CN114085171A

CN114085171A - Surfactant for natural gas hydrate clean fracturing fluid and preparation method thereof

Info

Publication number: CN114085171A
Application number: CN202111475013.0A
Authority: CN
Inventors: 张志庆; 王师修; 张桂珍; 王芳; 杨春天
Original assignee: Anqiu Plasticizer Factory; China University of Petroleum East China
Current assignee: Anqiu Plasticizer Factory; China University of Petroleum East China
Priority date: 2021-12-03
Filing date: 2021-12-03
Publication date: 2022-02-25
Anticipated expiration: 2041-12-03
Also published as: CN114085171B

Abstract

The invention provides a sulfate salt gemini viscoelastic surfactant represented by the following structural formula 1, a preparation method thereof and application of the surfactant in natural gas hydrate clean fracturing fluid, wherein the surfactant contains two sulfate hydrophilic head groups and two hydrophobic tail chains with long alkyl chains in molecules, and has good viscoelasticity and shear resistance at low temperatureClean fracturing fluid with excellent cutting and rheological properties, at 15 ℃ for 170s^‑1The viscosity can be maintained above 20 mPa.s after the lower shearing for two hours, the national oil and gas industry standard is met, the sand can be well suspended, the on-site preparation is convenient, the gel breaking is clean, the residual quantity after the fracturing operation is less due to the characteristics of anions, the adsorption to the stratum is small, the damage is low, and the fracturing mining method is suitable for fracturing and exploiting the natural gas hydrate at a low temperature.

Description

Surfactant for natural gas hydrate clean fracturing fluid and preparation method thereof

Technical Field

The invention belongs to the field of petrochemical industry, and particularly relates to a preparation method of an anionic gemini sulfate viscoelastic surfactant and application of the surfactant in natural gas hydrate fracturing in oilfield chemistry.

Background

Natural Gas Hydrate (Natural Gas Hydrate/Gas Hydrate) is an organic compound, like ice, in the form of a solid compound of the formula CH₄·nH₂O, which is naturally formed under certain pressure and temperature conditions, water molecules are bonded together through hydrogen bonds to form crystals, which are called hydrates or clathrates. They are also called "Combustible ice" (Combustible gas) or "solid gas" and "vapor ice" because they look like ice and burn on fire. At present, fossil fuels are increasingly exhausted, energy consumption is increased rapidly, the price of the fossil fuels is increased, and natural gas hydrate is praised as a new and promising unconventional alternative energy source due to wide distribution, rich reserves and high fuel utilization rate. Natural gas hydrate reservoirs are similar to storage tanks in the global carbon cycle, containing most of the methane in the world, accounting for one third of the organic carbon that earth moves. Natural gas hydrate is one of the largest methane reservoirs in the global carbon cycle, and 1 cubic meter of natural gas hydrate contains not less than 160 cubic meters of natural gas at standard temperature and pressure. This means that natural gas hydrate has twice the carbon content of the world's mixed fossil fuels.

The exploitation of combustible ice in the ocean is a world-level problem, and the principle of the existing exploitation method is mainly to change the temperature and pressure of a combustible ice reservoir, break the phase balance of the combustible ice reservoir and decompose the combustible ice to obtain methane gas. In the ocean, the main difficulties in mining are seam collapse and pore silt plugging. Fracturing is a technique for improving the seepage capability of a reservoir by applying pressure to form a seam. By combining the fracturing technology with the conventional exploitation method, the exploitation period can be prolonged, and a gas migration channel is provided. The fracturing fluid is a high-viscosity fluid, and can fracture an oil layer in fracturing construction to form cracks and carry sand into the cracks. The fracturing fluid can be divided into oil-based fracturing fluid, water-based fracturing fluid, foam fracturing fluid and clean fracturing fluid. The early fracturing fluids were gasoline as the dispersing medium, with a fluid of a certain viscosity added, which was the earliest oil-based fracturing fluid; later, with the increase of the well depth and the increase of the well temperature, the requirements on the viscosity and the temperature resistance of the fracturing fluid are increased, and natural vegetable gum fracturing fluid, cellulose fracturing fluid and synthetic polymer fracturing fluid, namely the traditional water-based fracturing fluid, are adopted; in the 80 s of the 20 th century, the foam fracturing fluid is widely applied because the foam fracturing fluid has little damage to the stratum; in the 90 s of the 20 th century, the viscoelastic surfactant (VES) carried proppant by virtue of the structural viscosity of the VES in the fracturing process, and the fracturing fluid system developed towards the VES clean fracturing fluid because the viscoelastic surfactant does not need to add a crosslinking agent, a gel breaker and other various chemical additives and has little damage to the stratum.

