A high-strength low-molecular polymer gel system, a preparation method and application thereof

Abstract

The application provides a high-strength low-molecular polymer gel system and a preparation method and application thereof, and raw materials of the polymer gel system include: 4-6% of a polymer emulsion, 0.4-0.8% of a phenolic crosslinking agent, 0-0.3% of a catalyst, 0.2-0.5% of an oxygen scavenger, 0.2-0.4% of a stabilizer, and the rest of water; the polymer emulsion is formed by copolymerization reaction of acrylamide monomers and sodium p-styrene sulfonate monomers, and the molar amount of the sodium p-styrene sulfonate monomers accounts for 8-15% of the total amount of monomers. In the polymer gel plugging agent system, a low-molecular polymer with good heat resistance is selected, the polymer usage is high, the viscosity of the gel liquid is low, the deep part of the formation has good injectivity, the polymer gel has high gel strength, the elastic modulus after gelation is large, the stability at high temperature is good, large channels can be effectively plugged, and the steam channeling effect of a heavy oil reservoir is improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of oilfield chemistry, and particularly relates to a high-strength low-molecular polymer gel system and a preparation method and application thereof. BACKGROUND

[0002] The development methods for heavy oil reservoirs mainly include steam flooding and steam huff and puff. However, the heavy oil reservoirs have the characteristics of complex geological structure, loose cementation, strong formation heterogeneity and the like. After long-term steam injection development, high-temperature steam has a huge impact on the geological structure, which aggravates the formation heterogeneity, forms high-permeability steam channeling channels, reduces the steam sweep efficiency, and reduces the steam utilization rate, which seriously restricts the development benefit of steam huff and puff. Steam channeling is a problem that needs to be solved urgently in the development of heavy oil reservoirs by steam injection. In order to control steam channeling, in addition to adjusting the steam injection process to improve steam channeling, steam channeling blocking is also an important measure in the anti-channeling technology, and the chemical agent channeling blocking technology is one of the most effective methods for blocking steam channeling and improving the steam sweep efficiency. The key to this technology is the development of high-temperature channeling blocking agents.

[0003] At present, the blocking agents for steam channeling in heavy oil reservoirs mainly include high-temperature foams, high-temperature gels, solid particles and resin-based agents. In the steam flooding process, usually multiple temperature zones are formed, including the ultra-high temperature zone (350℃-200℃) near the wellbore, the high temperature zone in the middle and deep part of the formation (200℃-110℃), and the low temperature zone (<100℃). The blocking agents for the ultra-high temperature zone mainly include inorganic particle blocking agents and high-temperature tannin systems, which have good high-temperature resistance. For the high temperature zone, the inorganic particle blocking agents have limited migration ability in the formation and poor solidification time adjustability, and it is difficult to achieve the channeling blocking in the middle and deep part of the formation; the tannin system can resist a temperature as high as 250℃, and the gelling liquid is a low-viscosity liquid (<10 mPa·s), which is easy to enter different scale channels when injected in large quantities into the formation, causing damage to the low-permeability oil layers, and it is difficult to handle after mis-blocking, affecting the subsequent development effect. Therefore, the channeling blocking in the middle high-temperature zone of the formation is mainly achieved by using high-temperature foam or high-temperature gel blocking agents. The high-temperature foam can be prepared by selecting a high-temperature resistant surfactant, which can improve the steam sweep efficiency, has good injectability, and causes little damage to the oil layer, but it has effective sealing ability for large channels and a short effective period. For the high-temperature gel system, it is mainly composed of high-concentration temperature-resistant polymers and cross-linking agents, and the polymer content is usually greater than 1%, which has high viscosity and limited injection ability, making it difficult to achieve steam channeling blocking in the middle and deep part of the formation, and the gel strength is limited, and higher polymer concentration is required to achieve high strength.

[0004] For the profile control and plugging in the middle and deep part of the oil layer, there is a contradiction between the formation damage, injectability and plugging ability, and this phenomenon is particularly prominent for steam plugging. SUMMARY

[0005] To solve the above technical problems, the present application aims to provide a high-strength low-molecular polymer gel system and a preparation method thereof, which has high gel strength and good injectability.

[0006] To achieve the above-mentioned purposes, the present application provides a high-strength low-molecular polymer gel system, which comprises, by mass percentage, 4-6% of a polymer emulsion, 0.4-0.8% of a phenolic crosslinking agent, 0-0.3% of a catalyst, 0.2-0.5% of an oxygen scavenger, 0.2-0.4% of a stabilizer, and the balance of water.

[0007] The polymer emulsion is obtained by copolymerization of acrylamide monomers and sodium p-styrene sulfonate monomers, wherein the sodium p-styrene sulfonate monomers account for 8-15% of the total monomers in terms of molar amount.

