A foam plugging agent suitable for nitrogen gas drive reservoirs and a method of preparing the same

Abstract

The present application relates to the field of oil field chemistry, and more particularly to a kind of foam blocking agent suitable for nitrogen gas drive reservoir and its preparation method, by mass percentage, including the following raw materials: polymer 0.2-0.5%, foaming agent 0.5-1.2%, crosslinking agent 0.1-0.4%, stabilizer 0.05-0.2%, polysaccharide polymer 0.5-2.5%, modified bentonite 0.2-1%, nanoparticle 0.05-0.3%, initiator 0.01-0.05%, and the balance is water.The foam generated by the blocking agent defoams when encountering oil, and is stable when encountering water, allowing the blocking agent to preferentially enter high water content channels while causing minimal damage to oil-containing channels.The blocking agent has low viscosity, good injectability, and is easy to pump.

Description

Technical Field

[0001] This invention relates to the field of oilfield chemistry, and more specifically, to a foam plugging agent suitable for nitrogen-driven oil reservoirs and its preparation method. Background Technology

[0002] Nitrogen flooding is a crucial technique for enhancing oil recovery, particularly in bottom-water reservoirs, fractured-vuggy reservoirs, and heavy oil reservoirs. However, due to formation heterogeneity and differences in gas and oil mobility, gas channeling is highly likely to occur during nitrogen flooding, leading to ineffective circulation of injected gas along high-permeability channels or large pores, thus reducing sweep efficiency and oil displacement effectiveness.

[0003] To address the problem of gas channeling, researchers have developed various plugging technologies. Among them, nitrogen foam control technology has attracted attention due to its selective plugging characteristics of "foaming upon contact with water and defoaming upon contact with oil." While conventional foam systems can plug high-permeability layers to some extent, they suffer from poor stability and insufficient plugging strength. Existing gel-based plugging agents, although possessing high plugging strength, suffer from poor injectability and cause irreversible damage to the oil reservoir.

[0004] While conventional foam systems (such as surfactant + nitrogen) can block high-permeability layers to some extent, they suffer from the following drawbacks: poor foam stability and short half-life under high temperature and high salinity conditions; insufficient plugging strength, making it difficult to withstand formation pressure differentials; and the foam's adhesion to the rock surface is merely physical, making it easily eroded and destroyed by displacing fluids. Existing gel-based plugging agents (such as polyacrylamide / organic chromium crosslinking systems) offer high plugging strength but suffer from poor injectability (high viscosity before gelation), are prone to causing irreversible damage to the oil reservoir, and lack selectivity.

[0005] In recent years, researchers have attempted to enhance the stability and interfacial bonding of foam gels by introducing nanomaterials or biomimetic modified bentonite. For example, Chinese patent CN121674040A discloses a carbon dot-hydrogel plugging agent that utilizes in-situ carbonization of polysaccharides to generate carbon dots, improving the temperature and salt resistance of the gel. However, this system does not address the synergistic foaming and selective plugging functions with nitrogen. Chinese patent CN121825511A proposes a biomimetic adhesion-type nano-plugging agent that utilizes dopamine acrylamide to enhance chemical bonding with rocks. However, this system is a plugging agent for drilling fluids, and dopamine monomers are expensive and complex to synthesize. Chinese patent CN119286494A relates to a CO2-responsive foam plugging agent that uses nano-calcium carbonate modification, but its response mechanism depends on CO2 and is not suitable for nitrogen-enhanced reservoirs.

[0006] Therefore, there is an urgent need to develop a nitrogen-driven plugging agent that combines high-strength plugging capability with good selectivity. Summary of the Invention

[0007] The purpose of this invention is to provide a foam plugging agent suitable for nitrogen-enhanced oil reservoirs. This plugging agent produces foam that defoams upon contact with oil and stabilizes upon contact with water, allowing it to preferentially enter high water-cut channels while causing minimal damage to oil-bearing channels. Furthermore, this plugging agent has low viscosity, good injectability, and is easy to pump.

[0008] Another objective of this invention is to provide a method for preparing a foam plugging agent suitable for nitrogen-driven reservoirs, which involves online mixing and has a simple process.

[0009] The technical problem solved by this invention is achieved by the following technical solution.

[0010] On one hand, embodiments of the present invention provide a foam plugging agent suitable for nitrogen-enhanced oil reservoirs, comprising the following raw materials by mass percentage: Polymer 0.2-0.5%, foaming agent 0.5-1.2%, crosslinking agent 0.1-0.4%, stabilizer 0.05-0.2%, polysaccharide polymer 0.5-2.5%, modified bentonite 0.2-1%, nanoparticles 0.05-0.3%, initiator 0.01-0.05%, balance is water.

[0011] The modified bentonite is polydopamine-modified bentonite; The nanoparticles are at least one of nano-silica and nano-calcium carbonate.

[0012] The polymer is at least one of partially hydrolyzed polyacrylamide and hydrophobically associating polyacrylamide. The polysaccharide polymer is a mixture of carboxymethyl chitosan, sodium alginate methacrylate, and sodium carboxymethyl cellulose.