The clean fracturing fluid is a viscoelastic surfactant (VES) formed by adding a surfactant into saline water, is formed by mutually dissolving the surfactant and the saline water, and does not need additives such as a cross-linking agent and the like. The viscoelastic surfactant can form rod-shaped micelles in saline (dispersion medium), and the rod-shaped micelles can be mutually entangled with each other along with the increase of the rod-shaped micelles, so that the rod-shaped micelles are converted into wormlike micelles to form a spatial network structure similar to crosslinked polymer macromolecules. The sand-carrying principle of VES fracturing fluid is fundamentally different from that of conventional water-based and oil-based fracturing fluid. Conventional fracturing fluids rely on the apparent viscosity of the fracturing fluid to carry the proppant, while VES fracturing fluids rely on the structural viscosity of the fluid to carry the proppant.

The clean fracturing fluid mainly comprises a cationic surfactant, particularly a quaternary ammonium salt gemini surfactant, and is mature in synthesis process, convenient to prepare, good in temperature resistance and suitable for high-temperature oil reservoirs, but under the condition of low temperature, the temperature is lower than the Kraft point, the solubility of the cationic surfactant is greatly reduced, the viscosity and the viscoelasticity of a prepared fracturing fluid system are greatly reduced, and the application effect is poor. In the constructed stratum, the cationic surfactant is positively charged, so that the cationic surfactant is easily adsorbed to the stratum and has large damage to the stratum.

At present, fracturing fluid systems at home and abroad are developing towards low damage, strong adaptability, low cost and environmental friendliness, so that the development of an anionic surfactant used at a low temperature as a natural gas hydrate fracturing fluid has important practical significance.

Disclosure of Invention

In view of the problems of the prior art, it is an object of the present invention to provide a sulfate gemini viscoelastic surfactant.

According to the technical purpose of the invention, the technical route adopted by the invention is as follows:

a sulfate salt type gemini anionic surfactant contains a hydrophobic tail chain with two sulfate hydrophilic head groups and two long alkyl chains in a molecule, and is represented by the following structural formula 1:

wherein n is an integer of 10 to 20, preferably an integer of 12 to 18, more preferably 12, 14, 16 or 18, more preferably 16.

X is-CH₂O(CH₂)_mOCH₂-wherein m is an integer from 1 to 6, preferably an integer from 1 to 4, more preferably 1, 2 or 3, more preferably 2.

One purpose of the invention is to provide a synthetic preparation method of the sulfate salt gemini anionic surfactant, which comprises the following steps:

(1) first, long-chain fatty alcohol is mixed with a Lewis acid catalyst at room temperature and stirred for 10 minutes.

(2) Adding the long-chain fatty alcohol (C) obtained in the step (1)_nH_2n+1OH) and a catalyst, dropwise adding an alkyl ether diepoxy ethane compound into a mixed system, continuously stirring, heating to 30-50 ℃ for reaction for 0.5-3 hours, then heating to 60-110 ℃ for reaction for 1-6 hours, washing after the reaction is finished, carrying out reduced pressure distillation to obtain colorless transparent liquid, namely, carrying out etherification reaction on long-chain fatty alcohol and the alkyl ether diepoxy ethane compound to obtain an alkyl ether glycol intermediate, wherein the reaction is shown in the following reaction formula 1,

(3) and (3) sulfonating the alkyl ether glycol intermediate obtained in the step (2) by using a sulfonating agent (sulfonate), controlling the temperature to be 35-55 ℃, and completing the reaction in 1-3 hours. The reaction is shown in equation 2 below:

(4) and (4) after the sulfonation reaction in the step (3) is finished, cooling the system temperature to room temperature, neutralizing the sulfonation product with an alkali solution until the solution is neutral, and drying to obtain a yellow solid. And recrystallizing with acetone to obtain light yellow solid which is the sulfate salt gemini anionic surfactant.