[0008] According to the specific embodiments of the present application, preferably, the relative molecular mass of the polymer in the polymer emulsion is 600-800 thousand.

[0009] According to the specific embodiments of the present application, preferably, the preparation method of the polymer emulsion is as follows:

[0010] Under nitrogen protection, the acrylamide monomers are mixed with the emulsifier in water and preheated to the polymerization temperature, part of the initiator is added, then the sodium p-styrene sulfonate monomers are added dropwise, after the dropwise addition is completed, N,N,N',N'-tetramethyl ethylenediamine and the remaining initiator are added, and the copolymerization reaction is carried out to obtain the polymer emulsion.

[0011] According to the specific embodiments of the present application, preferably, in the preparation method of the polymer emulsion, the amount of the emulsifier is 0.05-0.1% of the total mass of the monomers; and the emulsifier is compounded by anionic surfactant SDS and nonionic surfactant OP-10 at a mass ratio of 1-1.2:1.

[0012] According to the specific embodiments of the present application, preferably, in the preparation method of the polymer emulsion, preferably, the mass ratio of the total mass of the monomers to the mass of water is 1:1.5-2.

[0013] According to the specific embodiments of the present application, preferably, in the preparation method of the polymer emulsion, the amount of the initiator is 0.5-1% of the total mass of the monomers; and the initiator is ammonium persulfate.

[0014] According to the specific embodiments of the present application, preferably, in the preparation method of the polymer emulsion, the amount of the N,N,N',N'-tetramethyl ethylenediamine is 10-20% of the mass of the initiator.

[0015] According to the specific embodiment of the present application, preferably, in the preparation method of the polymer emulsion, the temperature of the copolymerization reaction is 60-65℃, and the reaction time is 8-10h.

[0016] The polymer emulsion prepared by the present application is a viscous liquid, which contains 32-35% (mass fraction) of polymers, and the polymers are low-molecular polymers with a relative molecular mass of 600-800 thousand.

[0017] According to the specific embodiment of the present application, more preferably, the preparation method of the polymer emulsion is as follows:

[0018] S1: acrylamide monomers and a certain amount of water are added to a four-necked flask equipped with a stirrer, nitrogen is introduced for purging, and the reaction solution is heated to 60-65℃;

[0019] S2: after stirring for 20-30min after adding a certain amount of emulsifier, 2 / 3 of ammonium persulfate initiator is added, then sodium p-styrenesulfonate is slowly added dropwise, after the dropwise addition is completed, N,N,N',N'-tetramethyl ethylenediamine is added;

[0020] S3: then the remaining ammonium persulfate initiator is added dropwise to the four-necked flask for mixing and stirring, after reaction at 60-65℃ for 8-10h, the stirring is stopped, the temperature is lowered, and the product is discharged, to obtain a polymer emulsion;

[0021] wherein the emulsifier is a mixture of anionic surfactant SDS and nonionic surfactant OP-10 in a mass ratio of 1-1.2:1, and the amount is 0.05-0.1% of the total mass of the monomers; the amount of ammonium persulfate initiator is 0.5-1% of the total mass of the monomers, and the amount of N,N,N',N'-tetramethyl ethylenediamine is 10-20% of the mass of the ammonium persulfate initiator.

[0022] According to the specific embodiment of the present application, preferably, the phenolic crosslinking agent is a water-soluble phenolic resin crosslinking agent, or a composite crosslinking agent of a phenolic crosslinking agent and an aldehyde crosslinking agent, and more preferably, the phenolic crosslinking agent is a water-soluble phenolic resin crosslinking agent.

[0023] According to the specific embodiment of the present application, preferably, the preparation method of the water-soluble phenolic resin crosslinking agent comprises the following steps:

[0024] The water-soluble phenolic resin crosslinking agent is obtained by condensation reaction of phenol and formaldehyde in a molar ratio of 1:2-3 under the catalysis of an alkaline catalyst.

[0025] According to the specific embodiment of the present application, preferably, in the preparation of the water-soluble phenolic resin crosslinking agent, the temperature of the condensation reaction is 80-90℃, and the time is 1.5-2h.

[0026] According to the specific embodiment of the present application, preferably, in the preparation of the water-soluble phenolic resin crosslinking agent, the basic catalyst comprises at least one of sodium hydroxide, sodium carbonate and ammonia water, and more preferably, the basic catalyst is sodium hydroxide.

[0027] According to the specific embodiment of the present application, preferably, in the preparation of the water-soluble phenolic resin crosslinking agent, the amount of the basic catalyst is 4-6% of the total mass of phenol and formaldehyde.

