A method for preparing a double-crosslinked polyacrylamide gel fracturing fluid

Abstract

This invention belongs to the field of tertiary oil recovery additives and relates to a method for preparing a double-crosslinked polyacrylamide gel fracturing fluid. This method employs a mixed double-crosslinking system of inorganic boron and phenolic resin to form a three-dimensional micro-crosslinked network structure within the polyacrylamide fracturing system, improving the stability of the fracturing system in high-temperature and high-salt environments. Compared to commonly used organic small-molecule crosslinking agents such as N,N-methylenebisacrylamide, the double-crosslinking system described in this invention exhibits stronger high-temperature resistance and hydrolysis resistance, resulting in better stability of the crosslinked polyacrylamide fracturing system in deep, high-temperature formations, and enabling more stable pressure transmission.

Description

Technical Field

[0001] This invention belongs to the field of tertiary oil recovery additives, specifically relating to a method for preparing a double-crosslinked polyacrylamide gel fracturing fluid. Background Technology

[0002] Fracturing technology, as one of the representative technologies of tertiary oil recovery, can further improve the recovery rate by 15-20 percentage points based on waterflooding in secondary oil recovery. In recent years, polymer fracturing fluid systems, represented by polyacrylamide, have attracted attention due to their strong viscosity-enhancing properties, low residue, low friction, easy gel breaking, and minimal damage to the reservoir. my country's major oilfields have undergone decades of long-term exploitation. With the continuous development of low-salt, low-temperature, high-quality oil reservoirs and the depletion of reserves, high-temperature, high-salt oil reservoirs will be the main replacement resource for increasing reserves and production for a considerable period. As formation depth increases, formation temperature also rises, currently reaching over 180℃ in ultra-deep oil reservoirs. At this temperature, the amide groups in ordinary polyacrylamide fracturing systems undergo hydrolysis, leading to rapid viscosity reduction or even flocculation and precipitation, gradually losing their pressure-transferring function and making it difficult to operate stably in high-temperature environments for extended periods. To adapt to the current oil production situation, there is an urgent need to develop high-temperature resistant polyacrylamide fracturing fluids to meet the needs of oilfield exploitation. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing a double-crosslinked polyacrylamide gel fracturing fluid, which solves the problem that the viscosity of the polyacrylamide fracturing system decreases or even flocculates and precipitates due to hydrolysis in high-temperature reservoirs, making it difficult to work for a long time in high-temperature environments.

[0004] To achieve the objective of this invention, a mixed double-crosslinking system of inorganic boron and phenolic resin is used to form a three-dimensional micro-crosslinked network structure with polyacrylamide in an aqueous solution, thereby improving the heat resistance of the polyacrylamide fracturing gel system. This is specifically achieved through the following steps:

[0005] Water was added to a container, inorganic boron was added and stirred until homogeneous, and the pH of the solution was adjusted to 8-13 using a pH adjuster for hydrolysis. After hydrolysis, phenolic resin was added to obtain an aqueous solution containing two crosslinking agents. Polyacrylamide was added to the above aqueous solution containing the two crosslinking agents, stirred thoroughly, and allowed to stand until the polyacrylamide was completely dissolved to obtain a pre-crosslinked gel system. The gel system was heated to further crosslink, forming a double-crosslinked polyacrylamide gel fracturing system.

[0006] The inorganic boron is one or a mixture of boric acid or sodium tetraborate decahydrate.

[0007] The phenolic resin is a liquid phenolic resin prepared by reacting phenol and formaldehyde aqueous solution under alkaline catalysis, and the molar ratio of phenol to formaldehyde is <1:1.5.

[0008] The pH adjuster is one or a mixture of more than one of NaOH, Na2CO3 or NaHCO3.

[0009] The polyacrylamide is one or a mixture of more than one of nonionic polyacrylamide, cationic polyacrylamide, anionic polyacrylamide, or amphoteric polyacrylamide.

[0010] The mass ratio of water:inorganic boron:phenolic resin:polyacrylamide is 10000:1-10:1-10:100-1000.

[0011] This invention employs a mixed dual-crosslinking system of inorganic boron and phenolic resin to form a three-dimensional micro-crosslinked network structure within a polyacrylamide fracturing system, thereby improving the system's stability in high-temperature environments. In this polyacrylamide gel fracturing system, inorganic boron is linked to the carboxyl groups on the polyacrylamide side chains via coordination bonds, while the phenolic resin is linked to the amide groups on the polyacrylamide side chains via covalent bonds, resulting in a dual-crosslinked polyacrylamide gel fracturing system that can operate stably at high temperatures. Compared to commonly used organic small-molecule crosslinking agents such as N,N-methylenebisacrylamide, the dual-crosslinking system selected in this invention exhibits stronger high-temperature resistance and hydrolysis resistance, better stability in high-temperature environments at greater depths, and more stable pressure transmission.

[0012] The advantages of this invention are:

[0013] (1) Single organic resin crosslinking agents are difficult to gel at room temperature or take a long time to gel, which affects the implementation effect of fracturing operations. Inorganic boron crosslinking agents can coordinate with polyacrylamide to form a gel system with a certain initial strength at room temperature, which improves the efficiency of fracturing operations. After the system is heated, it further crosslinks with covalent bonds to form a high-strength polyacrylamide gel fracturing system with better heat resistance, which can transmit pressure stably for a long time.