[0013] In some embodiments of the present invention, the foaming agent is at least one of sodium dodecylbenzenesulfonate, α-olefin sulfonate, and betaine.

[0014] In some embodiments of the present invention, the crosslinking agent is at least one of an organochromium crosslinking agent and a phenolic resin crosslinking agent.

[0015] In some embodiments of the present invention, the stabilizer is at least one of thiourea, sodium sulfite, and ammonium sulfite.

[0016] In some embodiments of the present invention, the mass ratio of carboxymethyl chitosan, sodium alginate methacrylate, and sodium carboxymethyl cellulose is 4:3:3.

[0017] In some embodiments of the present invention, the method for preparing the modified bentonite includes the following steps: The purified sodium bentonite was added to Tris buffer and ultrasonically dispersed for 30-40 min to obtain a suspension; the pH of the Tris buffer was 8.0-9.0 and the concentration of the Tris buffer was 10-50 mM. Dopamine hydrochloride was added to Tris buffer and stirred until homogeneous. Then, it was added dropwise to the suspension. The mixture was magnetically stirred for 12-24 h in an air atmosphere. After centrifugation, washing, drying, and grinding, the modified bentonite was obtained.

[0018] In some embodiments of the present invention, the mass ratio of sodium bentonite to dopamine hydrochloride is 1:(0.2-0.5).

[0019] On the other hand, embodiments of the present invention provide a method for preparing a foam plugging agent suitable for nitrogen-driven reservoirs, comprising the following steps: (1) Add the polymer to water, stir, and allow it to swell for 30-40 minutes to obtain liquid A; (2) Add the crosslinking agent to water and stir to obtain liquid B; (3) After adding the nanoparticles to water, disperse them at high speed for 10-15 minutes, then add foaming agent, polysaccharide polymer, modified bentonite, stabilizer and initiator in sequence, disperse evenly, and obtain liquid C; (4) Mix liquid A, liquid B and liquid C online according to the proportion of each raw material to obtain the sealing agent.

[0020] Compared with the prior art, the embodiments of the present invention have at least the following advantages or beneficial effects: The plugging agent provided by this invention can rapidly form a primary gel network under the action of a crosslinking agent. The amino groups of carboxymethyl chitosan participate in the condensation reaction of phenolic resin, further increasing the density of crosslinking points. The polysaccharide polymer undergoes dehydration and carbonization at high temperatures to generate carbon dots with a particle size of 2-5 nm. The high thermal decomposition temperature of these carbon dots can inhibit the thermal motion of polymer chains and free radical degradation at high temperatures. The active groups (-OH, -NH2, -COOH) on the surface of the carbon dots can form hydrogen bonds or covalent bonds with the polymer chains, enhancing the strength of the gel and prolonging the plugging time. Nanoparticles can be adsorbed at the bubble interface, stabilizing the foam and filling the micropores of the gel network, improving the compactness of the plugging layer. The polydopamine (PDA) on the surface of modified bentonite contains abundant catechol groups, which form hydrogen bonds with the silanol groups (Si-OH) on the rock surface at formation temperatures (≥70℃), and can dehydrate at higher temperatures (≥90℃) to form stable Si-OC covalent bonds. Furthermore, the amino and hydroxyl groups of polydopamine can form coordination bonds with metal ions, improving the bonding strength between the gel and the rock. On the other hand, the layered structure of bentonite provides physical protection for polydopamine, delaying its thermal oxidative degradation at high temperatures, allowing the gel to adhere well to the rock even at high temperatures, thus improving the gel's high-temperature stability.

[0021] The foam generated by this plugging agent defoams upon contact with oil and stabilizes in water, allowing it to preferentially enter high water-content channels while causing minimal damage to oil-containing channels. Furthermore, the plugging agent has low viscosity, good injectability, and is easy to pump. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to specific embodiments.

[0024] On one hand, embodiments of the present invention provide a foam plugging agent suitable for nitrogen-enhanced oil reservoirs, comprising the following raw materials by mass percentage: Polymer 0.2-0.5%, foaming agent 0.5-1.2%, crosslinking agent 0.1-0.4%, stabilizer 0.05-0.2%, polysaccharide polymer 0.5-2.5%, modified bentonite 0.2-1%, nanoparticles 0.05-0.3%, initiator 0.01-0.05%, balance is water.

[0025] In some embodiments of the present invention, the polymer is at least one of partially hydrolyzed polyacrylamide and hydrophobically associating polyacrylamide. It is the main material for forming the three-dimensional network framework of the gel. The amide groups (-CONH2) and carboxylate groups (-COO2) on the polyacrylamide molecular chain... - Polyacrylamide can undergo cross-linking reactions with cross-linking agents to construct hydrogel networks with elasticity and strength. Polyacrylamide, with a molecular weight of 8 million to 20 million, has a relatively long molecular weight and high cross-linking density, improving the strength of the gel formed by the blocking agent. Carboxylate groups can react with organochromium cross-linking agents (Cr... 3+ Coordination with the amide group allows for partial hydrolysis at high temperatures to generate more carboxylate groups, increasing the density of crosslinking points. On the other hand, the polyacrylamide molecular chains and polysaccharide molecular chains form an interpenetrating network through physical entanglement and hydrogen bonding, improving the gel's toughness and resistance to breakage. The polyacrylamide solution also exhibits pseudoplasticity, facilitating pumping and injection. After injection, the viscosity recovers at low flow rates in the formation, promoting gel formation.

