Foam stabilizer, foaming base fluid, foam fracturing fluid and preparation methods of foam stabilizer, foaming base fluid and foam fracturing fluid

Abstract

The invention discloses a foam stabilizer, a foaming base fluid, a foam fracturing fluid and a preparation method thereof, and relates to the technical field of foam fracturing fluids. The foam stabilizer comprises a viscoelastic surfactant and cationic quaternary ammonium salt modified cellulose nanofibrils. When the foam stabilizer provided by the invention is applied to the foam fracturing fluid, the viscoelastic surfactant forms a three-dimensional dynamic micelle network through self-assembly, so that the viscoelasticity and interfacial adsorption strength of a liquid film are remarkably improved, and bubble coalescence is effectively inhibited; the cellulose nanofibrils construct a three-dimensional nano skeleton network in the liquid film, a multiple anchoring effect is formed through surface functional groups and a three-dimensional dynamic micelle network, the mechanical strength and high-temperature tolerance of the liquid film are enhanced, and the integrity of the liquid film can still be maintained at 110 DEG C; the synergistic effect of the cellulose nanofibril skeleton and the three-dimensional dynamic micelle network can resist the compression double electric layer damage of high-salinity ions to an interfacial film, realizes long-acting stability in high-temperature and high-salt environments, and can be directly applied to offshore oilfields or deep high-temperature reservoirs.

Description

Technical Field

[0001] The present invention relates to the technical field of foam fracturing fluids, and in particular to a foam stabilizer, a foaming base fluid, a foam fracturing fluid and a preparation method thereof. Background Art

[0002] Shale oil and gas resources are rich and have great development potential. As a typical unconventional oil and gas reservoir, the pore structure of shale reservoirs is complex, fractures are underdeveloped, and the permeability is extremely low. Therefore, hydraulic fracturing technology is still a necessary means for its efficient development. However, at present, water-based fracturing fluids are still mainly used in hydraulic fracturing operations, resulting in extremely high water consumption and inevitably causing great pressure on the supply of fresh water resources. At the same time, the potential reservoir damage and water quality pollution caused by chemical reagents in water-based fracturing fluids have become increasingly prominent. Therefore, on the premise of ensuring the fracturing effect, the development of a less water-consuming fracturing fluid has become the primary requirement for the efficient and sustainable development of shale oil.

[0003] A foam fracturing fluid is a new type of fracturing system that adds a foaming agent, a foam stabilizer, etc. to a conventional water-based fracturing fluid, with gas as the internal phase and the foaming liquid as the external phase. For example, carbon dioxide (CO2) foam fracturing fluid. As a two-phase fluid, CO2 foam has a relatively high apparent viscosity, high sand-carrying capacity, and low filtration loss; the presence of a high component of CO2 effectively enhances the backflow efficiency and also greatly reduces the contact opportunity between water and the formation during the fracturing operation, thereby reducing reservoir damages such as water sensitivity and water blockage; the potential phase change of CO2 releases a large amount of energy, which helps to promote the penetration of fractures into the reservoir and improve the fracturing effect. In addition, for shale oil reservoirs, CO2 dissolves in the reservoir crude oil, which can effectively reduce the viscosity of the crude oil and improve its fluidity. Therefore, CO2 foam fracturing fluid can effectively reduce water resource consumption, and its many advantages are expected to solve various problems in current hydraulic fracturing operations and are also an effective way to carry out large-scale CO2 sequestration.

[0004] However, current research on foam fracturing fluids mainly focuses on heat resistance, and the application range of foam fracturing fluids is limited. Therefore, it is of great significance to provide a foam fracturing fluid with both high temperature resistance and high salt resistance to expand its application range. Summary of the Invention

[0005] Based on the above deficiencies of the prior art, the purpose of the present invention is to provide a foam stabilizer, a foaming base fluid, a foam fracturing fluid and a preparation method thereof, aiming to use this foam stabilizer to prepare a foam fracturing fluid with both high temperature resistance and high salt resistance so that it can be applicable to offshore oilfields or deep high-temperature reservoirs.

[0006] The technical solution of the present invention is as follows:

[0007] In the first aspect of the present invention, a foam stabilizer is provided, wherein the foam stabilizer includes a viscoelastic surfactant and cellulose nanofibrils modified with a cationic quaternary ammonium salt.

[0008] Optionally, the mass ratio of the viscoelastic surfactant to the cationic quaternary ammonium salt-modified cellulose nanofibrils is (0.1-1):(0.1-0.3).

[0009] Optionally, the viscoelastic surfactant includes at least one of a viscoelastic cationic quaternary ammonium salt surfactant and a viscoelastic zwitterionic surfactant.

