A foam system for reducing viscosity of heavy oil in fracture-cavity oil reservoirs and a method for improving recovery

Abstract

The present invention belongs to the technical field of oil and gas field development, and specifically relates to a foam system for reducing the viscosity of heavy oil in a fracture-cavity oil reservoir and a method for improving oil recovery. The foam system comprises a foaming base liquid and a gas phase in a high-salt environment, wherein the mass concentration of the foaming base liquid is 0.5%, and the foaming base liquid comprises an anionic surfactant and a zwitterionic surfactant in a mass ratio of (1-4):(4-1), wherein the specific proportion of the components is determined according to the viscosity of the crude oil at the oil field, and the remainder is (18×10 4 ‑22×10 4 ) mg / L of mineralized water. The viscosity-reducing foam system of the present invention has strong foaming and viscosity-reducing properties. After aging at 150°C for 7 days, the foaming multiple is 5.3-5.8, the liquid separation half-life is 11-13 minutes, and the viscosity reduction rate for heavy oil is 97.5%-99%. It can achieve stable foaming and efficient viscosity reduction under high temperature and high salinity conditions, ensuring the efficient recovery of heavy oil and extra-heavy oil in fracture-cavity reservoirs.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas field development, in particular to a foam system for reducing the viscosity of heavy oil in fracture-cavity oil reservoirs and a method for improving recovery. Background Art

[0002] Fracture-cavity reservoirs hold significant reserves of heavy / ultra-heavy oil and possess significant potential for recovery. After multiple phases of tectonic karstification, the reservoir spaces of these reservoirs are primarily composed of caves, fractures, and dissolution pores. This results in diverse spatial distribution patterns, complex types, and uneven connectivity between caves and fractures, typically characterized by fault-karst bodies and residual hillocks.

[0003] Due to the temperature sensitivity of heavy oil viscosity, steam thermal recovery is currently a primary development method for highly viscous oil reservoirs. Steam injection is primarily used to reduce the viscosity of heavy oil at high temperatures, thereby increasing its fluidity. However, for deeply buried, highly heterogeneous, fractured-cavity reservoirs, steam generation presents challenges, on the one hand, with channeling through high-permeability zones and difficulty injecting into low-permeability zones. On the other hand, due to the deep burial and high temperatures of fractured-cavity reservoirs, it is generally believed that ultra-heavy oil has good fluidity within the formation, making steam thermal recovery of limited value. Currently, post-water flooding gas injection is the most effective method for recovering heavy oil from fractured-cavity reservoirs. Its principle involves restoring formation pressure and activating attic oil through gravity separation. However, due to the low viscosity and high gas-oil mobility ratio of the gas, gas channeling can lead to a limited expansion of the flooding envelope, leaving a large amount of residual oil in the formation. Foam, as an effective method for sealing gas channeling pathways, can expand the flooding envelope of subsequent gas flooding and further enhance recovery. At the same time, whether using water or gas flooding, during the lifting process, as the wellbore temperature decreases, the viscosity of heavy oil gradually increases, and it loses its fluidity at around 3000m, which can easily lead to wellbore blockage and prevent smooth production. To improve the efficiency of heavy oil production, viscosity reduction is of great significance.

[0004] Chinese patent document CN118291107A (application number CN202410217493.8) provides a method for preparing and applying an in-situ self-emulsifying viscosity reducer. This viscosity reducer, containing sodium α-olefin sulfonate, struggles to meet the demanding conditions of high temperature, high pressure, and high salinity in fracture-cavity reservoirs. Furthermore, viscosity reduction and foam plugging significantly increase oilfield development costs. Against this backdrop, viscosity-reducing foams offer a new approach to developing heavy and extra-heavy oil in fracture-cavity reservoirs. For example, the master's thesis "Experimental Study on the Oil Displacement Mechanism of a Viscosity-Reducing Foam System in Block Z" (Qin Haoliang, Shandong, China University of Petroleum (East China), 2022) investigated a KN-3 viscosity-reducing foam system that simultaneously blocks crossflow channels and reduces the viscosity of heavy oil. However, this system exhibits good foaming properties only within 120°C, and its high viscosity reduction rate is limited to temperatures between 70 and 90°C and a water-oil ratio of 3:7, making it ineffective in the complex fluid system found in wellbores.

