person skilled in the art can ob- tain other drawings according to these drawings without creative efforts.

FIG. 1 is a schematic structural diagram of the device used by this disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The disclosure provides a self-cleaning and filtering method for inorganic ceramic membrane of shale gas fracturing fluid, com- prising the following steps: Sl: adding solid cleaning particles to the fracturing flow- back fluid; S2: filtering the fracturing flow-back fluid, during the pro- cess of filtration, solid cleaning particles are used to collide, break, absorb and carry the pollutants adsorbed on the surface of the inorganic ceramic membrane with the flow of the fracturing flow-back fluid, so as to remove the organic and inorganic pollu- tants on the surface of the ceramic membrane.

Different from the traditional use of acid solution, alkaline solution and organic solvent as cleaning materials, the invention is based on the characteristics of ceramic membrane fouling during the filtration of shale gas fracturing flow-back fluid, that is, 5 organic macromolecular groups such as polyacrylamide adhere to in- organic scales formed by the hydrolysis of metal ions such as cal- cium and magnesium. The special function of solid cleaning parti- cles can effectively remove the organic and inorganic pollutants on the surface of the ceramic membrane, and ensure that the ceram- ic membrane is not harmed.

The main organic pollutant in the fracturing fluid is poly- acrylamide, which has stable chemical properties and is not easy to be chemically oxidized or dissolved. At the same time, the mo- lecular weight of polyacrylamide is large, and the molecules on the surface of the ceramic membrane are entangled and agglomerated during filtration, forming a supramolecular cluster structure. The solid cleaning particles prepared by the preferred pecan shell ma- terial of the present invention have the characteristics of large surface porosity, with a porosity of 27% to 50%, uneven surface structure and many grooves, so the macromolecular organic matter and suspended matter can be wrapped around the solid cleaning par- ticles, therefore the organic pollutants on the surface of the ce- ramic membrane can be effectively removed.

Further, the solid cleaning particles are one of pecan shell, coconut shell, quartz sand, and activated carbon.

Preferably, the solid cleaning particles are pecan shells.

Further, before step S1, it also includes step S0: preparing solid cleaning particles, the specific method for preparing solid cleaning particles is: crushing and sieving pecan shells and plac- ing them in sulfuric acid with a concentration of 1% to 2% (v/v) and 0.5%~1% (v/v) nitric acid mixed acid, soaking for 1~2 h, then filtering and cleaning; raising the temperature to 300~400 °C at a heating rate of 10~15 °C/min, and heating for 3~4 h, during the heating process, the oxygen concentration is controlled from 1 to 5% (v/v) and taken out after cooling.

Preferably, the shape of the solid cleaning particles is po- lygonal, which can effectively remove the inorganic pollutants on the surface of the ceramic membrane through collision and fragmen- tation; the particle diameter of the solid cleaning particles is 1 to 2 mm, the particle density of the solid cleaning particles is

1.2~1.6 g/cm2, the particle porosity of the solid cleaning parti- cles is 27% to 50%, and the molar hardness of the solid cleaning particles is 3 to 4. Based on the analysis of the accumulation rate and distribution of inorganic scale on the surface of the ce- ramic membrane, it is preferred that the solid cleaning particles with a particle size of 1 to 2 mm and a particle density of 1.2 to

1.6 g/cm3 can efficiently remove the inorganic pollutants on the surface of the ceramic membrane. The hardness of the ceramic mem- brane is 6.0~7.0, and the preferred solid cleaning particles with a molar hardness of 3~4 will not damage the membrane structure of the ceramic membrane during the cleaning process, and can also en- sure the integrity of the particles during operation. Ceramic mem- brane pore size specifications are generally lnm-5um. The pre- ferred solid cleaning particle size is larger than the ceramic membrane pore size, which can prevent solid cleaning particles from blocking and damaging the ceramic membrane during operation. Further, the specific step of step S1 is: mixing the solid cleaning particles and the fracturing flow-back fluid in a buffer tank, and controlling the concentration of the solid cleaning par- ticles to 5-10 g/L, which can ensure that the solid cleaning par- ticles can fully impact, crush, adsorb, and carry the membrane contaminated surface, thereby reducing the adsorption or accumula- tion of macromolecular organic compounds and suspended matter in the fracturing flow-back fluid to the membrane surface, and achieving self-cleaning. Further, the specific step of step S2 is: controlling the circulating flow rate of fracturing flow-back fluid in the inor- ganic ceramic membrane filter, the ratio of circulating flow to inflow in the inorganic ceramic membrane filter is 30: 1-50: 1, the increase of the circulating flow rate increases the lateral shear force and the contact frequency of the solid cleaning parti- cles and the pollutants, thereby enhancing the scouring effect on organic pollutants attached to the surface of ceramic membranes, making it difficult to deposit on the surface of the membranes,