[0010] Optionally, the viscoelastic cationic quaternary ammonium salt surfactant includes at least one of cetyltrimethylammonium bromide, erucylamide propyl trimethyl ammonium chloride, and distearyldimethylammonium chloride; the viscoelastic zwitterionic surfactant includes at least one of carboxylic acid betaine and sulfonic acid betaine.

[0011] Optionally, the sulfonic acid betaine includes erucylamide propyl hydroxy sulfobetaine.

[0012] In a second aspect of the present invention, there is provided a foaming base fluid, which includes composite brine, a foaming agent, and the foam stabilizer as described above in the present invention; or, the foaming base fluid includes water, a foaming agent, and the foam stabilizer as described above in the present invention.

[0013] Optionally, in the foaming base fluid, the mass content of the foaming agent is 0.2%-1.2%, and the mass content of the foam stabilizer is 0.2%-1.3%.

[0014] Optionally, the foaming agent includes at least one of alkyl polyglycoside, laurylamide propyl hydroxy sulfobetaine, and coconut oil amide propyl hydroxy sulfobetaine;

[0015] The composite brine includes the following components by mass percentage:

[0016] NaCl 11.2%, MgCl 1.2%, CaCl 2.4%, and water 85.2%.

[0017] In a third aspect of the present invention, there is provided a foam fracturing fluid, where the foam fracturing fluid includes a gas phase and a liquid phase, and the liquid phase includes the foaming base fluid as described above in the present invention.

[0018] In a fourth aspect of the present invention, there is provided a preparation method of a foam fracturing fluid, which includes the following steps:

[0019] The foaming base fluid as described above in the present invention is foamed by mechanical foaming or gas diffusion to obtain the foam fracturing fluid.

[0020] Beneficial effects: When the foam stabilizer provided by the present invention is applied to foam fracturing fluid, the viscoelastic surfactant forms a three-dimensional dynamic micelle network through self-assembly, significantly enhancing the viscoelasticity of the liquid film and the interfacial adsorption strength, and effectively inhibiting the coalescence of bubbles; the cationic quaternary ammonium salt-modified cellulose nanofibrils construct a three-dimensional nano-skeleton network in the liquid film, and form multiple anchoring effects with the three-dimensional dynamic micelle network through surface functional groups, enhancing the mechanical strength and high-temperature tolerance of the liquid film, and still maintaining the integrity of the liquid film at 110 °C; the synergistic effect of the cationic quaternary ammonium salt-modified cellulose nanofibril skeleton and the three-dimensional dynamic micelle network can resist the compression double-layer destruction of the interfacial film by high salinity ions (total salinity 14.8%), realizing long-term stability in high-temperature (≤110 °C) and high-salt (salinity ≤14.8%) environments. The viscoelastic micelles of the foam stabilizer in the present invention can self-break gel, causing little damage to the formation, while cellulose nanofibrils are non-toxic and pollution-free, and have the advantages of environmental friendliness. Therefore, the foam system based on the foam stabilizer provided by the present invention has high efficiency in high-temperature (110 °C) and high-salt conditions, and can be directly applied to offshore oilfields (formation water with high salinity) or deep high-temperature reservoirs (such as deep gas reservoirs more than 8000 m in the Tarim Basin). Description of the Drawings

[0021] Figure 1 It is a diagram of the bubble stability mechanism of the foam fracturing fluid in the present invention.

[0022] Figure 2 It is a diagram of the foaming performance results of different foam base fluids in Example 1. Among them, (a) is the foaming base fluid containing only APG0810, (b) is the foaming base fluid containing only LSB, and (c) is the foaming base fluid containing APG0810 and LSB.

[0023] Figure 3 It is a diagram of the foaming performance results of different foam base fluids in Example 2. Among them, (a) is the foaming base fluid containing APG0810, LSB and different contents of EBS, and (b) is the foaming base fluid containing APG0810, LSB, EBS and different mass contents of ZCNF.

[0024] Figure 4 It is the high-temperature and high-pressure stability test results of different foam fracturing fluids in Example 3. Among them, (a) is the foam fracturing fluid a based on 0.3 wt% compound foaming agent system, (b) is the foam fracturing fluid b based on 0.3 wt% compound foaming agent + 0.6 wt% ESB system, and (c) is the foam fracturing fluid c based on 0.3 wt% compound foaming agent + 0.6 wt% ESB + 0.3% ZCNF system.