[0026] The foaming agent is at least one of sodium dodecylbenzenesulfonate, α-olefin sulfonate, and betaine. The foaming agent generates stable foam in the presence of nitrogen; it also acts as a surfactant to assist in wetting the rock surface. When using a compound foaming agent system, the foam volume is increased by more than 30% compared to a single surfactant, the foam half-life is prolonged, and the foam can effectively fill hyperpermeable channels. The foam generated by this plugging agent defoams under oil-containing conditions and remains stable under water-containing conditions, achieving high selectivity in plugging water without plugging oil, thus protecting oil-containing areas. Furthermore, the foaming agent can reduce the surface energy of polysaccharide polymers and nanoparticles, promoting their uniform dispersion in solution. It can also reduce the interfacial resistance between the fluid and the rock, improving injectability.

[0027] The crosslinking agent is at least one of an organochromium crosslinking agent and a phenolic resin crosslinking agent. The organochromium crosslinking agent forms coordination bonds with the carboxyl groups of polyacrylamide within 2-4 hours at 30-90℃, rapidly constructing a primary gel network and preventing the injection liquid from leaking out before gel formation. Meanwhile, the phenolic resin crosslinking agent undergoes a condensation reaction with the amide groups of polyacrylamide and the amino groups of carboxymethyl chitosan at ≥90℃, forming a denser covalent crosslinking network, significantly improving the temperature resistance and mechanical strength of the gel. That is, when both are used in combination, the compressive strength of the gel can be improved. Furthermore, the CN bonds in the phenolic resin crosslinking are more stable than the coordination bonds in the organochromium crosslinking agent, and are less prone to hydrolysis under high temperature and high salt conditions, extending the service life of the sealing agent during gel formation.

[0028] The stabilizer is at least one of thiourea, sodium sulfite, and ammonium sulfite. Sodium sulfite preferentially reacts with dissolved oxygen in the system, reducing the attack of hydroxyl radicals (·OH) and peroxy radicals (ROO·) on the polyacrylamide backbone, and also prevents excessive self-condensation of phenolic resin at high temperatures, ensuring its effective cross-linking with the polymer.

[0029] The nanoparticles are at least one of nano-silica and nano-calcium carbonate.

[0030] Nano-sized calcium carbonate (20-100 nm) adsorbs onto the surface of bubbles, inhibiting foam drainage and aggregation through steric hindrance, thus extending the half-life of the foam by 2-3 times at 130℃ and a mineralization of 210,000 mg / L. It can also fill the micropores (50-200 nm) in the gel network, improving the compactness of the sealing layer and reducing gas permeability. As stress concentration points, nanoparticles can disperse external forces and improve the compressive strength of the gel. Calcium carbonate dissolves slowly under acidic conditions, adjusting the local pH, which is beneficial for the cross-linking of phenolic resins and the hydrolysis and carbonization of polysaccharides.

[0031] The polysaccharide polymer comprises a mixture of carboxymethyl chitosan, sodium alginate methacrylate, and sodium carboxymethyl cellulose. Carboxymethyl chitosan is an amino-containing polysaccharide whose functional groups can participate in chemical cross-linking and also serve as a precursor for nitrogen-doped carbon dots. The amino groups (-NH2) of carboxymethyl chitosan undergo a condensation reaction with the hydroxymethyl groups of the phenolic resin, covalently linking the polysaccharide to the gel network and preventing polysaccharide detachment at high temperatures. Carbonization generates N-carbon dots with active groups such as pyridine nitrogen and pyrrole nitrogen on their surface, enhancing hydrogen bonding with the polymer chains and improving gel strength.

[0032] Sodium methacrylated alginate (MA-Alg) is a polymerizable polysaccharide containing vinyl double bonds, covalently linked to a polyacrylamide network via free radicals. Initiated by potassium persulfate, the double bonds of MA-Alg undergo free radical copolymerization with the polyacrylamide chains, forming the strongest C-C bonds that anchor the polysaccharide molecular chains within the gel network, enhancing the bond strength. MA-Alg is covalently anchored to the gel backbone, and the carbon dots generated during its carbonization are correspondingly distributed within the network, preventing carbon dot aggregation or migration.

[0033] Sodium carboxymethyl cellulose (CMC) acts as a thickener, physical entanglement enhancer, and auxiliary carbon dot precursor. The long-chain molecules of CMC form an inter-polymer entanglement network (IPN) with polyacrylamide, which improves the modulus and resistance to deformation of the gel. It also increases solution viscosity, reduces foam drainage rate, and prolongs foam half-life. The glucose units of CMC undergo dehydration and carbonization at high temperatures, synergistically forming multi-component carbon dots with CMCS and MA-Alg.