and making membranes less prone to blockages; The filtration pres- sure is 0.15~0.25 MPa, which not only ensures the effluent effi- ciency, but also reduces the deposition of organic matter on the membrane surface, and maximizes the self-cleaning effect of pollu- tants; The liquid flow rate is 10-20 L/s m2, At this flow rate, the fracturing flow-back fluid flows parallelly to the membrane surface under the impetus of the pump, which has a large scouring effect on the membrane surface. At the same time, the solid clean- ing particles collide, break, adsorb, and carry when flowing through the membrane surface, thereby bringing the removing con- taminants on the membrane surface.

Further, after step S2, it also includes step S3: reusing the solid cleaning particles after the solid cleaning particles are washed by an acid solution and an alkali solution, wherein the ac- id solution is 1% to 2% (v/v) hydrochloric acid, and the alkali solution is 1%~2% (v/v) sodium hydroxide solution, so the solid cleaning particles can be reused.

In order to verify the actual effect of the inorganic ceramic membrane self-cleaning and filtering method of the fracturing flow-back fluid provided by the present invention, three fractur- ing flow-back fluids from a well in the Sichuan shale gas field were self-cleaned and filtered: Embodiment 1 In this embodiment, an inorganic ceramic membrane filtering operation is performed on the fracturing flow-back fluid of a well in the Sichuan shale gas development zone. The COD of the fractur- ing flow-back fluid is 186 mg/L. The filtration effects of two ce- ramic membrane devices were compared, one of which adopted the technology provided by the present invention, and the other did not adopt the technology provided by the present invention. The fracturing flow-back fluid was firstly homogenized and the pH of the water sample remained unchanged. Then adding 10L of fracturing flow-back fluid to the two ceramic membrane filter 4 respectively.

During the fracturing fluid circulation filtration operation, 5 g/L of solid cleaning particles were added to one of the ceramic membrane filters 4 and the other was not added. When the ceramic membrane filter 4 starts to operate, adjusting the filtration pressure to 0.15 MPa for operation, making sure that the ratio of circulating flow to inflow in the inorganic ceramic membrane fil- ter is 50: 1, and the liquid flow rate is 10 L/s:m2. In the same 33-hour filtration time, the flow rate of the inorganic ceramic membrane with solid cleaning particles decreased from 605 L/(m2eh) to 492 L/(m2+h), and the flow rate of the inorganic ceramic mem- brane without solid cleaning particles decreased from 605 L/{m2sh) to 348 L/(m2+h). Taking the ceramic membrane filtration water flux dropping by 70% as a standard, the working time of a filter with- out solid cleaning particles is about 8-10 hours, and the working time of a filter with solid cleaning particles is about 116-122 hours.

Embodiment 2 In this embodiment, an inorganic ceramic membrane filtering operation is performed on the fracturing flow-back fluid of a well in the Sichuan shale gas development zone. The COD of the fractur- ing flow-back fluid is 656 mg/L. The filtration effects of two ce- ramic membrane devices were compared, one of which adopted the technology provided by the present invention, and the other did not adopt the technology provided by the present invention. The fracturing flow-back fluid was firstly homogenized and the pH of the water sample remained unchanged. Then adding 10L of fracturing flow-back fluid to the two ceramic membrane filter 4 respectively. During the fracturing fluid circulation filtration operation, 8 g/L of solid cleaning particles were added to one of the ceramic membrane filters 4 and the other was not added. When the ceramic membrane filter 4 starts to operate, adjusting the filtration pressure to 0.2 MPa for operation, making sure that the ratio of circulating flow to inflow in the inorganic ceramic membrane fil- ter is 40: 1, and the liquid flow rate is 15 L/s:m2. In the same 3-hour filtration time, the flow rate of the inorganic ceramic membrane with solid cleaning particles decreased from 560 L/(m2eh) to 469 L/(mZ+h), and the flow rate of the inorganic ceramic mem- brane without solid cleaning particles decreased from 560 L/{m2enh) to 357 L/{m2°h) . Taking the ceramic membrane filtration water flux dropping by 50% as a standard, the working time of a filter with- out solid cleaning particles is about 4-5 hours, and the working time of a filter with solid cleaning particles is about 72-76 hours.