[0025] Figure 5 It is a diagram of the free diffusion coefficient results of water molecules in the composite structure composed of micelles and ZCNF in Example 3. Detailed Embodiments

[0026] The present invention provides a foam stabilizer, a foaming base fluid, a foam fracturing fluid and a preparation method thereof. To make the objectives, technical solutions and effects of the present invention clearer and more definite, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0028] In order to prepare a foam fracturing fluid with both high temperature resistance and high salt resistance, the inventors of the present invention have conducted a lot of research. Through research, it is found that for salt tolerance, general anionic surfactants are not applicable, and although sulfuric acid type or sulfonic acid type surfactants have good salt tolerance, they can only be used in oil reservoirs below 90°C. Based on this, an embodiment of the present invention provides a foam stabilizer, wherein the foam stabilizer includes a viscoelastic surfactant and cellulose nanofibrils modified with a cationic quaternary ammonium salt (i.e., introducing a cationic quaternary ammonium salt group on the surface of cellulose nanofibrils).

[0029] When the foam stabilizer in the embodiment of the present invention is applied to a foam fracturing fluid, the viscoelastic surfactant forms a three-dimensional dynamic micelle network through self-assembly, significantly improving the viscoelasticity of the liquid film and the interfacial adsorption strength, effectively inhibiting the coalescence of bubbles, and improving the compressive performance; the cellulose nanofibrils modified with a cationic quaternary ammonium salt are uniformly dispersed in the cracking fluid and construct a three-dimensional nano-skeleton network in the liquid film, forming a multiple anchoring effect with the three-dimensional dynamic micelle network through surface functional groups, enhancing the mechanical strength and high temperature tolerance of the liquid film, and still maintaining the integrity of the liquid film at 110°C; the synergistic effect of the cellulose nanofibril skeleton modified with a cationic quaternary ammonium salt and the three-dimensional dynamic micelle network can resist the compression double-layer destruction of the interfacial film by high salinity ions (total salinity 14.8%), realizing long-term stability in high temperature (≤110°C) and high salt (salinity ≤14.8%) environments. In addition, the viscoelastic micelles of the foam stabilizer in the present invention can break the gel automatically, causing little damage to the formation, and the cellulose nanofibrils are non-toxic and pollution-free, having the advantages of environmental protection. Therefore, the foam system based on the foam stabilizer provided by the present invention has high efficiency stability under high temperature (110°C) and high salt conditions, and can be directly applied to offshore oilfields (formation water with high salinity) or deep high temperature reservoirs (such as deep gas reservoirs more than 8000 m in the Tarim Basin).

[0030] In some embodiments, the mass ratio of the viscoelastic surfactant to the cationic quaternary ammonium salt-modified cellulose nanofibrils is (0.1 - 1):(0.1 - 0.3), and for example, it can be 0.1:0.1, 0.1:0.2, 0.1:0.3, 0.5:0.1, 0.5:0.2, 0.5:0.3, 1:0.1, 1:0.2 or 1:0.3, etc. This ratio can better achieve the synergistic effect between the two.

[0031] In some embodiments, the viscoelastic surfactant includes at least one of a viscoelastic cationic quaternary ammonium salt surfactant and a viscoelastic zwitterionic surfactant, but is not limited thereto.

[0032] In some embodiments, the viscoelastic cationic quaternary ammonium salt surfactant includes at least one of cetyltrimethylammonium bromide, erucylamidopropyltrimethylammonium chloride, and distearyldimethylammonium chloride, but is not limited thereto; the viscoelastic zwitterionic surfactant includes at least one of carboxylic acid betaine and sulfonic acid betaine, but is not limited thereto.

[0033] In some embodiments, the sulfonic acid betaine is specifically a long-chain sulfonic acid betaine, and the long-chain sulfonic acid betaine includes erucylamidopropyl hydroxysulfobetaine.

[0034] In some embodiments, the cationic quaternary ammonium salt-modified cellulose nanofibrils include at least one of 2,3-epoxypropyltrimethylammonium chloride-modified cellulose nanofibrils, acryloyloxyethyltrimethylammonium chloride-modified cellulose nanofibrils, glycidyltrimethylammonium chloride-modified cellulose nanofibrils, and dimethyldiallylammonium chloride-modified cellulose nanofibrils, but is not limited thereto.

[0035] The cellulose nanofibrils are prepared by the following method: using softwood pulp as a raw material, mixing it with 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO), sodium bromide, and an aqueous solution of sodium hypochlorite, adjusting the pH value to about 10.5. After the pH value no longer changes, adjust the pH value to 7, and centrifuge and dialyze the mixed solution. The dialyzed solution is mechanically stirred to obtain a TEMPO-pretreated nanofibril suspension, that is, the cellulose nanofibrils. The obtained cellulose nanofibrils, that is, the cellulose nanofibrils treated with TEMPO, have partial carboxylate groups on the surface.