[0034] In some embodiments of the present invention, the mass ratio of carboxymethyl chitosan, sodium alginate methacrylate, and sodium carboxymethyl cellulose is 4:3:3.

[0035] In some embodiments of the present invention, the method for preparing the modified bentonite includes the following steps: The purified sodium bentonite was added to Tris buffer and ultrasonically dispersed for 30-40 min to obtain a suspension; the pH of the Tris buffer was 8.0-9.0 and the concentration of the Tris buffer was 10-50 mM. Dopamine hydrochloride was added to Tris buffer and stirred until homogeneous. Then, it was added dropwise to the suspension. The mixture was magnetically stirred for 12-24 h in an air atmosphere. After centrifugation, washing, drying, and grinding, the modified bentonite was obtained.

[0036] The catechol groups of polydopamine (PDA) form hydrogen bonds (70-90℃) and covalent bonds (≥90℃, dehydration to form Si-OC) with the Si-OH on the rock surface, ensuring the sealing layer adheres firmly to the rock pore surface. Bentonite sheets (approximately 1 nm thick, 100-500 nm transverse dimensions) are dispersed in the gel, enhancing the gel's modulus and fracture resistance through physical barrier and stress transfer. The interlayer confinement effect of bentonite protects PDA from high-temperature oxygen attack, maintaining the chemical anchoring effect for a long time at 150℃. Bentonite sheets can also adsorb at the gas-liquid interface, synergistically stabilizing bubbles with nano-calcium carbonate.

[0037] In some embodiments of the present invention, the mass ratio of sodium bentonite to dopamine hydrochloride is 1:(0.2-0.5).

[0038] On the other hand, embodiments of the present invention provide a method for preparing a foam plugging agent suitable for nitrogen-driven reservoirs, comprising the following steps: (1) Add the polymer to water, stir, and allow it to swell for 30-40 minutes to obtain liquid A; (2) Add the crosslinking agent to water and stir to obtain liquid B; (3) After adding the nanoparticles to water, disperse them at high speed for 10-15 minutes, then add foaming agent, polysaccharide polymer, modified bentonite, stabilizer and initiator in sequence, disperse evenly, and obtain liquid C; (4) Mix liquid A, liquid B and liquid C online according to the proportion of each raw material to obtain the sealing agent.

[0039] The features and performance of the present invention will be further described in detail below with reference to the embodiments. The sources of each raw material are shown in Table 1.

[0040] Table 1

[0041] Example 1 Preparation of modified bentonite: Sodium-based bentonite was added to deionized water at a mass ratio of 5% (i.e., 50 g / L), dispersed by high-speed stirring (1000 rpm) for 2 h, and allowed to stand for 24 h. The supernatant was collected and centrifuged (4000 rpm, 20 min). The precipitate was dried at 105℃ for 12 h, ground and passed through a 200-mesh sieve to obtain purified sodium-based bentonite.

[0042] Preparation of Tris buffer: Weigh 6.05 g Tris and dissolve it in about 800 mL of deionized water. Adjust the pH to 8.5 with 1 mol / L HCl and bring the volume to 1 L to obtain 50 mM Tris-HCl buffer (pH 8.5).

[0043] The purified sodium bentonite was added to Tris buffer at a concentration of 3 g / L and ultrasonically dispersed (power 200 W, frequency 40 kHz) for 30 min to obtain a suspension. Dopamine hydrochloride was added to Tris buffer and stirred until homogeneous. The mixture was then added dropwise to the suspension. The reaction was magnetically stirred for 24 h, followed by centrifugation, washing, drying, and grinding to obtain modified bentonite. The mass ratio of bentonite to dopamine hydrochloride was 1:0.3.

[0044] Prepare the raw materials according to the following proportions: Polymer (partially hydrolyzed polyacrylamide) 0.3%, foaming agent 1%, crosslinking agent (organic chromium crosslinking agent) 0.3%, stabilizer (ammonium sulfite) 0.1%, polysaccharide polymer 1.5%, modified bentonite 0.7%, nanoparticles (nano-calcium carbonate) 0.2%, initiator (potassium persulfate) 0.03%, balance being water.

[0045] The foaming agent is a mixture of α-olefin sulfonate and betaine in a mass ratio of 1:1. The polysaccharide polymer is a mixture of carboxymethyl chitosan, sodium alginate methacrylate, and sodium carboxymethyl cellulose in a mass ratio of 4:3:3.

[0046] The preparation method of sodium alginate methacrylate is as follows: Dissolve sodium alginate (SA) in deionized water to prepare a 3% solution. Adjust the pH to 8-9, and add glycidyl methacrylate (GMA) at a molar ratio of SA repeating unit:GMA = 1:0.3.