[0028] According to the specific embodiment of the present application, preferably, the water-soluble phenolic resin crosslinking agent is prepared from phenol and formaldehyde under the action of a basic catalyst, and the specific preparation method is as follows:

[0029] S1: phenol and a basic catalyst solution are put into a three-necked flask, stirred and the temperature is raised to 45℃, and the temperature is controlled to be not more than 45℃;

[0030] S2: 80% formaldehyde solution is added dropwise within 30 min, and the temperature is raised to 80℃, and the reaction is carried out for 60-80 min;

[0031] S3: the remaining 20% formaldehyde solution is added to the above reaction solution, and the temperature is raised to 90℃, and the reaction is carried out for 20-30 min under constant temperature and stirring, and a transparent brown-red liquid, i.e. the water-soluble phenolic resin crosslinking agent, is obtained after the reaction is completed.

[0032] According to the specific embodiment of the present application, preferably, the catalyst is acetic acid and / or ammonium chloride, and more preferably, the catalyst is ammonium chloride.

[0033] According to the specific embodiment of the present application, preferably, the oxygen scavenger is thiourea.

[0034] According to the specific embodiment of the present application, preferably, the stabilizer is polypropylene fiber and / or nano-silicon, and preferably, the stabilizer is nano-silicon.

[0035] According to the specific embodiment of the present application, preferably, the median particle size of the nano-silicon is 8-15 nm. More preferably, the nano-silicon is added in the form of nano-silicon sol, and the content of nano-silicon dioxide in the nano-silicon sol is 28-32%.

[0036] According to the specific embodiment of the present application, preferably, the water is clear water or treated oilfield reinjection sewage.

[0037] The present application also provides a preparation method of the high-strength low-molecular polymer gel system.

[0038] (1) the phenolic crosslinking agent, the catalyst and the oxygen scavenger are added to water in proportion and stirred uniformly to obtain solution A;

[0039] (2) Add stabilizer into the solution A in proportion, stir uniformly to obtain solution B;

[0040] (3) Under stirring condition, add polymer emulsion into the solution B, stir fully to obtain the high-strength low-molecular polymer gel system.

[0041] In the preparation method of the high-strength low-molecular polymer gel system, preferably, the stirring speed in step (3) is 150-200 rpm, and the stirring time is 20-30 min.

[0042] The application further provides an application of the high-strength low-molecular polymer gel system in plugging steam channeling in heavy oil reservoirs.

[0043] In the application, preferably, when the formation temperature is less than 80℃, the raw materials of the high-strength low-molecular polymer gel system include: 4-6% of polymer emulsion, 0.4-0.8% of phenolic crosslinking agent, 0.2-0.3% of catalyst, 0.2-0.3% of oxygen scavenger, 0.2-0.3% of stabilizer, and the rest of water.

[0044] In the application, preferably, when the formation temperature is 80-110℃, the raw materials of the high-strength low-molecular polymer gel system include: 4-6% of polymer emulsion, 0.4-0.8% of phenolic crosslinking agent, 0.1-0.2% of catalyst, 0.3-0.4% of oxygen scavenger, 0.2-0.3% of stabilizer, and the rest of water.

[0045] In the application, preferably, when the formation temperature is more than 110℃, the raw materials of the high-strength low-molecular polymer gel system include: 4-6% of polymer emulsion, 0.4-0.8% of phenolic crosslinking agent, 0-0.1% of catalyst, 0.3-0.5% of oxygen scavenger, 0.3-0.4% of stabilizer, and the rest of water.

[0046] The technical solution provided by the application has the following beneficial effects:

[0047] The polymer emulsion used in the high-strength low-molecular polymer gel system prepared by the application is a low-molecular polymer copolymerized from acrylamide and sodium p-styrene sulfonate, and the sodium p-styrene sulfonate monomer makes the low-molecular polymer have better temperature resistance; the molecular weight of the low-molecular polymer in the synthesized polymer emulsion is 600-800 thousand, the gel liquid viscosity is low under high polymer dosage, the gel liquid viscosity is less than 500 mPa·s under the temperature of wellbore or formation, the low-molecular polymer has good migration performance in the deep part of the formation during the injection process, the low-molecular polymer dosage is high, the polymer gel has high gel strength, the gel has large elastic modulus after gelation, the plugging strength is high, the large pore channels can be effectively plugged, and the steam channeling effect in heavy oil reservoirs can be improved.

[0048] The high-strength low-molecular polymer gel system prepared by the application has adjustable gelation time at different temperatures, controllable low viscosity of the gelled liquid and gelation time, and good injection property, good gelation performance, small damage to medium and low permeability oil layers, and good plugging effect on large channels after gelation. The high-strength high-temperature resistant polymer gel has controllable gelation time under the condition of low-temperature stratum without steam injection, good thermal stability during steam injection, and long-term plugging effect on steam channeling channels. The problems of poor gelation performance of conventional high-temperature resistant plugging agent systems under low-temperature conditions and poor injection property of plugging agents under high strength are effectively solved, and the high-strength low-molecular polymer gel system can be used for long-term channeling plugging in deep strata during steam injection. DETAILED DESCRIPTION

[0049] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the application, the technical solutions of the application will be described in detail below, but it cannot be understood as limiting the implementable scope of the application.