[0005] Therefore, the preparation of a temperature-resistant and salt-resistant viscosity-reducing foam system with the dual properties of stable foaming and effective viscosity reduction is of great significance for improving the recovery rate of heavy oil / super heavy oil in fracture-cavity reservoirs. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art and provide a fracture-cavity oil reservoir viscosity reduction foam system and production method. The foam system achieves stable foaming and efficient viscosity reduction under high temperature and high salinity conditions through the compounding of surfactants. At the same time, the production method provided by the present invention can significantly improve the crude oil recovery rate.

[0007] In order to achieve the above technical effects, the present invention adopts the following technical solutions:

[0008] A foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs, comprising a foaming base liquid and a gas phase, wherein the foaming base liquid is a mineralized aqueous solution of a surfactant.

[0009] The mass concentration of the surfactant is 0.5wt%, and the surfactant is composed of anionic surfactant and zwitterionic surfactant in a mass ratio of (1-4): (4-1). The mineralization degree of the mineralized water is (18×10 4 -22×10 4 )mg / L,

[0010] The anionic surfactant is alkylphenol polyoxyethylene ether carboxylate sodium (APEC-15Na), the zwitterionic surfactant is dodecyl dimethyl betaine (BS-12), and the gas phase is air or nitrogen.

[0011] After aging at 150°C for 7 days, the foaming multiple of the heavy oil viscosity reduction foam system for fracture-cavity reservoirs is 5.3-5.8, the liquid separation half-life is 11min-13min, and the viscosity reduction rate of the heavy oil is 97.5%-99%.

[0012] The present invention also provides a preparation method of a foam system for reducing the viscosity of heavy oil in fracture-hole oil reservoirs, comprising adding anionic surfactant and zwitterionic surfactant into mineralized water and stirring to obtain a foaming base liquid, and foaming the foaming base liquid to obtain the foam system for reducing the viscosity of heavy oil in fracture-hole oil reservoirs.

[0013] In the foaming base liquid, the mass concentration of the surfactant is 0.5wt%, and the surfactant is composed of anionic surfactant and zwitterionic surfactant in a mass ratio of (1-4): (4-1). The mineralization degree of the mineralized water is (18×10 4 -22×10 4 )mg / L,

[0014] The anionic surfactant used was alkylphenol polyoxyethylene ether carboxylate sodium (APEC-15Na), the zwitterionic surfactant used was dodecyl dimethyl betaine (BS-12), and the mineralization degree of the mineralized water was (18×10 4 -22×10 4 )mg / L.

[0015] Preferably, the stirring and mixing conditions are stirring at a speed of 500 rpm to 1000 rpm for 30 min to 60 min;

[0016] The foaming condition is to pass the foaming base liquid and air or nitrogen into a foam generator to generate foam, or the foaming condition is to stir the foaming base liquid at a speed of 8000 rpm for 3-5 minutes under air or nitrogen conditions to foam.

[0017] In the fracture-hole reservoir heavy oil viscosity reducing foam system provided by the present invention, the anion (-COO - ) group ionizes in the solution, giving the molecule a negative charge, which generates electrostatic repulsion when adsorbed at the gas-liquid interface, thereby inhibiting the close arrangement of molecules and resulting in poor foaming performance. + The (CH3)3 group binds to the anionic end of APEC-15Na through electrostatic attraction, forming an ion pair and significantly reducing intermolecular electrostatic repulsion. Simultaneously, the long-chain alkyl group of BS-12 fills the gaps between the hydrophobic benzene rings of APEC-15Na, enhancing the hydrophobicity of the foam film and thus improving its strength and stability. The combination of the two creates a tighter arrangement of molecules at the gas-liquid interface, forming a dense and stable foam film, effectively improving the foaming and stabilization properties of the system in high-salt environments.

[0018] Furthermore, the nonionic polyoxyethylene (EO) chains of the anionic surfactant APEC-15Na bind to water through hydrogen bonds, forming an oil-in-water emulsion with excellent viscosity reduction. The long-chain alkyl hydrophobic tail of BS-12 can intercalate into the oil phase, while its zwitterionic head group stabilizes the aqueous phase through electrostatic interactions. The combination of these two forms a denser interfacial film, significantly improving the stability of the emulsion and ensuring excellent viscosity reduction in the wellbore, ensuring smooth crude oil recovery.