[0036] The cationic quaternary ammonium salt-modified cellulose nanofibrils can be prepared by a chemical modification method. For example, using 2,3-epoxypropyltrimethylammonium chloride (EPTMAC) as a modifier, under alkaline conditions, the epoxy group in EPTMAC can undergo a ring-opening reaction with the hydroxyl group of the cellulose nanofibrils, thereby grafting the quaternary ammonium salt group onto the surface of the cellulose nanofibrils.

[0037] An embodiment of the present invention also provides a foaming base liquid (or the liquid to be foamed). Wherein, the foaming base liquid includes composite brine, a foaming agent, and the foam stabilizer as described above in the embodiments of the present invention; or, the foaming base liquid includes water, a foaming agent, and the foam stabilizer as described above in the embodiments of the present invention.

[0038] In some embodiments, in the foaming base liquid, the mass content of the foaming agent is 0.2% - 1.2% (preferably 0.25% - 0.4%), and the mass content of the foam stabilizer is 0.2% - 1.3% (preferably 0.2% - 0.6%). By way of example, in the foaming base liquid, the mass content of the foaming agent can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1% or 1.2%, etc., and the mass content of the foam stabilizer can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2% or 1.3%, etc.

[0039] In some embodiments, the foam stabilizer includes a viscoelastic surfactant and cellulose nanofibrils modified with cationic quaternary ammonium salts. In the foaming base liquid, the mass content of the viscoelastic surfactant is 0.1% - 0.8% (for example, it can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7% or 0.8%, etc.), and the mass content of the cellulose nanofibrils modified with cationic quaternary ammonium salts is 0.1% - 0.5% (for example, it can be 0.1%, 0.2%, 0.3%, 0.4% or 0.5%, etc.).

[0040] In some embodiments, in the foaming base liquid, the mass content of the foaming agent is 0.3%, and the mass content of the foam stabilizer is 0.9%. Among them, the mass content of the viscoelastic surfactant is 0.6%, and the mass content of the cellulose nanofibrils modified with cationic quaternary ammonium salts is 0.3%.

[0041] In some embodiments, the foaming agent includes at least one of alkyl polyglycoside, lauramidopropyl hydroxysulfobetaine, and coconut oil amide propyl hydroxysulfobetaine.

[0042] In some embodiments, the foaming agent includes alkyl polyglycoside and lauramidopropyl hydroxysulfobetaine. In some specific embodiments, the mass ratio of alkyl polyglycoside to lauramidopropyl hydroxysulfobetaine is 2:1.

[0043] In some embodiments, the foaming agent includes alkyl polyglycoside and cocamidopropyl hydroxysultaine. In some specific embodiments, the mass ratio of alkyl polyglycoside to cocamidopropyl hydroxysultaine is 2:1.

[0044] In some embodiments, the composite brine includes the following components in mass percentage:

[0045] NaCl 11.2%, MgCl 1.2%, CaCl 2.4% and water 85.2%.

[0046] The embodiment of the present invention also provides a foam fracturing fluid. Among them, the foam fracturing fluid includes a gas phase and a liquid phase. The liquid phase includes the foaming base fluid as described above in the present invention. The foaming base fluid includes composite brine, a foaming agent and a foam stabilizer. The foam stabilizer includes a viscoelastic surfactant and cellulose nanofibrils modified with cationic quaternary ammonium salts.

[0047] In this embodiment, the viscoelastic surfactant in the foam fracturing fluid forms a three-dimensional dynamic micelle network through self-assembly, significantly improving the viscoelasticity of the liquid film and the interfacial adsorption strength, effectively inhibiting the coalescence of bubbles, and improving the compressive performance; the cellulose nanofibrils modified with cationic quaternary ammonium salts construct a three-dimensional nano-skeleton network in the liquid film, and form multiple anchoring effects with the three-dimensional dynamic micelle network through surface functional groups, enhancing the mechanical strength and high-temperature tolerance of the liquid film, and still maintaining the integrity of the liquid film at 110°C; the synergistic effect of the cellulose nanofibril skeleton modified with cationic quaternary ammonium salts and the three-dimensional dynamic micelle network can resist the compression double-layer destruction of the interfacial film by high salinity ions (total salinity 14.8%), realizing long-term stability in high-temperature (≤110°C) and high-salt (salinity ≤14.8%) environments. The viscoelastic micelles of the foam stabilizer in the present invention can break the gel automatically, causing little damage to the formation, while the cellulose nanofibrils are non-toxic and pollution-free, having the advantages of environmental protection. Therefore, the foam fracturing fluid provided by the present invention has high efficiency stability under high-temperature (110°C), high-pressure (10 MPa), and high-salt (total salinity 14.8%) conditions, and can be directly applied to offshore oilfields (formation water with high salinity) or deep high-temperature reservoirs (such as deep gas reservoirs more than 8000 m in the Tarim Basin).