[0050] The medicines used in the application are all industrial products, wherein the formaldehyde solution is an industrial product with a mass fraction of 35%.

[0051] The phenolic crosslinking agent used in the embodiments of the application is prepared as follows:

[0052] 40g of phenol and 40mL of a 20% mass fraction sodium hydroxide solution are added to a three-necked flask, stirred and the temperature is raised to 45℃, and the temperature is controlled to be not more than 45℃; 80g of formaldehyde solution is added dropwise within 30min, and the temperature is raised to 80℃, and the reaction is carried out for 1.5h; then 20g of formaldehyde solution is added to the above reaction liquid, and the temperature is raised to 90℃, and the reaction is carried out for 30min under constant temperature and stirring, to obtain the water-soluble phenolic resin crosslinking agent used in the embodiments.

[0053] The embodiments and comparative examples of the application are all taken as an example of preparing 200g of a polymer gel system.

[0054] Embodiment 1

[0055] The embodiment provides a high-strength low-molecular polymer gel system, and the raw materials thereof include, in mass percentage: 5wt% of polymer emulsion, 0.5wt% of the water-soluble phenolic resin crosslinking agent prepared by the application, 0.2wt% of ammonium chloride (catalyst), 0.3wt% of thiourea (oxygen scavenger), 0.3wt% of nano silicon (stabilizer), and the balance is water.

[0056] The preparation method of the polymer emulsion is as follows:

[0057] Into a four-necked flask equipped with a stirrer, 90 g of acrylamide monomer and 180 g of water were added, purged with nitrogen, and heated to 60°C; after stirring for 20-30 min with 0.05% by mass of emulsifier, 0.4 g of ammonium persulfate initiator was added, then 10 g of sodium p-styrenesulfonate solution was slowly added dropwise, after the dropwise addition was completed, 0.06 g of N,N,N',N'-tetramethyl ethylenediamine was added; then the remaining 0.2 g of ammonium persulfate initiator was added dropwise into the four-necked flask for mixing and stirring, and after 8 h of reaction at 60°C, the stirring was stopped and the temperature was lowered to discharge the product, to obtain a polymer emulsion.

[0058] The mass fraction of low molecular weight polymer in the prepared polymer emulsion was 32%, and the relative molecular mass of the low molecular weight polymer was 600-800 thousand.

[0059] The high-strength low-molecular-weight polymer gel system was prepared using the above polymer emulsion, and the preparation method included the following steps:

[0060] 1 g of water-soluble phenolic resin crosslinking agent was added to an appropriate amount of water and stirred uniformly, then 0.4 g of ammonium chloride and 0.6 g of thiourea were added respectively and stirred uniformly, then 0.6 g of nano-silica sol was added and stirred uniformly to obtain a solution;

[0061] The above solution was transferred to an electric stirrer, 31.25 g of polymer emulsion was slowly added to the solution at a stirring speed of 150 rpm, then water was added to make up the solution to 200 g, and the solution was stirred for 30 min to obtain a high-strength low-molecular-weight polymer gel system.

[0062] 20 g of the high-strength low-molecular-weight polymer gel system prepared in this example was injected into an ampoule, which was placed in a high-pressure tank at 60°C, 90°C, 110°C and 130°C respectively to observe the gelation time, and after gelation, it was placed in a high-temperature oven at 160°C and 180°C to observe the stability and determine the gel strength.

[0063] The gelation time of the high-strength low-molecular-weight polymer gel system prepared in Example 1 at 60°C, 90°C, 110°C and 130°C was 26 h, 15 h, 6.5 h and 4 h respectively; the viscosity of the polymer gel system of Example 1 at 30°C and a stirring speed of 7.34 s -1 was 563.45 mPa·s. After gelation, the polymer gel system was placed at 160°C and 180°C, and the elastic modulus of the polymer gel system was 123.5 Pa and 117.6 Pa respectively; after aging for 120 days, the elastic modulus was 110.24 Pa and 102.8 Pa respectively, and the polymer gel did not show dehydration.