[0047] The reaction was carried out in the dark, under a nitrogen atmosphere, at 40-60℃ for 8 hours, with the pH value of the system controlled at 8-8.5 during the reaction. The system was then cooled to room temperature and the pH value was adjusted to 7 to terminate the reaction. The solution was then dialyzed and freeze-dried to obtain sodium alginate methacrylate (MA-Alg).

[0048] Prepare the sealing agent according to the following steps: (1) Add polyacrylamide to water, stir, and allow it to swell for 30 minutes to obtain liquid A; (2) Add the crosslinking agent to water and stir to obtain liquid B; (3) After adding the nanoparticles to water, disperse them at high speed for 10 minutes, and then add foaming agent, polysaccharide polymer, modified bentonite, stabilizer and initiator in sequence. Disperse evenly to obtain liquid C. (4) Mix liquid A, liquid B and liquid C online according to the proportion of each raw material to obtain the sealing agent.

[0049] Example 2 Based on Example 1, the mass ratio of bentonite to dopamine hydrochloride in the modified bentonite was adjusted to 1:0.2, while the remaining raw materials, proportions, and preparation methods were the same as in Example 1.

[0050] Example 3 Based on Example 1, the mass ratio of bentonite to dopamine hydrochloride in the modified bentonite was adjusted to 1:0.5, while the remaining raw materials, proportions, and preparation methods were the same as in Example 1.

[0051] Example 4 Based on Example 1, the proportions of the raw materials in the sealing agent were adjusted: Polymer (partially hydrolyzed polyacrylamide) 0.2%, foaming agent 0.5%, crosslinking agent (organic chromium crosslinking agent) 0.1%, stabilizer (ammonium sulfite) 0.05%, polysaccharide polymer 0.5%, modified bentonite 0.2%, nanoparticles (nano-calcium carbonate) 0.05%, initiator (potassium persulfate) 0.01%, balance being water.

[0052] Everything else is the same as in Example 1.

[0053] Example 5 Based on Example 1, the proportions of the raw materials in the sealing agent were adjusted: Polymer (partially hydrolyzed polyacrylamide) 0.5%, foaming agent 1.2%, crosslinking agent (organic chromium crosslinking agent) 0.4%, stabilizer (ammonium sulfite) 0.2%, polysaccharide polymer 2.5%, modified bentonite 1%, nanoparticles (nano-calcium carbonate) 0.3%, initiator (potassium persulfate) 0.05%, balance being water.

[0054] Everything else is the same as in Example 1.

[0055] Example 6 Based on Example 1, the proportions of the raw materials in the sealing agent were adjusted: The composition is as follows: 0.5% polymer, 0.5% foaming agent, 0.4% crosslinking agent, 0.1% stabilizer, 1% polysaccharide polymer, 0.2% modified bentonite, 0.3% nanoparticles, 0.01% initiator, and the balance is water.

[0056] Everything else is the same as in Example 1.

[0057] Example 7 Based on Example 1, the proportions of the raw materials in the sealing agent were adjusted: The composition is as follows: 0.5% polymer, 1.2% foaming agent, 0.4% crosslinking agent, 0.1% stabilizer, 1% polysaccharide polymer, 1% modified bentonite, 0.3% nanoparticles, 0.05% initiator, and the balance is water.

[0058] Everything else is the same as in Example 1.

[0059] Example 8 Based on Example 1, some raw materials in the sealing agent were adjusted: The crosslinking agent is a mixture of an organochromium crosslinking agent and a phenolic resin crosslinking agent, with a mass ratio of 1:1. Everything else is the same as in Example 1.

[0060] Example 9 Based on Example 1, some raw materials in the sealing agent were adjusted: The crosslinking agent is a phenolic resin crosslinking agent. Everything else is the same as in Example 1.

[0061] Example 10 Based on Example 1, some raw materials in the sealing agent were adjusted: The foaming agent was sodium dodecylbenzenesulfonate, and all other aspects were the same as in Example 1.

[0062] Example 11 Based on Example 1, some raw materials in the sealing agent were adjusted: The foaming agent is a mixture of sodium dodecylbenzenesulfonate and betaine in a mass ratio of 1:1, and all other aspects are the same as in Example 1.

[0063] Example 12 Based on Example 1, some raw materials in the sealing agent were adjusted: The foaming agent is a mixture of sodium dodecylbenzenesulfonate and α-olefin sulfonate in a mass ratio of 1:1, and all other aspects are the same as in Example 1.

[0064] Comparative Example 1 The difference from Example 1 is that purified sodium-based bentonite is used instead of the modified bentonite in Example 1, while the other raw materials, proportions and preparation methods are the same as in Example 1.

[0065] Comparative Example 2 The difference from Example 1 is that polydopamine is used instead of modified bentonite, while the other raw materials, proportions and preparation methods are the same as in Example 1.

[0066] Comparative Example 3 The difference from Example 1 is that the polysaccharide polymer is a single carboxymethyl chitosan, while the other raw materials, proportions and preparation methods are the same as in Example 1.

[0067] Comparative Example 4 The difference from Example 1 is that the polysaccharide polymer is a single sodium alginate methacrylate, while the other raw materials, proportions and preparation methods are the same as in Example 1.