[0048] The binding mechanism of the worm-like micelle and cellulose nanofibril composite structure to water molecules in the foam liquid film is further explained by molecular simulation. The foam liquid film is simplified to a single liquid film, and the stable structure of the system is as Figure 1 shown. The surfactant adsorption layer is distributed on both sides of the liquid film. The viscoelastic surfactant self-assembles into a worm-like micelle structure (i.e., the worm-like surfactant micelle in Figure 1 ), and combines with the cellulose nanofibrils modified with cationic quaternary ammonium salts ( Figure 1The cellulose nanofibrils therein are specifically cationic quaternary ammonium salt - modified cellulose nanofibrils) located in the middle of the surfactant adsorption layer, and the three form a three - dimensional composite structure, enabling the foam fracturing fluid to have high temperature, high pressure, and high salt resistance properties.

[0049] In some specific embodiments, the gas phase includes carbon dioxide, air, or nitrogen, but is not limited thereto. When the gas phase is carbon dioxide, the foam fracturing fluid is a carbon dioxide foam fracturing fluid.

[0050] In some embodiments, the mass content of the foam in the foam fracturing fluid is 80% - 95%, for example, it can be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, or 95%, etc.

[0051] In some specific embodiments, the mass content of the foam in the foam fracturing fluid is 85% - 95%.

[0052] The present invention also provides a preparation method of a foam fracturing fluid, which includes the following steps:

[0053] Using the mechanical foaming method or the gas diffusion method to foam the foaming base fluid as described above in the embodiments of the present invention to obtain the foam fracturing fluid.

[0054] Among them, the specific steps of preparing the atmospheric - pressure foam fracturing fluid by the mechanical foaming method are as follows:

[0055] Feed the foaming base fluid into a sealed foaming device. The foaming device is filled with a foaming gas (such as carbon dioxide, air, or nitrogen, etc.) and is connected to a gas source. Foam under high - speed stirring (12000 rpm) to obtain the pressure foam fracturing fluid.

[0056] The specific steps of preparing the high - pressure foam fracturing fluid by the gas diffusion method are as follows:

[0057] Pass the foaming base fluid and the foaming gas (such as carbon dioxide, air, or nitrogen, etc.) through a pressure - resistant foamer (a sand - carrying pipe containing glass beads with a particle size of 50 - 80 μm, and there are porous ceramic discs at both ends for blocking). Control the foaming by controlling the gas and liquid flow rates and achieve the control of the foam quality. The gas flow rate is regulated by a gas flow controller, and the liquid flow rate is controlled by a constant - flow pump to obtain the foam fracturing fluid. Among them, the foam quality corresponding to the gas flow rate and the liquid flow rate is shown in Table 1.

[0058] Table 1. Corresponding relationship between foam quality, gas flow rate, and liquid flow rate

[0059]

[0060]

[0061] The preparation method of the foaming base liquid comprises the following steps:

[0062] Add a foaming agent to water or composite brine, stir evenly, then add a viscoelastic surfactant, stir evenly, and then add cellulose nanofibrils modified with cationic quaternary ammonium salt. After ultrasonic dispersion, the foaming base liquid is obtained. Specifically, the foaming base liquid can be placed in a constant temperature phase at 60 °C and used for foaming after a stabilization time longer than 12 hours.

[0063] The present invention will be further described below through specific examples.

[0064] In the following examples, wt% represents mass percentage.

[0065] Example 1 Determination of the foaming agent and its dosage

[0066] Take 25 mL of pure water, add alkyl polyglycoside 0810 (APG0810, purchased from Shanghai Yincong New Material Technology Co., Ltd.) to it, and obtain foaming base liquids with APG0810 contents of 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, and 0.6 wt% respectively. Send this foaming base liquid to a sealed foaming device (which is filled with carbon dioxide foaming gas and connected to a gas source), stir and foam at high speed (12000 rpm). After completion, quickly transfer the foam to a graduated cylinder and measure the foam volume. The foam volume and drainage half-life results of the foaming base liquids with different APG0810 contents after foaming are as Figure 2 shown in (a) of the figure. After the APG0810 content in this foam system (APG0810) is higher than 0.3 wt%, it is basically unchanged, while the foaming property increases limitedly.

