Preparation method of low pour point invert emulsion fracturing thickener

Abstract

The present application relates to the technical field of inverse emulsion fracturing thickening agent, and particularly relates to a preparation method of low pour point inverse emulsion fracturing thickening agent, comprising: preparation of acrylamide and ammonium acrylate aqueous solution; preparation of polyoxyethylene sorbitol beeswax derivative and hydroxyl lanolin alcohol solution; homogenization of the aqueous solution and the alcohol solution into water-in-alcohol emulsion; initiation of polymerization of the water-in-alcohol inverse emulsion under anaerobic condition; cooling and addition of cosolvent. The polyoxyethylene sorbitol beeswax derivative and the hydroxyl lanolin alcohol solution are used as alcohol phase of the inverse emulsion, and the ammonium acrylate is used as anionic polymerization monomer of the inverse emulsion thickening agent, so that the pour point of the inverse emulsion fracturing thickening agent is greatly reduced. The quality indexes of the inverse emulsion fracturing thickening agent product are as follows: pour point: ≤-35 ℃, anionicity: ≥40%, molecular weight: ≥25 million, and solid content: ≥35%. The low pour point requirement of the inverse emulsion fracturing thickening agent for winter fracturing construction in high-latitude areas of China is fully met.

Description

Technical Field

[0001] This invention relates to the field of thickeners for reverse emulsion hydraulic cracking, and particularly to a method for preparing a low pour point reverse emulsion hydraulic cracking thickener. Background Technology

[0002] With the continuous deepening of oil and gas exploration and development, most of China's onshore oilfields have entered the middle and late stages of development. Low-permeability and ultra-low-permeability oil and gas reservoirs, with their enormous potential reserves, will gradually become the main battleground for future oil and gas development in China. In recent years, low-permeability reservoirs have accounted for as much as 67% of newly discovered proven geological reserves. Typical characteristics of low-permeability oil and gas reservoirs include small matrix pore radius, high interstitial material content, high displacement pressure, high clay content, low permeability, low oil saturation, and high sensitivity and complexity. Hydraulic fracturing is an effective measure for increasing production in oil and gas wells and for increasing injection in water injection wells. For the development of low-permeability oil and gas reservoirs, it has become an indispensable measure. Among these measures, fracturing fluid plays a crucial role in fracturing operations. It is present throughout the entire fracturing process and is a fundamental condition determining the success or failure and effectiveness of hydraulic fracturing operations.

[0003] Fracturing fluid is the working fluid used for creating fractures and carrying proppant during hydraulic fracturing in oil and gas fields. It is the "lifeblood" of hydraulic fracturing operations, and various fracturing processes have multiple performance requirements for it during field operations. Apparent viscosity is the most basic and important indicator of fracturing fluid, and it can be divided into the apparent viscosity of the base fluid and the apparent viscosity of the gel. Depending on the stage of fracturing operations, the function of the fracturing fluid varies, and the viscosity requirements of the liquid also differ. The base fluid is the most basic liquid in fracturing fluid. It is usually cross-linked to form a gel, and then various additives are added to become the fracturing fluid. The quality of the base fluid includes its apparent viscosity, pH value, and cross-linking ability, while apparent viscosity directly reflects the basic performance of the base fluid. If the base fluid viscosity is too low, it indicates that the base fluid has not met the design requirements, while if the base fluid viscosity is too high, it will affect the supply capacity of the fracturing fluid, which may lead to high friction in the pipeline, causing wellhead overpressure, and directly affecting the success or failure of the fracturing operation. For the pre-fracturing fluid, its main function is to fracture the formation and create fractures, facilitating the entry of subsequent proppant-carrying fluids. Therefore, the higher the viscosity of the fracturing fluid gel during fracture creation, the larger the fracture area, and the greater the width and height of the fracture. However, for the same fracture area, a higher fracture results in a shorter fracture length, which is not conducive to forming long and wide fractures, thus affecting the effectiveness of fracturing operations.