[0064] Example 2

[0065] The high-strength low-molecular polymer gel system of the present embodiment is prepared by using the polymer emulsion of the present embodiment, and the preparation method comprises the following steps:

[0066] The preparation method of the polymer emulsion is as follows:

[0067] 90 g of acrylamide monomer and 180 g of water are added to a four-necked flask equipped with a stirrer, purged with nitrogen, and heated to 65℃; after 20-30 min of stirring after adding 0.05% of emulsifier, 0.6 g of ammonium persulfate initiator is added, then 10 g of sodium p-styrenesulfonate solution is slowly added dropwise, after the dropwise addition is completed, 0.09 g of N,N,N',N'-tetramethyl ethylenediamine is added; then the remaining 0.3 g of ammonium persulfate initiator is added dropwise to the four-necked flask for mixing and stirring, and after 8 h of reaction at 65℃, the stirring is stopped and the temperature is lowered to discharge the material, to obtain the polymer emulsion.

[0068] The mass fraction of the low-molecular polymer in the prepared polymer emulsion is 35%, and the relative molecular mass of the low-molecular polymer is 600-800 thousand.

[0069] The high-strength low-molecular polymer gel system of the present embodiment is prepared by using the polymer emulsion of the present embodiment, and the preparation method comprises the following steps:

[0070] 1 g of water-soluble phenolic resin crosslinking agent is added to an appropriate amount of water and stirred uniformly, then 0.6 g of ammonium chloride and 0.6 g of thiourea are added respectively and stirred uniformly, and then 0.6 g of nano-silica sol is added and stirred uniformly to obtain a solution;

[0071] The above solution is transferred to an electric stirrer, 31.25 g of polymer emulsion is slowly added to the solution at a speed of 150 rad / s, then water is added to make up the solution to 200 g, and the solution is stirred for 30 min to obtain 200 g of high-strength low-molecular polymer gel system.

[0072] 20 g of the high-strength low-molecular polymer gel system prepared in the present embodiment is injected into an ampoule, respectively placed in a high-pressure tank at 60℃, 90℃ and 110℃ to observe the gelation time, and after gelation, placed in a high-temperature oven at 160℃ and 180℃ to observe the stability and measure the gel strength.

[0073] The gelation time of the high-strength low-molecular polymer gel system prepared in Example 2 at 60℃, 90℃ and 110℃ is 18 h, 8 h and 1.5 h respectively; the polymer gel system of Example 2 is placed in a 30℃ water bath at a speed of 7.34 s -1The viscosity of the polymer emulsion prepared in Example 1 is 502.23 mPa-s. After gelling, the polymer gel system is placed at 160°C and 180°C, and the elastic modulus of the polymer gel system is 118.5 Pa and 108.4 Pa, respectively. After aging for 120 days, the elastic modulus is 101.24 Pa and 92.7 Pa, respectively, and the polymer gel does not dehydrate.

[0074] Comparing Example 1 with Example 2, it can be seen that increasing the amount of initiator slightly reduces the viscosity of the polymer gel system prepared, mainly because increasing the amount of initiator increases the free radicals in the polymer synthesis process, increasing the reaction active center, which reduces the molecular weight of the polymer, thereby reducing the viscosity of the system. In addition, comparing Example 1 with Example 2, the amount of catalyst ammonium chloride has a greater effect on the gelling time of the polymer gel at low temperatures, but has a smaller effect on the gelling strength after gelling.

[0075] Example 3

[0076] This example provides a high-strength low-molecular-weight polymer gel system, which includes, in mass percentage: 5 wt% of a polymer emulsion, 0.5 wt% of a water-soluble phenolic resin crosslinking agent prepared by the present application, 0.05 wt% of ammonium chloride, 0.3 wt% of thiourea, 0.3 wt% of nano-silica sol, and the balance being water. The polymer emulsion is the polymer emulsion prepared in Example 1, and the method of preparing the polymer gel is the same as in Example 1.

[0077] 20 g of the high-strength low-molecular-weight polymer gel system prepared in this example is injected into an ampoule, which is placed in a high-pressure tank at 110°C and 130°C to observe the gelling time. After gelling, the system is placed in a high-temperature oven at 160°C and 180°C to observe the stability and measure the gelling strength.

[0078] The high-strength low-molecular-weight polymer gel prepared in Example 3 has a gelling time of 18 h and 10 h at 110°C and 130°C, respectively. The viscosity of the polymer gel system of Example 3 is 559.65 mPa-s at 30°C and a rotation speed of 7.34 s -1 After gelling, the polymer gel system is placed at 160°C and 180°C, and the elastic modulus of the polymer gel system is 132.9 Pa and 118.1 Pa, respectively. After aging for 120 days, the elastic modulus is 101.24 Pa and 96.5 Pa, respectively, and the polymer gel does not dehydrate.