[0019] However, at high temperatures, the EO chains of APEC-15Na undergo dehydration and contraction, and excessive contraction reduces its wettability with heavy oil. In contrast, the hydrophobic chains of BS-12 stretch at high temperatures and synergize with those of APEC-15Na to form a dynamic interfacial film. This synergistic effect not only enhances adsorption capacity but also effectively maintains wettability. The combination of the two exhibits excellent foaming properties and good viscosity reduction in high-temperature, high-salt environments.

[0020] The present invention also provides a method for improving crude oil recovery, comprising the following steps:

[0021] Fracture-cavity reservoirs are subjected to alternating water and gas flooding. The viscosity of the produced crude oil is monitored during the flooding process. The ratio of anionic surfactants to zwitterionic surfactants in the foaming base fluid of the heavy oil viscosity reduction foam system for fracture-cavity reservoirs is determined based on the viscosity of the produced crude oil. Formation water is used in the first water flooding round, and the water used in subsequent water flooding rounds is composed of foaming base fluid and formation water in a mass ratio of 1:9. When the viscosity of the produced crude oil is less than 10,000 mPa·s, viscosity reduction foam flooding is performed until production is terminated.

[0022] The viscosity-reducing foam system and foaming base fluid for heavy oil in fracture-cavity reservoirs are both the viscosity-reducing foam system and foaming base fluid for heavy oil in fracture-cavity reservoirs provided by the above-mentioned scheme; and the viscosity-reducing foam flooding is to inject the viscosity-reducing foam system for heavy oil in fracture-cavity reservoirs into the fracture-cavity reservoirs for displacement.

[0023] In the method provided by the present invention, anionic surfactants and amphoteric surfactants are rationally proportioned according to the viscosity range of heavy oil, which not only effectively reduces the viscosity of crude oil but also maximizes economic benefits. In addition, during multiple rounds of water-gas alternating displacement, a small amount of foaming base liquid is added each time the water drive is injected to pre-reduce viscosity. By injecting a small amount multiple times, the viscosity of the crude oil is gradually reduced, thereby reducing the difficulty of mining. During the water-gas alternating injection stage, the surfactant carried in the foaming base liquid generates foam under the shear action of the gas, and forms a local blockage in the fracture-cavity oil reservoir, expanding the scope of gas drive. In the viscosity-reducing foam drive stage, the foam blocks the high-permeability channels in the fracture-cavity oil reservoir on the one hand, and on the other hand reduces the viscosity of the crude oil, maintaining its good fluidity, thereby smoothly extracting it from the wellbore, significantly improving the crude oil recovery rate of the fracture-cavity oil reservoir.

[0024] Preferably,

[0025] When the viscosity of crude oil is less than 10000 mPa·s, the mass ratio of anionic surfactant to zwitterionic surfactant in the foaming base liquid is 1:4;

[0026] When the viscosity of crude oil ranges from 10000mPa·s to 50000mPa·s, the mass ratio of anionic surfactant to zwitterionic surfactant in the foaming base liquid is 2:3;

[0027] When the viscosity of crude oil is in the range of 50,000 mPa·s-100,000 mPa·s, the mass ratio of anionic surfactant to zwitterionic surfactant in the foaming base liquid is 3:2;

[0028] When the viscosity of crude oil is in the range of 100,000 mPa·s-150,000 mPa·s, the mass ratio of anionic surfactant to zwitterionic surfactant in the foaming base liquid is 4:1.

[0029] Preferably, the foaming condition of the above-mentioned fracture-cavity reservoir heavy oil viscosity reduction foam system is to pass the foaming base liquid and air or nitrogen into the foam generator to generate foam.

[0030] Preferably, water flooding is injected at a rate of 0.1 PV to 0.2 PV at a time and at a rate of 1 mL / min; gas flooding is injected at a rate of 0.1 PV to 0.2 PV at a time and at a rate of 1 mL / min; and viscosity-reducing foam flooding is injected at a rate of 1.5 PV at a time and at a rate of 1 mL / min.

[0031] Preferably, the water used for water flooding is formation water, and the air or nitrogen is used for gas flooding.

[0032] The beneficial effects of the present invention are as follows:

[0033] 1. The present invention provides a foam system for reducing viscosity of heavy oil in fracture-hole reservoirs. By utilizing the synergistic effect of anionic surfactants and zwitterionic surfactants in salt resistance and temperature resistance, the foam system for reducing viscosity of heavy oil in fracture-hole reservoirs can be used at 150°C and a salinity of 18×10 4 -22×10 4 mg / L, it still maintains good foaming performance and foam stabilization performance.