[0004] To address the conflict between fracture height and width caused by high-viscosity fracturing fluids, fracture height control techniques are frequently employed in fracturing. Slurry suspension capacity is the most crucial indicator of fracturing fluids. For proppant-carrying fluids, their primary function is to carry proppant into the formation. Therefore, the lurry suspension capacity of fracturing fluids directly affects the proppant packing method, and consequently, the conductivity of the propped fracture. Stronger lurry suspension capacity facilitates more uniform proppant distribution within the fracture, making it easier to form highly conductive propped fractures. Insufficient lurry suspension capacity can lead to large proppant particles settling first, followed by smaller particles, causing discontinuous proppant settling and negatively impacting fracturing effectiveness. It is generally believed that higher fracturing fluid gel viscosity emulsions have stronger proppant carrying capacity. However, research indicates that the lurry suspension capacity of fracturing fluids is not solely a function of apparent viscosity. This is because, during field operations, there have been instances where fracturing fluid systems with high apparent viscosity failed to carry proppant. The filtration efficiency of fracturing fluid also significantly impacts fracture formation. The lower the filtration efficiency of fracturing fluid during fracturing operations, the larger the fracture area produced at the same flow rate, resulting in better fracturing performance. Besides being affected by formation fluid compression, the filtration efficiency of fracturing fluid is also influenced by its wall-building properties and viscosity. Stronger wall-building properties result in a denser filter cake, and higher fluid viscosity leads to greater flow resistance through the filter cake and formation channels, all of which help reduce filtration efficiency. For displacement fluids, their primary function is to completely fill (displace) the proppant-carrying fluid into the fracture. In this case, low friction is the primary performance requirement, rather than proppant-carrying capacity. The friction of the fracturing fluid has a significant impact on the normal progress of the fracturing process, and its importance increases with well depth. Higher friction results in more energy loss in the wellbore, reducing the energy available for fracturing the formation and forming fractures. At a given equipment power, higher frictional resistance of the fracturing fluid results in lower effective displacement, which can affect fracture formation and propagation, potentially forcing the termination of fracturing operations. The frictional resistance of fracturing fluid is typically related to its composition, viscosity, and rheological properties. The compatibility of the fracturing fluid significantly impacts its effectiveness against the formation. The components and additives of the fracturing fluid should exhibit good compatibility, preventing chemical reactions due to changes in downhole temperature and pressure. The fracturing fluid should also have good compatibility with the reservoir rock, avoiding hydration swelling, exfoliation, and migration with clay minerals that could decrease formation permeability. Furthermore, the fracturing fluid should have good compatibility with formation fluids, preventing reactions with formation water and the formation of precipitates that could block flow channels. In addition, the stability of the fracturing fluid, including thermal and shear stability, greatly affects the fracturing operation. Fracturing fluids with good thermal and shear stability will not experience a significant decrease in viscosity due to increases in temperature and shear rate. In summary, a qualified fracturing fluid should have the following characteristics: moderate viscosity, good sand suspension performance, low filtration loss, low friction, low residue, and good compatibility.

[0005] Thickeners are the most important additives in fracturing fluids. Their main function is to increase the viscosity of the aqueous solution to form the base fluid for fracturing, while also reducing filtration loss and friction. Thickeners are generally required to achieve the viscosity requirements for fracturing operations with the lowest possible dosage. This facilitates the breaking down of the fracturing fluid into a hydrated state, reduces costs, and minimizes damage to the formation. The thickening properties of a thickener are related to its molecular weight, the functional groups on its molecular chain, and the morphology of the molecular chain. Generally, the larger the molecular weight of the thickener, the stronger its thickening ability, and the less thickener is needed to achieve a given viscosity.

[0006] Currently, reverse emulsion fracturing thickeners are typically prepared by mixing acrylamide and sodium acrylate to form monomer solutions of suitable concentration, ratio, and temperature. Using stirring and a pump, under the action of an emulsifier, the monomer solution is homogenized with the polymeric oil to form a reverse emulsion monomer. The reverse emulsion monomer is then polymerized under nitrogen atmosphere (oxygen-free) using an initiator to synthesize a reverse emulsion polymer. Adding a phase-inversion agent to the reverse emulsion polymer yields the reverse emulsion fracturing thickener. This method uses sodium acrylate as the monomer and a high-flash-point solvent oil as the oil phase. Due to the low solubility of sodium salts, the polymer easily condenses at low temperatures. The high pour point of the solvent oil results in a high pour point for the synthesized reverse emulsion fracturing thickener, causing it to solidify in winter in high-latitude regions, rendering it unusable and significantly limiting the application season and application area. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing a low-pour-point reverse emulsion hydraulic fracturing thickener, which greatly reduces the pour point of the reverse emulsion hydraulic fracturing thickener and meets the process requirements for low-pour-point reverse emulsion hydraulic fracturing thickeners in winter hydraulic fracturing operations in high-latitude regions of my country.