[0067] Take another 25 mL of pure water, add lauramidopropyl hydroxysulfobetaine (LSB) to it, and obtain foaming base liquids with LSB contents of 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, and 0.6 wt% respectively. Using the same method as above, the foam volume and drainage half-life results of the foaming base liquids with different LSB contents after foaming are obtained. Specifically, as Figure 2 shown in (b) of the figure. The performance of this foaming system (LSB) is also relatively stable after the LSB content is higher than 0.2 wt%.

[0068] In the present invention, APG0810 is used as the main agent and LSB as the auxiliary agent to form a compound system. To control costs, the content of APG0810 is set at 0.2 wt%. Specifically, APG0810 is added to 25 mL of pure water to form a foaming base liquid with an APG0810 content of 0.2 wt%. Then, different contents of the auxiliary agent LSB are added to the foaming base liquid with an APG0810 content of 0.2 wt%. After foaming using the above method, the foam volume and drainage half-life are tested. The results are as Figure 2 shown in (c) of [reference]. When the content of the auxiliary agent LSB is higher than 0.1 wt%, the foaming performance of the compound system tends to be stable. Therefore, a composite foaming agent based on APG0810 and LSB is selected, with a total content of 0.3 wt%. The mass ratio of APG0810 to LSB is 2:1, that is, the content of APG0810 is 0.2 wt% and the content of LSB is 0.1 wt%.

[0069] Test the salt tolerance performance of the composite foaming agent of APG0810 and LSB:

[0070] Prepare a composite brine, and the composite brine includes the following components in mass percentage:

[0071] NaCl 11.2%, MgCl 1.2%, CaCl2 2.4% and water 85.2%.

[0072] Mix APG0810 and LSB with 25 mL of the above composite brine to obtain a foaming base liquid. The content of APG0810 in the foaming base liquid is 0.2 wt% and the content of LSB is 0.1 wt%.

[0073] Use the above method to foam the foaming base liquid, and the foam volume still remains at about 280 mL. The foaming property is stable in this composite brine, indicating that APG0810 and LSB are salt-tolerant composite foaming agents.

[0074] Example 2 Determination of the foam stabilizer and its dosage

[0075] Prepare a composite brine, and the composite brine includes the following components in mass percentage:

[0076] NaCl 11.2%, MgCl2 1.2%, CaCl2 2.4% and water 85.2%.

[0077] In 25 mL of the composite brine, add APG0810 and LSB to obtain a foaming base liquid with an APG0810 content of 0.2 wt% and an LSB content of 0.1 wt%.

[0078] Different masses of erucylamidopropyl hydroxy sulfobetaine (ESB) were added to the above-mentioned foaming base liquid to obtain foaming base liquids with ESB contents of 0.0 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt% and 1 wt% respectively.

[0079] After foaming using the method in Example 1, the foam properties were as shown in Figure 3 (a) below. It can be seen that as the mass content of ESB increases, the viscosity of the mixed system increases, and the stability of the foam significantly improves after the ESB content reaches 0.5 wt%. The drainage half-life is as high as more than 1.5 hours, but the foaming property decreases relatively. When the ESB content is above 0.6 wt%, the base liquid is too viscous. Therefore, the ESB content is controlled at about 0.6 wt%.

[0080] Based on 0.3 wt% compound foaming agent + 0.6 wt% ESB + composite brine, cationic quaternary ammonium salt-modified cellulose nanofibrils (purchased from Wood Elf (Tianjin) New Materials Co., Ltd., model: ZCNF, and its structural formula is: (The preparation method can also be referred to "Zwitterionic Cellulose Nanofibrils with High Salt Sensitivity and Tolerance, An Wang, Biomacromolecules 2020, 21, 1471 - 1479") were added to obtain a foaming base liquid, in which the content of APG0810 was 0.2 wt%, the content of LSB was 0.1 wt%, the content of ESB was 0.6 wt%, and the content of ZCNF was 0.0 wt%, 0.1 wt%, 0.2 wt% and 0.3 wt% respectively. After foaming using the method in Example 1, the foam properties were as shown in Figure 3 (b) below. The foaming property of this system remained stable, the foam stability was further improved, and the drainage half-life increased to more than 2.5 hours. Finally, the content of ZCNF was determined to be 0.3 wt%.

[0081] Stability test of the foam in Example 3 under high temperature and high pressure

[0082] APG0810 and LSB were added to the composite brine (the composition and dosage were the same as in Example 2) to obtain a foaming base liquid a with a content of APG0810 of 0.2 wt% and a content of LSB of 0.1 wt% (also known as the foaming base liquid a based on the 0.3 wt% compound foaming agent system).