[0034] 2. The present invention utilizes the synergistic emulsification and viscosity reduction effects of anionic surfactants and zwitterionic surfactants. The resulting viscosity-reducing foam system achieves a viscosity reduction rate of 97.5%-99% after aging for 7 days at 150°C. This can effectively improve the fluidity of highly viscous crude oil and achieve long-term sustainable viscosity reduction, allowing for smooth recovery of heavy oil in the wellbore.

[0035] 3. The present invention innovatively couples viscosity reducer and viscosity reducing foam technology, which is divided into three key stages:

[0036] 1. According to the viscosity of the crude oil in the oil field, the mass concentration of the viscosity reducing foam system is determined based on the criteria of low economic benefit, high viscosity reducing effect and strong foaming performance;

[0037] Second, during the water-gas alternation phase, the foaming base fluid is injected concurrently. First, the early intervention of the viscosity reduction system can effectively reduce the viscosity of the formation crude oil. Second, the injected gas and the viscosity reduction foam system shear to generate the viscosity reduction foam in situ, improving gas mobility and enhancing the gas flooding effect. Third, the water-gas alternation injection method allows the viscosity reducer to contact and dissolve with the formation crude oil multiple times, thereby improving the distribution balance of the viscosity reducer in the reservoir. Fourth, the excellent temperature, salt, and aging resistance of the viscosity reduction foam system gives it long-term sustainable viscosity reduction ability, ensuring that its viscosity reduction effect can be maintained in the wellbore and ensuring smooth crude oil recovery.

[0038] 3. During the viscosity-reducing foam injection stage, the crossflow channels are blocked and the viscosity of the crude oil is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The viscosity reduction effect of the foam system for heavy oil in fracture-cavity reservoirs with different surfactant ratios is shown. The original viscosity of the crude oil is 8030 mPa·s.

[0040] Figure 2 The viscosity reduction effect of the foam system for heavy oil in fracture-cavity reservoirs with different surfactant ratios is shown. The original viscosity of the crude oil is 43980 mPa·s.

[0041] Figure 3 The viscosity reduction effect of the foam system for heavy oil in fracture-cavity reservoirs with different surfactant ratios is shown. The original viscosity of the crude oil is 89210 mPa·s.

[0042] Figure 4 The viscosity reduction effect diagram of the foam system for heavy oil in fracture-cavity reservoirs with different surfactant ratios is shown. The original viscosity of the crude oil is 103500 mPa·s.

[0043] Figure 5 This is the recovery curve of each stage after the completion of water flooding, gas flooding and foam flooding in the visualization fracture-cavity model. DETAILED DESCRIPTION

[0044] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0045] The raw materials or instruments used in the embodiments, comparative examples and experimental examples are all commercially available raw materials or instruments. For example, APEC-15Na was purchased from Zibo Yonghong New Materials Co., Ltd. and has the following structural formula:

[0046]

[0047] In formula I, n=15;

[0048] BS-12 was purchased from Beijing Yinuokai Technology Co., Ltd. and has the following structural formula:

[0049]

[0050] Mineralized water: Prepared by adding various ions of formation water into ultrapure water according to their content, for example, 22×10 4 mg / L of mineralized water, the amount of NaCl is 102g / L, the amount of CaCl2 is 84.8g / L, and the amount of MgCl2·6H2O is 71.1g / L;

[0051] The sources of other raw materials or instruments are not described here.

[0052] Experimental Example 1

[0053] Salt resistance test of heavy oil viscosity reduction foam system in fracture-cavity reservoirs

[0054] (1) At a mineralization of 10×10 4 mg / L、18×10 4 mg / L、20×10mg / L 4 and 22×10 4 The foaming base liquids of 0.1wt% APEC-15Na and 0.4wt% BS-12, 0.2wt% APEC-15Na and 0.3wt% BS-12, 0.3wt% APEC-15Na and 0.2wt% BS-12, and 0.4wt% APEC-15Na and 0.1wt% BS-12 were prepared in 100mL mineral water with an average concentration of 1 mg / L.

[0055] (2) The foaming base liquid was stirred in a high-speed stirrer at a speed of 8000 r / min for 3 min to obtain a foam system for reducing the viscosity of heavy oil in fracture-cavity reservoirs. The foam system was poured into a 1000 mL graduated cylinder and the timer was started. The volume of foam generated and the time required for half of the liquid in the foam to precipitate were recorded.