Preferably, the long-chain fatty alcohol (C) described in step (1)_nH_2n+1OH) is an integer from 10 to 20, preferably an integer from 12 to 18, more preferably the long-chain fatty alcoholIs one selected from lauryl alcohol, myristyl alcohol, cetyl alcohol and stearyl alcohol, and more preferably cetyl alcohol.

Preferably, the alkyl ether diepoxide ethane compound is one of a propyl ether diepoxide ethane compound, a butyl ether diepoxide ethane compound and a pentyl ether diepoxide ethane compound.

Preferably, the Lewis acid catalyst described in step (1) is selected from AlCl₃、FeCl₃、BF₃The dosage of the catalyst is 2.5-3% of the volume of the alkyl ether diepoxide ethane compound.

Preferably, the molar ratio of alkyl ether glycol intermediate to sulfonating agent in step (2) is from 1:2 to 1:2.5, preferably 1:2.

Preferably, in the step (2), the temperature is raised to 50 ℃ for reaction for 1 hour, and then the temperature is raised to 60-90 ℃ for reaction for 3 hours.

Preferably, this step of the reaction in step (3) is completed within 3 hours.

Preferably, the sulfonating agent in step (3) is selected from concentrated sulfuric acid, fuming sulfuric acid, sulfur trioxide, chlorosulfonic acid.

Preferably, the sulfonating agent in step (3) is used in the amount of reactants

Twice the molar amount.

Preferably, the alkali solution in the step (4) is selected from a sodium hydroxide solution with a mass percent concentration of 20% to 40%, a potassium hydroxide solution with a mass percent concentration of 20% to 40%, an ammonia water solution with a mass percent concentration of 20% to 40%, and a potassium hydroxide solution with a mass percent concentration of 20% to 40%.

The invention also aims to provide the application of the sulfate salt type gemini surfactant in clean fracturing fluid.

Another object of the present invention is to provide a clean fracturing fluid comprising the sulfate salt type gemini surfactant according to the present invention at a concentration of 1.5% by weight and an inorganic salt counter ion at a concentration of 0.4% by weight, the other component being water.

Preferably, the clean fracturing fluid according to the present invention, wherein the inorganic salt counter ion is selected from ammonium ion, alkaline earth metal ion, alkali metal ion, preferably ammonium ion.

The invention has the beneficial effects that:

1. the invention provides a method for preparing an anionic sulfate salt type viscoelastic gemini surfactant (AG) by using a long-chain fatty alcohol and an alkyl ether diepoxide compound, and the synthetic method is environment-friendly and has no industrial waste residue.

2. AG and inorganic salt counter ions are compounded to obtain clean fracturing fluid with good viscoelasticity, shear resistance and rheological property at low temperature of 170s at 15 DEG C^-1The viscosity can be maintained above 20 mPa.s after the lower shearing for two hours, the national oil and gas industry standard is met, the sand can be well suspended, the on-site preparation is convenient, the gel breaking is clean, the residual quantity after the fracturing operation is less due to the characteristics of anions, the adsorption to the stratum is small, the damage is low, and the fracturing mining method is suitable for fracturing and exploiting the natural gas hydrate at a low temperature.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.

FIG. 1 is a chart of an infrared spectrum of AG14 in example 1;

FIG. 2 is a NMR chart of AG14 in example 1;

FIG. 3 is a chart of an infrared spectrum of AG16 in example 2;

FIG. 4 is a NMR spectrum of AG16 in example 2;

FIG. 5 is a graph of the clean fracturing fluid of example 3 formulated with 1.5% AG16 and 0.4% ammonium chloride at 15 deg.C for 170s^-1Rheology profile under shear for 2 hours.