[0068] Comparative Example 5 The difference from Example 1 is that the polysaccharide polymer is a single sodium carboxymethyl cellulose, while the other raw materials, proportions and preparation methods are the same as in Example 1.

[0069] Experimental Example The sealing agents used in the examples and comparative examples were tested as follows.

[0070] Viscosity before gelation: Brookfield DV2T viscometer, rotor No. 2, 12 rpm; measured immediately after preparation of the sealant at 25°C, in accordance with GB / T 10247-2008.

[0071] Viscosity after gelation: Brookfield DV2T viscometer, rotor No. 7, 0.3 rpm, rotor factor of 4000. Viscosity displayed by the viscometer = torque percentage (%) × rotor factor. Viscosity = torque percentage (%) × rotor factor. According to GB / T10247-2008, the sealant was aged at 120℃ for 7 days and then cooled to 25℃ for measurement.

[0072] Breakthrough pressure: Core displacement experiment, record the inflection point of the pressure-flow curve. Referring to SY / T 5590-2018 "Core Displacement Experiment Specification", the permeability of the artificial high-permeability core is about 50 μm². Water injection displacement, the pressure gradient rises to the plateau pressure, which is the breakthrough pressure.

[0073] Core plugging rate: Permeability change rate before and after plugging η = (K1-K2) / K1×100%, SY / T 5590-2018 "Core Displacement Test Specification", the permeability of artificial high-permeability core is about 50 μm², the water permeability K1 is measured before plugging, and K2 is measured after plugging (after waiting for 24h of solidification).

[0074] The retention rate of plugging rate after high temperature aging: According to SY / T 5590-2018 "Specification for Core Displacement Test", the plugged core was placed in a high temperature and high pressure reactor and aged at 150℃ for 7 days, and the plugging rate was re-measured.

[0075] Scour resistance: Water and nitrogen (0.5 PV / cycle each) were alternately injected into the core displacement device for a total of 100 cycles, and the percentage decrease in plugging rate was measured.

[0076] Volume of foam produced upon contact with nitrogen: Graduated cylinder method (gas-liquid ratio 1:1), graduated cylinder volume 1000mL, accuracy 5mL. Refer to GB / T13173-2021 "Test Methods for Surfactants and Detergents", take 100 mL of sealing agent, and introduce nitrogen gas at a rate of 100 mL / min at 40℃, and record the reading when the foam reaches its maximum volume.

[0077] Foam half-life: Referring to GB / T 7462-94, nitrogen gas is introduced at 40℃ at a rate of 100 mL / min. After the foam reaches its maximum volume, the time required for the foam volume to decay to half of its initial volume is recorded after the gas is introduced and the gas is stopped.

[0078] Core parameters: Artificial sandstone core, length 40-45 cm, diameter 2.5 cm, pore volume 65-70 mL, initial water permeability 50-55 μm², simulating high permeability large pores.

[0079] Aging conditions: The gelled sealing agent (or the sealed core) is placed in a high-temperature and high-pressure reactor and aged at 150℃ and a mineralization of 21×10⁻⁶. 4 Aging for 7 days under conditions simulating the Tarim Oilfield (mg / L).

[0080] The test results are shown in Table 2.

[0081] Table 2

[0082] Table 2 shows that, considering overall performance (plugging rate, breakthrough pressure, temperature resistance, erosion resistance, and foaming properties), Example 5 exhibits the strongest overall performance. Its formulation consists of 0.5% polymer, 2.5% polysaccharide, and 1.0% modified bentonite, making it suitable for reservoirs with extreme high temperature, high salinity, and strong erosion. Example 8, employing a dual crosslinking agent (organic chromium + phenolic resin), demonstrates superior temperature resistance. Example 3 (high dopamine ratio) shows slightly better erosion resistance than Example 1. Example 6 (lower foaming agent ratio) exhibits limited foaming performance. Example 2 (lower dopamine ratio) shows decreased erosion resistance.

[0083] In Example 2, the dopamine to bentonite mass ratio was 1:0.2. After 100 cycles of scouring resistance, the plugging rate decreased by 6.5%, and after aging at 150°C for 7 days, the plugging rate remained at 93.8%. This indicates that the dopamine dosage was too low, resulting in incomplete polydopamine coating on the bentonite surface, insufficient chemical anchoring points, and poor scouring resistance. When the ratio was increased to 1:0.3 (Example 1), the plugging rate decreased by 4.2%, and the aging retention rate was 96.2%, reaching the optimal balance point. Further increasing the ratio to 1:0.5 (Example 3), the plugging rate decreased by 3.8%, and the aging retention rate remained at 96.8%, showing a slight improvement in performance.