[0083] In the composite brine (with the same composition and dosage as in Example 2), add APG0810, LSB, and ESB to obtain foaming base fluid b with an APG0810 content of 0.2 wt%, an LSB content of 0.1 wt%, and an ESB content of 0.6 wt% (also known as foaming base fluid b based on the 0.3 wt% compound foaming agent + 0.6 wt% ESB system).

[0084] In the composite brine (with the same composition and dosage as in Example 2), add APG0810, LSB, ESB, and ZCNF to obtain foaming base fluid c with an APG0810 content of 0.2 wt%, an LSB content of 0.1 wt%, an ESB content of 0.6 wt%, and a ZCNF content of 0.3 wt% (also known as foaming base fluid c based on the 0.3 wt% compound foaming agent + 0.6 wt% ESB + 0.3 wt% ZCNF system).

[0085] Respectively pass the above-mentioned foaming base fluid a, foaming base fluid b, and foaming base fluid c and carbon dioxide through a pressure-resistant foaming device (a sand-carrying pipe containing glass beads with a particle size of 50 - 80 μm and a porous ceramic disc barrier at each end). By controlling the gas flow rate at 100.00 mL / min and the liquid flow rate at 11.11 mL / min, foam fracturing fluid a with a foam quality of 90%, foam fracturing fluid b with a foam quality of 90%, and foam fracturing fluid c with a foam quality of 90% are obtained.

[0086] Pump the above-mentioned foam fracturing fluid a (based on the 0.3 wt% compound foaming agent system), foam fracturing fluid b (based on the 0.3 wt% compound foaming agent + 0.6 wt% ESB system), and foam fracturing fluid c (based on the 0.3 wt% compound foaming agent + 0.6 wt% ESB + 0.3% ZCNF system) into a pressure-resistant visualization container. Under the conditions of 110 °C and 10 MPa, monitor the dynamic coalescence process of the foam bubbles. The results are as Figure 4 shown. It can be seen that during the 24-hour monitoring process, it is found that for foam fracturing fluid a based on the 0.3 wt% compound foaming agent system, the bubble coalescence is the most obvious, and its high-temperature and pressure-resistant performance is poor; for foam fracturing fluid c based on the 0.3 wt% compound foaming agent + 0.6 wt% ESB + 0.3 wt% ZCNF system, the bubble coalescence is not obvious, and it has the best high-temperature and pressure-resistant performance.

[0087] The root-mean-square displacement and self-diffusion coefficient can be used to evaluate the motion state of molecules. In the present invention, for the composite brine (with the same composition and dosage as in Example 2), foam fracturing fluid b, and foam fracturing fluid c, calculate their diffusion coefficients based on the root-mean-square displacement of water molecules around the micelle-ZCNF composite structure (within the range of 1 - 3 nm). The results are as Figure 5 shown. The result of the composite brine corresponds to Figure 5 the brine film in Figure 5In the foaming agent + ESB system, the results of the foam fracturing fluid c correspond to Figure 5 In the foaming agent + ESB + ZCNF system, it can be seen that for the brine film without any additives, the molecular movement of water is significantly reduced by more than 50% after adding the foaming agent and stabilizer. At the same time, there are a large number of hydrophilic groups on the surface of ZCNF, and the binding of water is further enhanced after adding.

[0088] Example 4

[0089] This example provides a foaming base fluid, which is only different from the foaming base fluid c based on the 0.3wt% compound foaming agent + 0.6wt% ESB + 0.3wt% ZCNF system in Example 3 in that LSB therein is replaced by cocamidopropyl hydroxysulfobetaine (CSB).

[0090] Example 5

[0091] This example provides a foaming base fluid, which is only different from the foaming base fluid c based on the 0.3wt% compound foaming agent + 0.6wt% ESB + 0.3wt% ZCNF system in Example 3 in that ESB therein is replaced by cetyltrimethylammonium bromide, and ZCNF therein is replaced by cellulose nanofibrils modified with acryloyloxyethyltrimethylammonium chloride.

[0092] Example 6

[0093] This example provides a foaming base fluid, which is only different from the foaming base fluid c based on the 0.3wt% compound foaming agent + 0.6wt% ESB + 0.3wt% ZCNF system in Example 3 in that ESB therein is replaced by erucamidopropyltrimethylammonium chloride, and ZCNF therein is replaced by cellulose nanofibrils modified with glycidyltrimethylammonium chloride.

[0094] Example 7

[0095] This example provides a foaming base fluid, which is only different from the foaming base fluid c based on the 0.3wt% compound foaming agent + 0.6wt% ESB + 0.3wt% ZCNF system in Example 3 in that ZCNF therein is replaced by cellulose nanofibrils modified with dimethyldiallylammonium chloride.