FIG. 6 is the viscoelasticity curve at 5, 10, 15 ℃ of a clean fracturing fluid formulated from 1.5% AG16 and 0.4% ammonium chloride in example 3.

FIG. 7 is a viscosity-temperature curve of 5-15 ℃ of a clean fracturing fluid prepared by compounding 1.5% AG16 and 0.4% ammonium chloride in example 4.

FIG. 8 is a curve showing the change of modulus at 5-15 ℃ of the clean fracturing fluid obtained by compounding 1.5% AG16 and 0.4% ammonium chloride in example 4.

FIG. 9 is a photograph of a clean fracturing fluid formulated with 1.5% AG16 and 0.4% ammonium chloride from example 5, suspended for two hours at 20 ℃.

Detailed Description

Hereinafter, the present invention will be described in detail. Before the description is made, it should be understood that the terms used in the present specification and the appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Accordingly, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.

In this document, the terms "comprising," "including," "having," "containing," or any other similar term, are intended to be open-ended franslational phrase (open-ended franslational phrase) and are intended to cover non-exclusive inclusions. For example, a composition or article comprising a plurality of elements is not limited to only those elements recited herein, but may include other elements not expressly listed but generally inherent to such composition or article. In addition, unless expressly stated to the contrary, the term "or" is intended to mean an inclusive "or" rather than an exclusive "or". For example, the condition "a or B" is satisfied in any of the following cases: a is true (or present) and B is false (or not present), a is false (or not present) and B is true (or present), both a and B are true (or present). Furthermore, in this document, the terms "comprising," including, "" having, "" containing, "and" containing "are to be construed as specifically disclosed and to cover both closed and semi-closed conjunctions, such as" consisting of … "and" consisting essentially of ….

All features or conditions defined herein as numerical ranges or percentage ranges are for brevity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to have covered and specifically disclosed all possible subranges and individual numerical values within the ranges, particularly integer numerical values. For example, a description of a range of "1 to 8" should be considered to have specifically disclosed all subranges such as 1 to 7, 2 to 8, 2 to 6, 3 to 6, 4 to 8, 3 to 8, and so on, particularly subranges bounded by all integer values, and should be considered to have specifically disclosed individual values such as 1, 2, 3, 4, 5, 6, 7, 8, and so on, within the range. Unless otherwise indicated, the foregoing explanatory methods apply to all matters contained in the entire disclosure, whether broad or not.

If an amount or other value or parameter is expressed as a range, preferred range, or a list of upper and lower limits, it is to be understood that all ranges subsumed therein for any pair of that range's upper or preferred value and that range's lower or preferred value, whether or not such ranges are separately disclosed, are specifically disclosed herein. Further, when a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range.

For example, the sulfate salt type gemini anionic surfactant according to the present invention is represented by the following structural formula 1:

where n is an integer of 10 to 20, for example, may be an integer of 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20, more preferably an integer of 12 to 18, for example, may be 12, 13, 14, 15, 16, 17, and 18, more preferably 12, 14, 16, or 18, more preferably 16.

X is-CH₂O(CH₂)_mOCH₂The ether structure of (a), wherein m is an integer of 1 to 6, for example, may be 1, 2, 3, 4, 5 and 6, more preferably an integer of 1 to 4, for example, may be 1, 2, 3 and 4, more preferably 1, 2 or 3, more preferably 2.

The synthesis preparation method of the sulfate salt gemini anionic surfactant comprises the following steps:

(1) first a long chain fatty alcohol is contacted with a Lewis acid catalyst (e.g., AlCl)₃、FeCl₃Or BF₃) Mix well at room temperature and stir for 10 minutes. The dosage of the catalyst is only a few drops, and the dosage of the catalyst is 2.5-3% of the volume of the alkyl ether diepoxide compound. When the dosage of the catalyst is too small, the polymerization is insufficient, and a gemini surfactant cannot be formed; too much amount will cause the epoxy compound to self-polymerize and also affect the synthesis product.

in the step, the reaction temperature is low, so that the epoxy compound cannot be polymerized without ring opening; the reaction temperature is high, and the dropwise added epoxy compound is self-polymerized. In addition, the polymerization effect is poor when the reaction time is too short, and a single-chain surfactant is formed; too long a reaction time may also cause chain scission of the polymerized double-chain surfactant, which affects the yield.