[0008] The technical solution adopted by this invention to solve its technical problem is: a method for preparing a low pour point reverse emulsion hydraulic cracking thickener, the steps of which are as follows:

[0009] (1) Preparation of aqueous solutions of acrylamide and ammonium acrylate: Under ice-water bath, ammonia water is slowly added to the aqueous solutions of acrylamide AM and acrylic acid AA, the temperature is controlled between 10 and 15℃, and the pH is adjusted to 6.00 to 8.00 to obtain aqueous solutions of acrylamide and ammonium acrylate monomers;

[0010] (2) Preparation of polyoxyethylene sorbitol beeswax derivative and hydroxylated lanolin alcohol solution: Polyoxyethylene sorbitol beeswax derivative and hydroxylated lanolin are added to alcohol respectively, the temperature is controlled between 18 and 25°C, and the mixture is stirred until completely dissolved.

[0011] (3) The aqueous solution and alcohol solution are homogenized to form an alcohol-in-water emulsion: The aqueous solution of step (1) is added to the alcohol solution of step (2) and placed in a high shear force mixer to emulsify and obtain a uniform and stable alcohol-in-water reverse emulsion with a viscosity between 600 cP and 1500 cP.

[0012] (4) Initiation of polymerization of alcohol-in-water reverse emulsion under oxygen-free conditions: Pour 1400g of the homogeneous and stable alcohol-in-water reverse emulsion from step (3) into a 2000mL four-necked flask, lower the temperature to 5-8℃, purge with nitrogen for 30-40min, and then add 2mL of potassium chlorate aqueous solution with a concentration of 1.0-5.0wt% and 2mL of sodium thiosulfate aqueous solution with a concentration of 0.2-0.5wt% to initiate polymerization until the temperature stops rising, and the polymerization is complete;

[0013] (5) Cooling and adding cosolvent: After the polymerization is completed, the temperature of the alcohol-in-water reverse emulsion is reduced to 15-30℃. Then, 0.5-1% of the mass of the alcohol-in-water reverse emulsion, 0.8-1.2% of the mass of the alcohol-in-water reverse emulsion, and 23E9 cosolvent are slowly added to the polymerized reverse emulsion and stirred evenly to obtain a low pour point reverse emulsion cracking thickener.

[0014] Furthermore, in step (1), the mass ratio of acrylamide, acrylic acid, and deionized water is 3-4:3:2-3.

[0015] Furthermore, in step (2), the mass ratio of polyoxyethylene sorbitol beeswax derivative, hydroxylated lanolin and alcohol is 0.5-1:1:10-17.

[0016] Furthermore, the alcohol in step (2) is either mono-n-heptanol or mono-isoheptanol.

[0017] Furthermore, in step (3), the mass ratio of the aqueous solution to the alcohol solution is 2 to 5:1.

[0018] The present invention has the following beneficial effects:

[0019] Due to variations in manufacturing processes, reverse emulsion fracturing thickeners generally have high pour points, typically above -10°C. This causes them to solidify easily in winter in high-latitude regions, rendering them unusable and significantly limiting the seasons and application areas for fracturing operations. The method of this invention uses polyoxyethylene sorbitol beeswax derivatives and hydroxylated lanolin alcohol solution as the alcohol phase of the reverse emulsion, and ammonium acrylate as the anionic polymeric monomer of the reverse emulsion thickener, thus significantly reducing the pour point of the reverse emulsion fracturing thickener. The quality indicators of the reverse emulsion fracturing thickener product manufactured by this invention are: pour point: ≤-35°C, anionicity: ≥40%, molecular weight: ≥25 million, solid content: ≥35%. This fully meets the process requirements for low pour point reverse emulsion fracturing thickeners in winter fracturing operations in high-latitude regions of my country. Attached Figure Description

[0020] Figure 1 These are morphological images of the inverse emulsion hydraulic cracking thickener products prepared in Examples 1-3 of this invention at -35°C. Detailed Implementation

[0021] The following are specific embodiments of the present invention, which further describe the technical solution of the present invention. However, the scope of protection of the present invention is not limited to these embodiments. All changes or equivalent substitutions that do not depart from the concept of the present invention are included within the scope of protection of the present invention.