[0084] Comparing the data from Example 4 (polymer 0.2%, polysaccharide 0.5%, modified bentonite 0.2%), Example 1 (polymer 0.3%, polysaccharide 1.5%, modified bentonite 0.7%), Example 6 (polymer 0.5%, polysaccharide 1.0%, modified bentonite 0.2%), Example 7 (polymer 0.5%, polysaccharide 1.0%, modified bentonite 1.0%), and Example 5 (polymer 0.5%, polysaccharide 2.5%, modified bentonite 1.0%), it can be found that: increasing the polymer content within the range of 0.3% to 0.5% increases the gel strength and breakthrough pressure. Increasing the polysaccharide content within the range of 1.0% to 2.5% improves the carbon dot reinforcement effect. Increasing the modified bentonite content within the range of 0.2% to 1.0% enhances the erosion resistance.

[0085] The breakthrough pressure of a single organochromium crosslinking agent (Example 1, 0.3%) was 17.5 MPa, and the aging retention rate at 150°C was 96.2%. The breakthrough pressure of a single phenolic resin crosslinking agent (Example 9, 0.3%) was 15.2 MPa, and the aging retention rate was 94.8%, indicating that some parts of the system were displaced before complete crosslinking, resulting in a lower breakthrough pressure. The breakthrough pressure of the dual crosslinking agent (Example 8, 0.15% organochromium and 0.15% phenolic resin each) was 18.1 MPa, and the aging retention rate was 97.0%, exhibiting the best performance. The organochromium rapidly formed a primary network to fix the system, while the phenolic resin underwent secondary crosslinking at high temperature, forming a denser network.

[0086] Sodium dodecylbenzenesulfonate alone (Example 10) had a foaming volume of 480 mL, a half-life of 32 minutes, and a blocking rate of 98.0%, exhibiting the worst foaming performance. The combination of sodium dodecylbenzenesulfonate and betaine (Example 11) had a foaming volume of 530 mL, a half-life of 40 minutes, and a blocking rate of 98.4%, but the synergistic effect of betaine and sodium dodecylbenzenesulfonate was not as good as that with α-olefin sulfonates. The combination of sodium dodecylbenzenesulfonate and α-olefin sulfonate (Example 12) had a foaming volume of 545 mL, a half-life of 42 minutes, and a blocking rate of 98.1%. The combination of α-olefin sulfonate and betaine (Example 1) had a foaming volume of 560 mL, a half-life of 45 minutes, and a blocking rate of 98.3%, exhibiting the best performance. α-olefin sulfonates possess excellent calcium soap dispersing power and hard water resistance, while betaine provides amphoteric properties. Together, they form a tighter molecular arrangement at the gas-liquid interface, resulting in a more significant reduction in surface tension.

[0087] The viscosity of sodium carboxymethyl cellulose (Comparative Example 5) after gelation was 8900 mPa·s, with a breakthrough pressure of 12.5 MPa and an aging retention rate of only 72.5% at 150℃. This is because it only exhibits physical entanglement without chemical cross-linking, resulting in low carbon dot yield and easy degradation at high temperatures. Sodium alginate esterified with monomethyl methacrylate (Comparative Example 4) after gelation had a viscosity of 9800 mPa·s, a breakthrough pressure of 14.2 MPa, and an aging retention rate of 85.3%. It can form covalent bonds through free radical copolymerization, but lacks nitrogen-doped carbon dots and physical entanglement, limiting its reinforcing effect. Chitosan (Comparative Example 3) after gelation had a viscosity of 10200 mPa·s, a breakthrough pressure of 14.8 MPa, and an aging retention rate of 88.6%. Although it can provide amino chemical cross-linking sites and nitrogen-doped carbon dots, it lacks covalent bonds and physical entanglement. The ternary mixture (Example 1, carboxymethyl chitosan: sodium alginate methacrylate: sodium carboxymethyl cellulose = 4:3:3) had a gelled viscosity of 11800 mPa·s, a breakthrough pressure of 17.5 MPa, and an aging retention rate of 96.2%. Carboxymethyl chitosan provides nitrogen-doped carbon dots and chemical crosslinking, sodium alginate methacrylate provides covalent bonding, and sodium carboxymethyl cellulose provides physical entanglement and thickening / foam stabilization, forming a multi-layered network of chemical crosslinking, covalent bonding, and physical entanglement. The carbon dots are anchored within this network, resulting in the highest performance retention rate after high-temperature aging. This ternary polysaccharide mixture achieves high strength, high temperature resistance, and erosion resistance.