(3) Sulfonating the alkyl ether glycol intermediate obtained in the step (2) by using a sulfonating agent, controlling the temperature to be 35-55 ℃, and completing the reaction in 1-3 hours. The reaction is shown in equation 2 below:

Preferably, the long-chain fatty alcohol (C) described in step (1)_nH_2n+1OH) is an integer from 10 to 20, preferably an integer from 12 to 18, and the long-chain fatty alcohol is more preferably one of lauryl alcohol, myristyl alcohol, cetyl alcohol and stearyl alcohol, and is more preferably cetyl alcohol.

Preferably, the molar ratio of alkyl ether glycol intermediate to sulfonating agent in step (3) is from 1:2 to 1:2.5, preferably 1:2.

Meanwhile, in the present invention, the weight average molecular weight (Mw) and the number average molecular weight (Mn) are polystyrene converted molecular weights analyzed by Gel Permeation Chromatography (GPC), and the molecular weight distribution can be calculated from the ratio. Mw/Mn.

Reagents were purchased from Aladdin, IR by Fourdrimer using an American platinum ElmerThe measurement of a vertical leaf transformation infrared spectrometer,¹HNMR was measured by using a 400MHZ NMR spectrometer from Bruker, Switzerland, and rheological properties were determined by using a modular intelligent advanced rheometer (MCR-302) from Antopa.

The following examples are given by way of illustration of embodiments of the invention and are not to be construed as limiting the invention, and it will be understood by those skilled in the art that modifications may be made without departing from the spirit and scope of the invention. Unless otherwise specified, reagents and equipment used in the following examples are commercially available products.

Example 1:

(1) 21.4g (0.1mol) of myristyl alcohol is weighed, heated to 40 ℃ to be melted, and a few drops of catalyst BF are added₃And stirred well for 10 minutes to allow thorough mixing. After mixing, heating to 50 ℃, dropwise adding 8.7g (0.05mol) of butyl ether diepoxy ethane compound into the system while stirring, reacting for 1 hour after all dropwise adding, heating to 80 ℃ again, reacting for 3 hours, washing with water after the reaction is finished, and distilling under reduced pressure to obtain a colorless transparent liquid butyl ether glycol intermediate.

(2) SO with dry inert gas nitrogen₃Diluting the concentration to 10% by volume, controlling the temperature to 35-55 ℃ and sulfonating the butyl ether glycol intermediate synthesized in the last step according to the molar ratio of the sulfonating agent to the intermediate of 2:1, wherein the reaction is completed within 3 hours. After the reaction is finished, 30% sodium hydroxide aqueous solution is used for adjusting the pH value to be neutral, and after drying, acetone is used for recrystallization to obtain light yellow solid which is the anionic gemini sulfate surfactant AG 14.

The obtained infrared spectrum of Gemini anionic viscoelastic surfactant AG14 is shown in figure 1, wherein 2856 and 2922cm^-1Is the stretching vibration absorption peak of saturated carbon-hydrogen bond, which indicates that the product has-CH₃，-CH₂-a structure; 1451cm^-1The position is an asymmetric stretching vibration absorption peak of a sulfur-oxygen double bond in a sulfate ester group, 616cm^-1Is a characteristic absorption peak of sulfate ester group, which indicates that the product has sulfate ester bond; 1130cm^-1Stretching vibration of the joint C-O-CThe absorption peak is the characteristic absorption peak of the aliphatic ether bond, and shows that the product has C-O-C.

The NMR spectrum of the Gemini anionic viscoelastic surfactant AG14 is shown in figure 2.¹H NMR(400MHz,CDCl3)δ4.62(s,1H),3.91–3.32(m,8H),1.66–1.49(m,2H),1.26(s,22H),0.88(t,J＝6.7Hz,3H).