[0022] Example 1

[0023] 1) Add 300g of high-purity acrylic acid, 200g of deionized water, and 300g of acrylamide to a 2000mL beaker. After complete dissolution, place the beaker in an ice-water bath and slowly add about 283g of 25% ammonia water to adjust the pH to 6.00. Keep the temperature at 10℃ to obtain an aqueous solution of acrylamide and ammonium acrylate.

[0024] 2) Add 300g of mono-n-heptanol, 15g of polyoxyethylene sorbitol beeswax derivative and 30g of hydroxylated lanolin to a 2000ml beaker and stir until fully dissolved to obtain a polyoxyethylene sorbitol beeswax derivative and hydroxylated lanolin alcohol solution.

[0025] 3) Add the aqueous solution from step (1) to the alcohol solution from step (2), place it in a high-shear mixer, stir for about 6 minutes, and emulsify to obtain a uniform and stable water-in-alcohol emulsion with a viscosity of 890 cP.

[0026] 4) Pour about 1400g of the homogeneous and stable alcohol-in-water emulsion from step (3) into a 2000mL four-necked flask, lower the temperature to 5℃, purge with nitrogen for 30min, and then add 2mL of 2wt% potassium chlorate aqueous solution and 2mL of 0.3wt% sodium thiosulfate aqueous solution to initiate polymerization until the temperature stops rising.

[0027] 5) After polymerization, lower the temperature to 20℃, and slowly add 8g of oleyl alcohol polyether-20 and 12g of 23E9 to the reverse emulsion, stirring until homogeneous. This yields a reverse emulsion hydraulic cracking thickener with a solid content of 48.5%, a molecular weight of 26.5 million, anionicity of 50.0%, and a pour point of -36℃.

[0028] Example 2

[0029] 1) Add 300g of high-purity acrylic acid, 300g of deionized water, and 400g of acrylamide to a 2000mL beaker. After complete dissolution, place the beaker in an ice-water bath and slowly add about 285g of 25% ammonia water to adjust the pH to 8.00. Keep the temperature at 15℃ to obtain an aqueous solution of acrylamide and ammonium acrylate.

[0030] 2) Add 300g of monoisoheptyl alcohol, 10g of polyoxyethylene sorbitol beeswax derivative and 20g of hydroxylated lanolin to a 2000mL beaker and stir until fully dissolved to obtain an alcoholic solution of polyoxyethylene sorbitol beeswax derivative and hydroxylated lanolin.

[0031] 3) Add the aqueous solution from step (1) to the alcohol solution from step (2), place it in a high-shear mixer, stir for about 8 minutes, and emulsify to obtain a uniform and stable water-in-alcohol reverse emulsion with a viscosity of 1200 cP.

[0032] 4) Pour about 1400g of the homogeneous and stable alcohol-in-water emulsion from step (3) into a 2000mL four-necked flask, lower the temperature to 6℃, purge with nitrogen for 35min, and then add 2mL of 3wt% potassium chlorate and 2mL of 0.4wt% sodium thiosulfate aqueous solution to initiate polymerization until the temperature stops rising.

[0033] 5) After polymerization, lower the temperature to 25℃, and slowly add 10g of oleyl alcohol polyether-20 and 12g of 23E9 to the emulsion, stirring until homogeneous. This yields a reverse emulsion hydraulic cracking thickener with a solid content of 48.6%, a molecular weight of 28.07 million, anionicity of 42.8%, and a pour point of -36℃.

[0034] Example 3

[0035] 1) Add 300g of high-purity acrylic acid, 400g of deionized water, and 350g of acrylamide to a 2000mL beaker. After complete dissolution, place the beaker in an ice-water bath and slowly add about 284g of 25% ammonia water to adjust the pH to 7.00. Keep the temperature at 12℃ to obtain an aqueous solution of acrylamide and ammonium acrylate.

[0036] 2) Add 300g of mono-n-heptyl alcohol, 15g of polyoxyethylene sorbitol beeswax derivative and 18g of hydroxylated lanolin to a 2000ml beaker and stir until fully dissolved to obtain an alcoholic solution of polyoxyethylene sorbitol beeswax derivative and hydroxylated lanolin.

[0037] 3) Add the aqueous solution from step (1) to the alcohol solution from step (2), place it in a high-shear mixer, stir for about 9 minutes, and emulsify to obtain a uniform and stable alcohol-in-water reverse emulsion with a viscosity of 1275 cP.