[0014] (2) The two crosslinking agents introduced are connected to polyacrylamide by coordination bonds and covalent bonds respectively. Compared with the fracturing system that produces crosslinking by a single chemical bond, the dual crosslinking fracturing system can further improve the high temperature resistance of the polyacrylamide gel fracturing system and ensure that it does not lose water and viscosity rapidly in the high temperature environment of deep formations. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the reaction principle structure of the double crosslinked polyacrylamide gel fracturing system of the present invention. Detailed Implementation

[0016] The following are implementation examples of the present invention, used to provide a more comprehensive description of the present invention, and should not be construed as limiting the present invention in any way.

[0017] Example 1:

[0018] Add 500g of water to a 1000mL beaker, then add 0.5g of boric acid and stir until homogeneous. Adjust the pH of the boric acid aqueous solution to 9 using Na2CO3 and perform hydrolysis. After hydrolysis, add 0.5g of phenolic resin to dissolve it, obtaining an aqueous solution containing two crosslinking agents. Add 20g of anionic polyacrylamide to the above aqueous solution containing the two crosslinking agents, stir for 10min, and let stand for 12h until the polyacrylamide is completely dissolved, obtaining a preliminary crosslinked gel system. Transfer the gel system to a 50℃ oven for further crosslinking to obtain a polyacrylamide gel fracturing system.

[0019] Example 2:

[0020] 8 kg of water was added to a 10 L reactor equipped with a stirrer and a heating device, followed by 6 g of sodium tetraborate decahydrate, and the mixture was stirred until homogeneous. The pH of the sodium tetraborate decahydrate aqueous solution was adjusted to 12 using NaOH, and hydrolysis was performed. After hydrolysis, 8 g of phenolic resin was added to dissolve the solution, resulting in an aqueous solution containing two crosslinking agents. 800 g of nonionic polyacrylamide was added to the aqueous solution containing the two crosslinking agents, and the mixture was stirred for 30 min and then allowed to stand for 24 h until the polyacrylamide was completely dissolved, resulting in a pre-crosslinked gel system. The gel system was then heated to 60 °C in the reactor for further crosslinking, resulting in a polyacrylamide gel fracturing system.

[0021] Example 3:

[0022] 600 kg of water was added to a 1000 L dissolving tank, followed by 0.5 kg of boric acid, and the mixture was stirred until homogeneous. The pH of the boric acid solution was adjusted to 8 using NaHCO3, and hydrolysis was performed. After hydrolysis, 0.4 kg of phenolic resin was added to dissolve the solution, resulting in an aqueous solution containing two crosslinking agents. 40 kg of zwitterionic polyacrylamide was added to the aqueous solution containing the two crosslinking agents, and the mixture was stirred thoroughly for 2 hours and then allowed to stand for 36 hours until the polyacrylamide was completely dissolved, resulting in a pre-crosslinked gel system. After the gel system was injected into the formation, it was further crosslinked at a formation temperature of 80 °C to obtain a double-crosslinked polyacrylamide gel fracturing system that can operate stably in high-temperature formations.

[0023] Performance evaluation experiment:

[0024] (1) Apparent viscosity test:

[0025] The apparent viscosity of the polyacrylamide gel fracturing system prepared in Example 1 was tested at 50°C at different time points, and the results are as follows:

[0026] Table 1. Viscosities of the polyacrylamide gel fracturing system in Example 1 at different times at 50°C.

[0027] time 0h 2h 12h 24h 7d 30d 60d 90d Viscosity (mPa·s) 128 452 689 703 748 755 739 744

[0028] (2) Temperature stability test

[0029] The polyacrylamide gel fracturing system prepared in Example 1 was fully crosslinked at 50°C for 24 hours, and its apparent viscosity μ1 was tested at 30°C under normal pressure. Then, the sample was placed in a 1000mL ground glass bottle, sealed, and placed in an oven at 120°C for 24 hours. After being removed and allowed to cool naturally to 30°C, its apparent viscosity μ1 was tested. Stability is expressed as W = μ2 / μ1 × 100%.

[0030] Table 2. Results of Temperature Resistance Stability Test in Example 1

[0031] sample <![CDATA[μ1 / mPa·s]]> <![CDATA[μ2 / mPa·s]]> W / ％ Example 1 981 802 81.75

[0032] It can be seen that the double-crosslinked polyacrylamide gel fracturing fluid prepared by this invention has good stability at high temperatures.

Claims

1. A method for preparing a dual cross-linked polyacrylamide gel fracturing fluid, characterized in that: The method comprises the following steps: 1) adding water into a container, then adding inorganic boron and stirring until uniform, adjusting the pH value of the inorganic boron aqueous solution to 8-13 with a pH regulator, hydrolyzing, adding phenolic aldehyde resin after the hydrolysis is completed to dissolve, and obtaining an aqueous solution containing two cross-linking agents; 2) adding polyacrylamide into the aqueous solution containing two cross-linking agents, stirring until the polyacrylamide is completely dissolved, and obtaining a weak gel system after standing; 3) heating the weak gel system to further cross-link, and obtaining a double-cross-linked polyacrylamide gel fracturing fluid; the inorganic boron is a mixture of boric acid or sodium tetraborate decahydrate or both; the phenolic aldehyde resin is a liquid phenolic aldehyde resin prepared from an aqueous solution of phenol and formaldehyde under alkaline catalysis, and the molar ratio of phenol to formaldehyde is less than 1:1.5; the polyacrylamide is one or a mixture of more than one of non-ionic polyacrylamide, cationic polyacrylamide, anionic polyacrylamide or zwitterionic polyacrylamide; the mass ratio of water, inorganic boron, phenolic aldehyde resin and polyacrylamide is 10000:1-10:1-10:100-1000.

Images