Example 2:

(1) taking 24.2g (0.1mol) of cetyl alcohol, heating to 50 ℃ to melt the cetyl alcohol, adding a few drops of catalyst BF₃And stirred well for 10 minutes to allow thorough mixing. After mixing, heating to 65 ℃, dropwise adding 8.7g (0.05mol) of butyl ether diepoxy ethane compound into the system while stirring, reacting for 1 hour after all dropwise adding, heating to 80 ℃ again, reacting for 3 hours, washing with water after the reaction is finished, and distilling under reduced pressure to obtain a colorless transparent liquid butyl ether glycol intermediate.

(2) According to the mol ratio of 2:1 of a sulfonating agent to the raw materials, sulfonating the butyl ether glycol intermediate synthesized in the last step by using 98% concentrated sulfuric acid, controlling the temperature to be 35-55 ℃, and completing the reaction within 3 hours. After the reaction is finished, 30% sodium hydroxide aqueous solution is used for adjusting the pH value to be neutral, and after drying, acetone is used for recrystallization to obtain light yellow solid which is the anionic gemini sulfate surfactant AG 16-1.

The obtained Gemini anionic viscoelastic surfactant AG16-1 has an infrared spectrum of 2920, 2851cm as shown in figure 3^-1The position is an expansion vibration absorption peak of a saturated carbon-hydrogen bond, 722cm^-1Is a deformation vibration absorption peak of a saturated carbon-hydrogen bond long chain, which indicates that the product has-CH₃，-CH₂-a structure; 1468cm^-1And 1253, 1271cm^-1The peak is respectively an absorption peak of asymmetric stretching vibration and symmetric stretching vibration of a sulfur-oxygen double bond in sulfate group, and the absorption peak is 623cm^-1Is a characteristic absorption peak of sulfate ester group, which indicates that the product has sulfate ester bond; 1125cm^-1Is a stretching vibration absorption peak of C-O-C and is a characteristic absorption peak of fatty ether bond, and indicates that the product has C-O-C.

The NMR spectrum of Gemini anionic viscoelastic surfactant AG16-1 is shown in FIG. 4.¹H NMR(400MHz,CDCl3)δ4.29–3.83(m,1H),3.83–3.10(m,8H),1.57(dt,J＝13.7,6.6Hz,2H),1.43–1.11(m,26H),0.87(q,J＝6.7Hz,3H).

Example 3:

(2) According to the mol ratio of 2:1 of a sulfonating agent to the raw materials, sulfonating the butyl ether glycol intermediate synthesized in the last step by using chlorosulfonic acid with the mass fraction of more than 99.7%, controlling the temperature to be 35-55 ℃, and completing the reaction in 3 hours. After the reaction is finished, 30% sodium hydroxide aqueous solution is used for adjusting the pH value to be neutral, and after drying, acetone is used for recrystallization to obtain light yellow solid which is the anionic gemini sulfate surfactant AG 16-2.

Test example 1

The surfactant AG16-2 obtained in example 3 was prepared into an aqueous solution with a mass percent concentration of 1.5%, and ammonium chloride solids were added thereto to make the final mass percent concentration of 0.4%, i.e., the surfactant and the counter ion were compounded into a clean fracturing fluid.

The clean fracturing fluid system is at 15 ℃ for 170s^-1The viscosity can be maintained above 20 mPa.s after the lower shearing for two hours, and the national oil and gas industry standard is met. As shown in fig. 5.

The clean fracturing fluid system is subjected to viscoelasticity tests at 5, 10 and 15 ℃, and the system has good viscoelasticity. As shown in fig. 6.

Test example 2

The AG16-1 synthesized in example 2 and counter ion ammonium chloride were compounded to prepare a clean fracturing fluid, wherein the mass percentage concentration of AG16-1 was 1.5%, and the mass percentage concentration of ammonium chloride was 0.4%. And testing the change of the viscosity of the clean fracturing fluid at 5-15 ℃ along with the temperature, wherein the viscosity is 22mPa & s at 15 ℃. As shown in fig. 7.

The change of the storage modulus and the loss modulus of a clean fracturing fluid system along with the temperature is tested, and the fracturing fluid has good viscoelastic property at the temperature of 5-15 ℃. As shown in fig. 8.