[0056] The results of the test showed that the above four foam systems for reducing viscosity of heavy oil in fracture-cavity reservoirs with different proportions can effectively reduce the viscosity of heavy oil in fracture-cavity reservoirs with salinity of 10×10 4 , 18×10 4 , 20×10 4 , 22×10 4 The data of foaming volume and elution half-life under the conditions of mg / L are shown in Table 1;

[0057] Table 1 Foaming system and liquid separation half-life of different solutions

[0058]

[0059]

[0060] As can be seen from the data in Table 1, the foaming volume of the foaming base fluid provided by the present invention reaches more than 530 mL under low and high salinity conditions, and the liquid precipitation half-life reaches more than 660 s. However, the foaming performance and foam stability performance are better in a high-salinity environment, and it can adapt to the conditions of high-salinity fracture-cavity oil reservoirs.

[0061] Experimental Example 2

[0062] Temperature aging performance test of heavy oil viscosity reduction foam system in fracture-cavity reservoirs:

[0063] (1) At a mineralization of 22×10 4A foaming base solution of 0.1 wt% APEC-15Na and 0.4 wt% BS-12, 0.2 wt% APEC-15Na and 0.3 wt% BS-12, 0.3 wt% APEC-15Na and 0.2 wt% BS-12, or 0.4 wt% APEC-15Na and 0.1 wt% BS-12 was prepared in 100 mL of mineral water with a concentration of 1 mg / L;

[0064] (2) Place the foaming base liquid in an oven at 130°C and 150°C respectively and take it out after 7 days;

[0065] (3) The foaming base liquid was stirred at 8000 r / min for 3 min in a high-speed stirrer to form foam. All the foam was poured into a 500 mL graduated cylinder and the timer was started. The volume of foam produced and the time required for half of the liquid in the foam to precipitate were recorded.

[0066] Table 2 Temperature aging performance test

[0067] temperature 20℃ 130℃ 150℃ Maximum foaming volume / mL 565 570 560 Minimum foaming volume / mL 555 555 550 Average foaming volume / mL 560 561.25 553.75 Maximum effusion half-life / s 739 732 735 Minimum effusion half-life / s 712 702 704 Average effusion half-life / s 723.75 719.25 717.75

[0068] It can be seen from the data in Table 2 that, compared with 20°C, the foaming volume and liquid separation half-life of the foaming base fluid provided by the present invention remain relatively stable under high temperature conditions of 130°C and 150°C, and the heat resistance is good, which can adapt to the conditions of high temperature and high salinity fracture-cavity oil reservoirs.

[0069] Experimental Example 3

[0070] Viscosity reduction performance test of aging-induced viscosity reduction foam system for heavy oil in fracture-cavity reservoirs

[0071] (1) At a mineralization of 22×10 4 mg / L of mineral water to prepare foaming base solutions of 0.1wt% APEC-15Na and 0.4wt% BS-12, 0.2wt% APEC-15Na and 0.3wt% BS-12, 0.3wt% APEC-15Na and 0.2wt% BS-12, and 0.4wt% APEC-15Na and 0.1wt% BS-12;

[0072] (2) Place the foaming base liquid in a 150°C oven and take it out after 7 days;

[0073] (3) Use MCR 302 rotational rheometer to measure the viscosity-temperature curve of crude oil in situ (temperature 20-150°C);

[0074] (4) The evaluation of the foam system for viscosity reduction of heavy oil in fracture-cavity reservoirs complies with the Chinese Petroleum and Natural Gas Industry Standard SY 17118-2018. 210.00 g ± 1.00 g of field crude oil and 90.42 g ± 1.01 g of the foaming base fluid sample were added to a 500 ml low-profile beaker. A two-blade stirring paddle on a constant-speed electric stirrer was placed into the low-profile beaker containing the above mixture, with the stirring paddle blades 2.5-3 cm from the bottom of the beaker. The mixture in the low-profile beaker was stirred at a speed of 400 r / min for 1 min to obtain an oil-in-water system.

[0075] (5) The viscosity of the oil-in-water system was measured using an MCR 302 rotational rheometer. The data were recorded every 1 min until the relative deviation of adjacent viscosity values ​​was less than 1%. The test was stopped and the arithmetic mean of the adjacent data was taken as the test result.