[0079] Comparing Example 1 with Example 3, it can be seen that adding a small amount of catalyst at a temperature higher than 110°C still provides the polymer gel with good gelling performance, and the elastic modulus after gelling is slightly improved. By comparing Example 1, Example 2, and Example 3, it can be seen that within a reasonable range of initiator amount, the low-molecular-weight polymer prepared has little effect on the gelling performance of the gel.

[0080] Example 4

[0081] The high-strength low-molecular polymer gel system of the present example is prepared by using 5wt% of the polymer emulsion, 0.8wt% of the water-soluble phenolic resin crosslinking agent prepared in the present application, 0.2wt% of ammonium chloride, 0.3wt% of thiourea, and 0.3wt% of nano-silica sol, with the balance being water.

[0082] The high-strength low-molecular polymer gel system prepared in the present example is taken in 20g and injected into an ampoule, which is then placed in a high-pressure tank at 60°C, 90°C, and 110°C, respectively, to observe the gelation time. After gelation, the system is placed in a high-temperature oven at 160°C and 180°C to observe the stability and determine the gel strength.

[0083] The high-strength low-molecular polymer gel system prepared in Example 4 has a gelation time of 19h, 11h, and 3h at 60°C, 90°C, and 110°C, respectively. The polymer gel system of Example 3 has a viscosity of 513.8mPa·s at 30°C and a rotational speed of 7.34s -1 After gelation, the system is placed in a high-temperature oven at 160°C and 180°C, and the elastic modulus of the polymer gel system is 152.2Pa and 136.5Pa, respectively. After aging for 120 days, the elastic modulus is 153.3Pa and 128.6Pa, respectively, and the polymer gel does not dehydrate.

[0084] Comparing Example 1 with Example 4, it can be seen that increasing the amount of the self-prepared water-soluble phenolic resin crosslinking agent shortens the gelation time of the system and slightly increases the elastic modulus of the polymer gel after gelation.

[0085] Example 5

[0086] The high-strength low-molecular polymer gel system of the present example is prepared by using 5wt% of the polymer emulsion, 0.8wt% of the water-soluble phenolic resin crosslinking agent prepared in the present application, 0.2wt% of ammonium chloride, 0.3wt% of thiourea, and 0.3wt% of nano-silica sol, with the balance being water.

[0087] The high-strength low-molecular polymer gel system prepared in the present example is taken in 20g and injected into an ampoule, which is then placed in a high-pressure tank at 60°C, 90°C, and 110°C, respectively, to observe the gelation time. After gelation, the system is placed in a high-temperature oven at 160°C and 180°C to observe the stability and determine the gel strength.

[0088] The high-strength low-molecular-weight polymer gel system prepared in Example 1 had gelation times of 26 h, 15 h, and 7 h at 60 °C, 90 °C, and 110 °C, respectively; the polymer gel system in Example 5 had gelation times of 30 °C and 7.34 s at a rotation speed of 7.34 s. -1 The viscosity was 556.4 mPa·s. After gelation, the elastic moduli of the polymer gel system were 130.8 Pa and 119.4 Pa at 160℃ and 180℃, respectively; after aging for 120 days, the elastic moduli were 121.3 Pa and 109.8 Pa, respectively, and the polymer gel did not show any dehydration.

[0089] Comparing Example 1 and Example 5, it can be seen that increasing the amount of the oxygen scavenger thiourea can slightly improve the strength and long-term stability of the system.

[0090] Comparative Example 1

[0091] This comparative example provides a polymer gel system that is the same as that in Example 1, except that the 5 wt% polymer emulsion in Example 1 is replaced with 2 wt% acrylamide and 2-acrylamido-2-methylpropanesulfonic acid binary copolymer AM / AMPS. The 2-acrylamido-2-methylpropanesulfonic acid AMPS monomer content in the binary copolymer accounts for 30% of the total monomer content, and the molecular weight is 6.5 million.

[0092] The tests were conducted using the same method as in Example 1. The results showed that the viscosity of the polymer gel system in this comparative example was 1873.53 mPa·s at 30°C and a rotation speed of 7.34 s⁻¹, indicating high viscosity and limited injection capacity. Furthermore, the polymer gel in this comparative example exhibited poor gelling performance at temperatures below 100°C. After continued gelling at 160°C, the elastic modulus was 18.5 Pa, and after 60 days of aging, the elastic modulus decreased to 6.75 Pa.

[0093] Compared with Example 1, it can be seen that although the viscosity of the gelling liquid is still very high when using a heat-resistant binary copolymer, the gel strength is low and the thermal stability is poor after gelation, making it unsuitable for sealing gas channeling in medium and deep parts.

[0094] Comparative Example 2

[0095] This comparative example provides a polymer gel system that is the same as that in Example 1, except that the water-soluble phenolic resin crosslinking agent in Example 1 is replaced by a combination of hydroquinone and hexamethylenetetramine, wherein hydroquinone is 0.2 wt% and hexamethylenetetramine is 0.3 wt%. Other preparation and testing were performed as in Example 1.