Test example 3

The synthetic AG16 in example 3 and counter-ion ammonium chloride are compounded into a clean fracturing fluid, wherein the mass percentage concentration of AG16 is 1.5%, and the mass percentage concentration of ammonium chloride is 0.4%.

At room temperature (20 ℃), a ceramsite proppant (40/70 mesh) was selected to observe the sedimentation rate of the sand. The settling rate of the fracturing fluid proppant is generally considered to be 0.08-0.18 mm · s^-1The time performance is good, and from the effect of the clean fracturing fluid of the test embodiment, the sand can be effectively suspended, sand grains hardly settle within two hours, wherein the volume fractions of the propping agents are 15%, 20%, 25%, 30% and 35%, respectively. As shown in fig. 9.

The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A sulfate salt gemini viscoelastic surfactant contains two sulfate hydrophilic head groups and two hydrophobic tail chains with long alkyl chains in the molecule, and is represented by the following structural formula 1:

wherein n is an integer from 10 to 20, preferably an integer from 12 to 18, more preferably 12, 14, 16 or 18, more preferably 16;

x is-CH₂O(CH₂)_mOCH₂-wherein m is an integer from 1 to 6, preferably an integer from 1 to 4,more preferably 1, 2 or 3, more preferably 2.

2. The synthetic process for the preparation of sulfate salt gemini viscoelastic surfactants as claimed in claim 1, said process comprising the steps of:

(1) firstly, fully mixing and stirring long-chain fatty alcohol and a Lewis acid (Lewis acid) catalyst at room temperature;

reaction scheme 1

(3) Sulfonating the alkyl ether glycol intermediate obtained in the step (2) by using a sulfonating agent (sulfonate), controlling the temperature to be 35-55 ℃, and completing the reaction within 1-3 hours, wherein the reaction is shown as the following reaction formula 2:

reaction formula 2

(4) After the sulfonation reaction in the step (3) is finished, cooling the system temperature to room temperature, neutralizing the sulfonation product with an alkali solution until the solution is neutral, drying to obtain a light yellow solid, recrystallizing with acetone to obtain the sulfate salt gemini anionic surfactant,

reaction formula 3.

3. The method according to claim 2, wherein the long-chain fatty alcohol (C) in the step (1)_nH_2n+ ₁OH) is an integer of 10 to 20, preferably an integer of 12 to 18, more preferably one of lauryl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol, and more preferably cetyl alcohol.

4. The method according to claim 2, wherein the alkyl ether diepoxide compound is one of a propyl ether diepoxide compound, a butyl ether diepoxide compound, and a pentyl ether diepoxide compound.

5. The method of claim 2, wherein the Lewis acid catalyst in step (1) is selected from AlCl₃、FeCl₃、BF₃The dosage of the catalyst is 2.5-3% of the volume of the alkyl ether diepoxide ethane compound.

6. The process of claim 2, wherein the molar ratio of alkyl ether glycol intermediate to sulfonating agent in step (2) is from 1:2 to 1:2.5, preferably 1: 2;

7. The process according to claim 2, wherein the reaction in the step (3) is completed within 3 hours;

preferably, the sulfonating agent in step (3) is selected from concentrated sulfuric acid, fuming sulfuric acid, sulfur trioxide, chlorosulfonic acid;

Twice the molar amount.

8. The method according to claim 2, wherein the alkali solution in the step (4) is selected from the group consisting of a sodium hydroxide solution having a concentration of 20 to 40% by mass, a potassium hydroxide solution having a concentration of 20 to 40% by mass, an aqueous ammonia solution having a concentration of 20 to 40% by mass, and a potassium hydroxide solution having a concentration of 20 to 40% by mass.

9. The use of a sulfate gemini surfactant according to claim 1 in clean fracturing fluids.

10. A clean fracturing fluid comprising the sulfate salt type gemini surfactant according to claim 1 in a concentration of 1.5% by weight and an inorganic salt counterion in a concentration of 0.4% by weight, the other component being water;

preferably, the inorganic salt counter ion is selected from ammonium ions, alkaline earth metal ions, alkali metal ions, preferably ammonium ions.