[0038] 4) Pour about 1400g of the homogeneous and stable alcohol-in-water reverse emulsion from step (3) into a 2000mL four-necked flask, lower the temperature to 7℃, purge with nitrogen for 40min, and then add 2mL of 5wt% potassium chlorate and 2mL of 0.5wt% sodium thiosulfate aqueous solution to initiate polymerization until the temperature stops rising.

[0039] 5) After polymerization, lower the temperature to 20℃, and slowly add 9g of oleyl alcohol polyether-20 and 13g of 23E9 to the emulsion, stirring until homogeneous. This yields a reverse emulsion hydraulic cracking thickener with a solid content of 44.1%, a molecular weight of 29.5 million, anionicity of 46.2%, and a pour point of -36℃.

[0040] Comparative Example 1

[0041] The steps are basically the same as those in the example, except that mono-n-heptanol is replaced with ordinary white oil to prepare the thickener product.

[0042] The morphology of the inverse emulsion hydraulic cracking thickeners prepared in Examples 1-3 and the thickener prepared in Comparative Example 1 was tested at -35°C. Figure 1 As shown in the figure, the three test tubes on the left contain the finished products of the reverse emulsion hydraulic cracking thickener prepared in Examples 1-3, respectively. It can be seen that the finished thickener products are still in a fluid state at -35℃. The inverted test tube on the far right contains the finished product of the thickener prepared in Comparative Example 1, which has solidified at -35℃.

[0043] This invention is not limited to the above-described embodiments. Anyone should know that any structural changes made under the guidance of this invention, and any technical solutions that are the same as or similar to this invention, fall within the protection scope of this invention.

[0044] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A method for preparing a low pour point reverse emulsion hydraulic cracking thickener, characterized in that, The steps are as follows: (1) Preparation of aqueous solutions of acrylamide and ammonium acrylate: Under ice-water bath, ammonia water is slowly added to the aqueous solutions of acrylamide AM and acrylic acid AA, the temperature is controlled between 10 and 15℃, and the pH is adjusted to 6.00 to 8.00 to obtain aqueous solutions of acrylamide and ammonium acrylate monomers; (2) Preparation of polyoxyethylene sorbitol beeswax derivative and hydroxylated lanolin alcohol solution: Polyoxyethylene sorbitol beeswax derivative and hydroxylated lanolin are added to alcohol, wherein the alcohol is monohydric n-heptol or monohydric isoheptol, the temperature is controlled between 18 and 25°C, and the mixture is stirred until completely dissolved. (3) The aqueous solution and alcohol solution are homogenized to form an alcohol-in-water emulsion: The aqueous solution of step (1) is added to the alcohol solution of step (2) and placed in a high shear force mixer to emulsify and obtain a uniform and stable alcohol-in-water reverse emulsion with a viscosity between 600 cP and 1500 cP. (4) Initiation of polymerization of alcohol-in-water reverse emulsion under oxygen-free conditions: Pour 1400g of the homogeneous and stable alcohol-in-water reverse emulsion from step (3) into a 2000mL four-necked flask, lower the temperature to 5-8℃, purge with nitrogen for 30-40min, and then add 2mL of 1.0-5.0wt% potassium chlorate aqueous solution and 2mL of 0.2-0.5wt% sodium thiosulfate aqueous solution to initiate polymerization until the temperature stops rising, and the polymerization is complete; (5) Cooling and addition of cosolvent: After the polymerization is completed, the temperature of the alcohol-in-water reverse emulsion is reduced to 15-30℃. Then, 0.5-1% of the mass of the alcohol-in-water reverse emulsion, 0.8-1.2% of the mass of the alcohol-in-water reverse emulsion, and 23E9 cosolvent are slowly added to the polymerized reverse emulsion. The mixture is stirred evenly to obtain a low pour point reverse emulsion cracking thickener with a pour point ≤-35℃.

2. The preparation method of the low pour point reverse emulsion hydraulic cracking thickener as described in claim 1, characterized in that, In step (2), the mass ratio of polyoxyethylene sorbitol beeswax derivative, hydroxylated lanolin and alcohol is 0.5-1:1:10-17.

3. The method for preparing the low pour point reverse emulsion hydraulic cracking thickener as described in claim 1, characterized in that, In step (3), the mass ratio of the aqueous solution to the alcohol solution is 2 to 5:1.

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