[0088] Comparing Example 1 with Comparative Example 1 (using unmodified bentonite instead of modified bentonite): the plugging rate of the modified bentonite system decreased by 4.2% after 100 scour cycles, while that of the unmodified bentonite system decreased by 28.5%, indicating that polydopamine chemical anchoring can improve the plugging rate of the plugging agent. Comparing Example 1 with Comparative Example 2 (using free polydopamine instead of modified bentonite): the plugging rate of the modified bentonite system decreased by 4.2% after 100 scour cycles, while that of the free polydopamine system decreased by 18.2%, indicating that the bentonite carrier can provide protection and enhancement for polydopamine. Comparing Example 1 with Comparative Examples 3 to 5 (using single carboxymethyl chitosan, single sodium alginate methacrylate, and single sodium carboxymethyl cellulose, respectively): the ternary polysaccharide system maintained a 96.2% retention rate at 150°C, while the single polysaccharide systems maintained 88.6%, 85.3%, and 72.5%, respectively. This indicates that the synergistic effect of the ternary polysaccharides can improve the carbon dot enhancement effect and network stability. Comparing Example 1 with Example 8 (dual crosslinking agent): the breakthrough pressure (18.1 MPa) and aging retention rate (97.0%) of the dual crosslinking agent system were both superior to those of the single organic chromium system (17.5 MPa, 96.2%), indicating that the dual crosslinking agent system is better. Comparing Example 1 with Examples 10 to 12 (different foaming agent combinations): the foaming volume (560 mL) and half-life (45 minutes) of the α-olefin sulfonate + betaine combination were superior to the other combinations (480 to 545 mL, 32 to 42 minutes), indicating that the α-olefin sulfonate + betaine combination has the best foaming effect.

[0089] In summary, dopamine-modified bentonite can form covalent bonds with the rock surface through catechol groups, increasing its erosion resistance by more than 5 times. After 100 cycles, the plugging rate decreases less, while the unmodified system shows a higher decrease. Ternary polysaccharides exhibit in-situ carbon dot enhancement. A mixture of carboxymethyl chitosan, sodium alginate methacrylate, and sodium carboxymethyl cellulose in a 4:3:3 ratio, carbonized at high temperature to generate carbon dots, increases the plugging rate retention after aging at 150℃ for 7 days. The synergistic effect of the dual cross-linked network (organic chromium + phenolic resin) increases the breakthrough pressure. The synergistic effect of chemical cross-linking, covalent bonding, and physical entanglement of ternary polysaccharides increases the viscosity after gelation from 10800 mPa·s to 11800-15500 mPa·s. The foaming agent, a combination of α-olefin sulfonate and betaine, increases the foaming volume from 420 mL to 560-610 mL and extends the half-life from 30 minutes to 45-52 minutes, resulting in good foaming performance.

[0090] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A foam plugging agent suitable for nitrogen-driven oil reservoirs, characterized in that, By weight percentage, it includes the following raw materials: Polymer 0.2-0.5%, foaming agent 0.5-1.2%, crosslinking agent 0.1-0.4%, stabilizer 0.05-0.2%, polysaccharide polymer 0.5-2.5%, modified bentonite 0.2-1%, nanoparticles 0.05-0.3%, initiator 0.01-0.05%, balance water; The polymer is at least one of partially hydrolyzed polyacrylamide and hydrophobically associating polyacrylamide; The polysaccharide polymer is a mixture of carboxymethyl chitosan, sodium alginate methacrylate, and sodium carboxymethyl cellulose; The modified bentonite is polydopamine-modified bentonite; The nanoparticles are at least one of nano-silica and nano-calcium carbonate.

2. The foam plugging agent suitable for nitrogen-driven reservoirs according to claim 1, characterized in that, The foaming agent is at least one of sodium dodecylbenzenesulfonate, α-olefin sulfonate, and betaine.

3. The foam plugging agent suitable for nitrogen-driven reservoirs according to claim 1, characterized in that, The crosslinking agent is at least one of organic chromium crosslinking agents and phenolic resin crosslinking agents.

4. The foam plugging agent suitable for nitrogen-driven reservoirs according to claim 1, characterized in that, The stabilizer is at least one of thiourea, sodium sulfite, and ammonium sulfite.

5. The foam plugging agent suitable for nitrogen-driven reservoirs according to claim 1, characterized in that, The mass ratio of carboxymethyl chitosan, sodium alginate methacrylate, and sodium carboxymethyl cellulose is 4:3:

3.

6. The foam plugging agent suitable for nitrogen-driven reservoirs according to claim 1, characterized in that, The method for preparing the modified bentonite includes the following steps: The purified sodium bentonite was added to Tris buffer and ultrasonically dispersed for 30-40 min to obtain a suspension. Dopamine hydrochloride was added to Tris buffer and stirred until homogeneous. Then, it was added dropwise to the suspension. The mixture was magnetically stirred for 12-24 h, centrifuged, washed, dried, and ground to obtain modified bentonite.

7. The foam plugging agent for nitrogen-driven reservoirs according to claim 6, characterized in that, The mass ratio of sodium bentonite to dopamine hydrochloride is 1:(0.2-0.5).

8. A method for preparing a foam plugging agent suitable for nitrogen-driven reservoirs as described in any one of claims 1-7, characterized in that, Includes the following steps: (1) Add the polymer to water, stir, and allow it to swell for 30-40 minutes to obtain liquid A; (2) Add the crosslinking agent to water and stir to obtain liquid B; (3) After adding the nanoparticles to water, disperse them at high speed for 10-15 minutes, then add foaming agent, polysaccharide polymer, modified bentonite, stabilizer and initiator in sequence, disperse evenly, and obtain liquid C; (4) Mix liquid A, liquid B and liquid C online according to the proportion of each raw material to obtain the sealing agent.

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