[0096] The phenolic combination crosslinker system has little effect on the viscosity of the gelled fluid, the gelling time of the prepared polymer gel system, but has great effect on the long-term stability and strength of the polymer gel. The polymer gel of the present comparative example has an elastic modulus of 75.5 Pa at 160°C, and after aging for 120 days, the elastic modulus of the gel decreases by 20%; the polymer gel has an elastic modulus of 63.4 Pa at 180°C, and after aging for 120 days, the elastic modulus of the gel decreases by 36%.

[0097] Comparative Example 3

[0098] The present comparative example provides a polymer gel system which is the same as that of Example 1, except that the amount of the oxygen scavenger in Example 1 is reduced to 0.1 wt%. Other preparations and tests are performed according to Example 1.

[0099] The amount of the oxygen scavenger has little effect on the viscosity of the gelled fluid, the gelling time and the initial gelling strength of the prepared polymer gel system, but has great effect on the long-term stability and strength of the polymer gel. The polymer gel of the present comparative example has an elastic modulus of 75.5 Pa at 160°C, and after aging for 120 days, the elastic modulus of the gel decreases by 20%; the polymer gel has an elastic modulus of 63.4 Pa at 180°C, and after aging for 120 days, the elastic modulus of the gel decreases by 36%.

[0100] Comparative Example 4

[0101] The present comparative example provides a polymer gel system which is the same as that of Example 1, except that the stabilizer in Example 1 is removed. Other preparations and tests are performed according to Example 1.

[0102] Comparing with the polymer gel prepared in Example 1, the polymer gel prepared in Comparative Example 3 has slightly longer gelling time at different temperatures, the viscosity of the gel at 30°C and a rotation speed of 7.34 s -1 is 581.5 mPa·s, the initial elastic modulus (aging for 1 day) of the gel placed at 160°C and 180°C is 92.3 Pa and 86.4 Pa respectively, and after aging for 30 days, the polymer gel is completely degraded. The polymer gel system of the present comparative example has poor stability.

[0103] Experimental Example 1

[0104] The present experimental example is used to evaluate the injectivity and plugging performance of the polymer gel systems prepared in Example 1 and Comparative Example 1 in the sandpack displacement experiment.

[0105] Two sandpacks with similar permeability were prepared, and the injection pressure was measured by using oilfield sewage, and the permeability of the sandpacks was about 1120 mD and 1230 mD, respectively. The polymer gel prepared in Example 1 and Comparative Example 1 was injected into the sandpacks with the permeability of about 1120 mD and 1230 mD, respectively, at a rate of 1 ml / min, and the injection pressure was recorded. Then, the sandpacks treated with the polymer gel were sealed and placed in an oven (160°C) for aging for 10 days, and the plugging effect was measured by injecting steam, and the residual resistance factor was calculated.

[0106] The experimental results are as follows: the injection equilibrium pressure of the polymer gel prepared in Example 1 is 0.23 MPa, and the injection equilibrium pressure of the polymer gel prepared in Comparative Example 1 is 0.89 MPa. After aging for 10 days, the residual resistance factor of Example 1 is 28.2, and the residual resistance factor of Comparative Example 1 is 11.4.

[0107] The sandpack treated in Example 1 was continuously placed in an oven (160°C), and the residual resistance factor was measured periodically. The residual resistance factor after aging for 30 days is 24, and the residual resistance factor after aging for 45 days is 21.4. It can be seen from the evaluation of the plugging performance of the sandpack that the high-strength temperature-resistant polymer gel prepared in Example 1 can be used for plugging steam channeling in the middle and deep parts.

Claims

1. A high-strength, low-molecular-weight polymer gel system, comprising, by weight percentage: Polymer emulsion 4-6%, phenolic crosslinking agent 0.4-0.8%, catalyst 0-0.3%, oxygen scavenger 0.2-0.5%, stabilizer 0.2-0.4%, balance water; The polymer emulsion is formed by copolymerization of acrylamide monomer and sodium p-styrene sulfonate monomer, wherein the molar amount of sodium p-styrene sulfonate monomer accounts for 8-15% of the total monomer amount; The polymer in the polymer emulsion has a relative molecular mass of 600,000 to 800,000. The phenolic crosslinking agent is a water-soluble phenolic resin crosslinking agent. The preparation method of the water-soluble phenolic resin crosslinking agent includes the following steps: under the catalysis of an alkaline catalyst, phenol and formaldehyde with a molar ratio of 1:2-3 undergo a condensation reaction to obtain the water-soluble phenolic resin crosslinking agent.