Embodiment 3 In this embodiment, an inorganic ceramic membrane filtering operation is performed on the fracturing flow-back fluid of a well in the Sichuan shale gas development zone. The COD of the fractur- ing flow-back fluid is 1228 mg/L. The filtration effects of two ceramic membrane devices were compared, one of which adopted the technology provided by the present invention, and the other did not adopt the technology provided by the present invention. The fracturing flow-back fluid was firstly homogenized and the pH of the water sample remained unchanged. Then adding 10L of fracturing flow-back fluid to the two ceramic membrane filter 4 respectively. During the fracturing fluid circulation filtration operation, 10 g/L of solid cleaning particles were added to one of the ceramic membrane filters 4 and the other was not added. When the ceramic membrane filter 4 starts to operate, adjusting the filtration pressure to 0.25 MPa for operation, making sure that the ratio of circulating flow to inflow in the inorganic ceramic membrane fil- ter is 30: 1, and the liquid flow rate is 20 L/s m2. In the same 3-hour filtration time, the flow rate of the inorganic ceramic membrane with solid cleaning particles decreased from 525 L/(m2eh) to 437 L/(m2+h), and the flow rate of the inorganic ceramic mem- brane without solid cleaning particles decreased from 525 L/(m2+h) to 261 L/(m2+h). Taking the ceramic membrane filtration water flux dropping by 50% as a standard, the working time of a filter with- out solid cleaning particles is about 3-4 hours, and the working time of a filter with solid cleaning particles is about 66-68 hours.

From the above three embodiments, it can be concluded that the self-cleaning and filtering method for inorganic ceramic mem- brane of shale gas fracturing fluid is effective to remove the or- ganic and inorganic pollutants on the surface of the ceramic mem- brane.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is il- lustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the prin- ciples of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are ex- pressed.

Claims (8)

CONCLUSIONS A self-cleaning and filtering method for an inorganic ceramic membrane of shale gas fracking fluid, comprising the following steps: S1: adding solid cleaning particles to the fracking backflow fluid; S2: Filtering the fracking backflow liquid, during the filtration process, using solid cleaning particles to collide, cleave, absorb and transport the contaminants adsorbed on the surface of the inorganic ceramic membrane with the flow of the fracturing reflux liquid, so that the organic and inorganic contaminants on the surface of the ceramic membrane are removed. The self-cleaning and filtering method for an inorganic ceramic membrane of shale gas fracking fluid according to claim 1, wherein the solid cleaning particles consist of pecan husks, coconut shells, quartz sand, and activated carbon. The self-cleaning and filtering method for inorganic ceramic membrane of shale gas fracking fluid according to claim 2, wherein the solid cleaning particles are pecan shells. The self-cleaning and filtering method for inorganic ceramic membrane of shale gas fracking fluid according to claim 3, wherein before step S1, it also includes step SO: preparing solid cleaning particles, wherein the specific method for preparing solid cleaning particles is: crushing and sieving pecan shells and placing them in a mixed acid of sulfuric acid at a concentration of 1% to 2% (v/v) and 0.5% to 1% (v/v) nitric acid, soak for 1 to 2 h, then filter and clean; increasing the temperature to 300 to 400 °C at a heating rate of 10 to 15 °C/min, and heating for 3 to 4 hours, during the heating process, the oxygen concentration is controlled from 1 to 5% (v/v) and removed after cooling. 5. Self-cleaning and filtering method for the inorganic ceramic membrane of shale gas fracking fluid according to claim 4, in which the shape of the cleaning solid particles is polygonal, the particle diameter of the cleaning solid particles is 1 to 2 mm, the particle density of the cleaning solid particles 1.2 to 1.6 g/cm? is, the particle porosity of the cleaning solid particles is 27% to 503, and the molar hardness of the cleaning solid particles is 3 to 4. The self-cleaning and filtering method for an inorganic ceramic membrane of shale gas fracking liquid according to claim 1, wherein the specific step of step S1 consists of: mixing the solid cleaning particles and the fracking reflux liquid in a buffer tank, and controlling the concentration of the solid cleaning particles to 5 to 10 g/l. The self-cleaning and filtering method for the inorganic ceramic membrane of shale gas fracking liquid according to claim 1, wherein the specific step of step S2 is: controlling the circulating flow of fracking backflow liquid in the inorganic ceramic membrane filter, the ratio of the circulating flow to the inflow into the inorganic ceramic membrane filter is 30:1 to 50:1, the filtration pressure is 0.15 to 0.25 MPa, and the liquid flow rate is 10 to 20 l/sm. The self-cleaning and filtering method for the inorganic ceramic membrane of shale gas fracking fluid according to claim 1, wherein after step S2, it also includes step S3: reuse of the cleaning solid particles after the cleaning solid particles are washed by an acid solution and an alkali solution, wherein the acid solution consists of 1% to 2% (v/v) hydrochloric acid, and the alkali solution consists of 1% to 2% (v/v) sodium hydroxide solution.