[0096] The foaming base fluids in Examples 4 to 7 are foamed by the foaming method in Example 3, and the obtained foam fracturing fluids have similar properties to the foam fracturing fluid c in Example 3, and can remain stable at 110 °C and 10 MPa, and have good high-temperature and pressure-resistant properties.

[0097] In summary, the present invention provides a foam stabilizer, a foaming base fluid, a foam fracturing fluid and a preparation method thereof. When the foam stabilizer provided by the present invention is applied to the foam fracturing fluid, the viscoelastic surfactant forms a three-dimensional dynamic micelle network through self-assembly, significantly improving the viscoelasticity of the liquid film and the interfacial adsorption strength, and effectively inhibiting the coalescence of bubbles; the cellulose nanofibrils modified with cationic quaternary ammonium salts construct a three-dimensional nano-skeleton network in the liquid film, and form multiple anchoring effects with the three-dimensional dynamic micelle network through surface functional groups, enhancing the mechanical strength and high-temperature tolerance of the liquid film, and still maintaining the integrity of the liquid film at 110 °C; the synergistic effect of the cationic quaternary ammonium salt-modified cellulose nanofibril skeleton and the three-dimensional dynamic micelle network can resist the compression double-layer destruction of the interfacial film by high salinity ions (total salinity 14.8%), realizing long-term stability in high-temperature (≤110 °C) and high-salt (salinity ≤14.8%) environments. The viscoelastic micelles of the foam stabilizer in the present invention can break the gel automatically, causing little damage to the formation, while the cellulose nanofibrils are non-toxic and pollution-free, and have the advantages of environmental protection. Therefore, the foam system based on the foam stabilizer provided by the present invention has high efficiency and stability under high-temperature (110 °C) and high-salt conditions, and can be directly applied to offshore oilfields (formation water with high salinity) or deep high-temperature reservoirs (such as deep gas reservoirs more than 8000 m in the Tarim Basin).

[0098] It should be understood that the application of the present invention is not limited to the above examples. For those of ordinary skill in the art, improvements or changes can be made according to the above description, and all such improvements and changes should fall within the protection scope of the appended claims of the present invention.

Claims

1. A foam stabilizer, characterized in that, The foam stabilizer includes a viscoelastic surfactant and cellulose nanofibrils modified with a cationic quaternary ammonium salt.

2. The foam stabilizer according to claim 1, characterized in that, The mass ratio of the viscoelastic surfactant to the cellulose nanofibrils modified with a cationic quaternary ammonium salt is (0.1 - 1):(0.1 - 0.3).

3. The foam stabilizer according to claim 1, wherein The viscoelastic surfactant includes at least one of a viscoelastic cationic quaternary ammonium salt surfactant and a viscoelastic zwitterionic surfactant.

4. The foam stabilizer according to claim 3, wherein The viscoelastic cationic quaternary ammonium salt surfactant includes at least one of cetyltrimethylammonium bromide, erucamide propyltrimethylammonium chloride, and didodecyldimethylammonium chloride; the viscoelastic zwitterionic surfactant includes at least one of carboxylic acid betaine and sulfonic acid betaine.

5. The foam stabilizer according to claim 4, wherein The sulfonic acid betaine includes erucamide propyl hydroxy sulfo betaine.

6. A foaming base liquid, characterized in that, The foaming base liquid includes composite brine, a foaming agent, and the foam stabilizer according to any one of claims 1 - 5; or, the foaming base liquid includes water, a foaming agent, and the foam stabilizer according to any one of claims 1 - 5.

7. The foaming base liquid according to claim 6, wherein, In the foaming base liquid, the mass content of the foaming agent is 0.2% - 1.2%, and the mass content of the foam stabilizer is 0.2% - 1.3%.

8. The foaming base liquid according to claim 6, wherein The foaming agent includes at least one of alkyl polyglycoside, lauramide propyl hydroxy sulfo betaine, and coconut oil amide propyl hydroxy sulfo betaine; The composite brine includes the following components by mass percentage: NaCl 11.2%, MgCl 1.2%, CaCl 2.4%, and water 85.2%.

9. A foam fracturing fluid, characterized in that, The foam fracturing fluid includes a gas phase and a liquid phase, and the liquid phase includes the foaming base liquid according to any one of claims 6 - 8.

10. A preparation method of a foam fracturing fluid, characterized in that, It includes the following steps: The foaming base liquid according to any one of claims 6 - 8 is foamed by mechanical foaming method or gas diffusion method to obtain the foam fracturing fluid.

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