2. The high-strength low-molecular-weight polymer gel system according to claim 1, wherein, The preparation method of the polymer emulsion is as follows: Under nitrogen protection, acrylamide monomer and emulsifier are mixed in water and preheated to the polymerization reaction temperature. A portion of the initiator is added, followed by the dropwise addition of sodium p-styrene sulfonate monomer. After the dropwise addition is complete, N,N,N',N'-tetramethylethylenediamine and the remaining initiator are added to carry out a copolymerization reaction to obtain the polymer emulsion.

3. The high-strength low-molecular-weight polymer gel system according to claim 2, wherein, The amount of emulsifier used is 0.05-0.1% of the total mass of monomers; the emulsifier is a compound of anionic surfactant SDS and nonionic surfactant OP-10 in a mass ratio of 1-1.2:

1.

4. The high-strength low-molecular-weight polymer gel system according to claim 3, wherein, The mass ratio of the total mass of the monomers to the mass of water is 1:1.5-2.

5. The high-strength low-molecular-weight polymer gel system according to claim 3, wherein, The amount of the initiator is 0.5-1% of the total mass of the monomer; the initiator is ammonium persulfate.

6. The high-strength low-molecular-weight polymer gel system according to claim 3, wherein, The amount of N,N,N',N'-tetramethylethylenediamine used is 10-20% of the initiator mass.

7. The high-strength low-molecular-weight polymer gel system according to claim 2, wherein, The copolymerization reaction is carried out at a temperature of 60-65℃ for 8-10 hours.

8. The high-strength low-molecular-weight polymer gel system according to claim 1, wherein, The condensation reaction takes place at a temperature of 80-90℃ for 1.5-2 hours.

9. The high-strength low-molecular-weight polymer gel system according to claim 8, wherein, The alkaline catalyst includes at least one of sodium hydroxide, sodium carbonate, and ammonia water.

10. The high-strength low-molecular-weight polymer gel system according to claim 9, wherein, The alkaline catalyst is sodium hydroxide.

11. The high-strength low-molecular-weight polymer gel system according to claim 1, wherein, The amount of alkaline catalyst used is 4-6% of the total mass of phenol and formaldehyde.

12. The high-strength low-molecular-weight polymer gel system according to claim 1, wherein, The catalyst is acetic acid and / or ammonium chloride.

13. The high-strength low-molecular-weight polymer gel system according to claim 12, wherein, The catalyst is ammonium chloride.

14. The high-strength low-molecular-weight polymer gel system according to claim 1, wherein, The oxygen scavenger is thiourea.

15. The high-strength low-molecular-weight polymer gel system according to claim 1, wherein, The stabilizer is polypropylene fiber and / or nano-silica.

16. The high-strength low-molecular-weight polymer gel system according to claim 15, wherein, The median particle size of nano-silica is 8-15 nm.

17. The high-strength low-molecular-weight polymer gel system according to claim 15, wherein, The nano-silica is added in the form of nano-silica sol.

18. A method for preparing a high-strength low-molecular-weight polymer gel system according to any one of claims 1-17, comprising the following steps: (1) Add the phenolic crosslinking agent, catalyst, and oxygen scavenger to water in proportion and stir evenly to obtain solution A; (2) Add the stabilizer to solution A in proportion and stir until homogeneous to obtain solution B; (3) Under stirring conditions, the polymer emulsion is added to the solution B and stirred thoroughly to obtain the high-strength low-molecular polymer gel system.

19. The preparation method according to claim 18, wherein, In step (3), the stirring speed is 150-200 rpm and the stirring time is 20-30 min.

20. The application of the high-strength low-molecular-weight polymer gel system according to any one of claims 1-17 in plugging gas channeling in heavy oil reservoirs.

21. The application according to claim 20, wherein, When the formation temperature is <80℃, the raw materials of the high-strength low-molecular-weight polymer gel system include: 4-6% polymer emulsion, 0.4-0.8% phenolic crosslinking agent, 0.2-0.3% catalyst, 0.2-0.3% oxygen scavenger, 0.2-0.3% stabilizer, and the balance being water.

22. The application according to claim 20, wherein, When the formation temperature is 80-110℃, the raw materials of the high-strength low-molecular polymer gel system include: 4-6% polymer emulsion, 0.4-0.8% phenolic crosslinking agent, 0.1-0.2% catalyst, 0.3-0.4% oxygen scavenger, 0.2-0.3% stabilizer, and the balance being water.

23. The application according to claim 20, wherein, When the formation temperature is >110℃, the raw materials of the high-strength low-molecular-weight polymer gel system include: 4-6% polymer emulsion, 0.4-0.8% phenolic crosslinking agent, 0-0.1% catalyst, 0.3-0.5% oxygen scavenger, 0.3-0.4% stabilizer, and the balance being water.