[0076] The viscosity reduction effect of foaming base liquid is as follows Figure 1-Figure 4 As shown in the figure, the polyethylene oxide (EO) chains of APEC-15Na combine with water through hydrogen bonds to form a hydration layer that wraps the oil droplets, reducing the viscosity of the heavy oil. While BS-12 has basically no viscosity reduction ability, the long-chain alkyl groups of BS-12 and the alkylphenol hydrophobic chains (rigid benzene rings) of APEC-15Na can fill each other, stabilizing the emulsification ability of APEC-15Na and maintaining the long-term stability of the emulsion, effectively reducing the viscosity of the heavy oil and ensuring that the crude oil still has a low viscosity and good fluidity in the wellbore.

[0077] Table 3 Elastic viscosity reduction range of heavy oil viscosity reduction foam system in fracture-cavity reservoirs

[0078]

[0079]

[0080] Based on viscosity reduction evaluation experiments, the elastic viscosity reduction range of the fracture-hole reservoir heavy oil viscosity reduction foam system is summarized in Table 3. Selecting an appropriate surfactant ratio based on crude oil viscosity can achieve good viscosity reduction while saving raw material costs. The excellent high-salt and high-temperature aging resistance of the fracture-hole reservoir heavy oil viscosity reduction foam system provides long-term sustainable viscosity reduction capabilities, ensuring that the emulsion maintains good fluidity in the wellbore and ensuring smooth crude oil recovery.

[0081] Example 1

[0082] A foam system for reducing the viscosity of heavy oil in fracture-cavity reservoirs, comprising an anionic surfactant, a zwitterionic surfactant and mineralized water.

[0083] The anionic surfactant used was alkylphenol polyoxyethylene ether carboxylate sodium (APEC-15Na), the zwitterionic surfactant used was dodecyl dimethyl betaine (BS-12), and the mineralization degree of the mineralized water was 22×104 mg / L,

[0084] The mass concentration of the anionic surfactant is 0.2%, the mass concentration of the zwitterionic surfactant is 0.3%, and the balance is mineralized water.

[0085] In this embodiment, the foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs is prepared by the following method:

[0086] Anionic surfactant and zwitterionic surfactant were added into mineralized water and stirred at 500 rpm for 60 min to obtain foaming base liquid, and the solution was stirred at 8000 r / min for 3 min to obtain a foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs.

[0087] Example 2

[0088] A foam system for reducing the viscosity of heavy oil in fracture-cavity reservoirs, comprising an anionic surfactant, a zwitterionic surfactant and mineralized water.

[0089] The anionic surfactant used was alkylphenol polyoxyethylene ether carboxylate sodium (APEC-15Na), the zwitterionic surfactant used was dodecyl dimethyl betaine (BS-12), and the mineralization degree of the mineralized water was 18×10 4 mg / L,

[0090] The mass concentration of the anionic surfactant is 0.4%, the mass concentration of the zwitterionic surfactant is 0.1%, and the balance is mineralized water.

[0091] In this embodiment, the foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs is prepared by the following method:

[0092] Anionic surfactant and zwitterionic surfactant were added into mineralized water and stirred at 1000 rpm for 30 min to obtain foaming base liquid, and the solution was stirred at 8000 r / min for 3 min to obtain the foam system for reducing viscosity of heavy oil in fracture-hole reservoirs.

[0093] Example 3

[0094] A foam system for reducing the viscosity of heavy oil in fracture-cavity reservoirs, comprising an anionic surfactant, a zwitterionic surfactant and mineralized water.

[0095] The anionic surfactant used was alkylphenol polyoxyethylene ether carboxylate sodium (APEC-15Na), the zwitterionic surfactant used was dodecyl dimethyl betaine (BS-12), and the mineralization degree of the mineralized water was 20×10 4 mg / L,

[0096] The mass concentration of the anionic surfactant is 0.1%, the mass concentration of the zwitterionic surfactant is 0.4%, and the balance is mineralized water.

[0097] In this embodiment, the foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs is prepared by the following method:

[0098] Anionic surfactant and zwitterionic surfactant were added into mineralized water and stirred at 800 rpm for 45 min to obtain a foaming base liquid. The solution was stirred at 8000 r / min for 3 min to obtain a foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs.

[0099] Comparative Example 1

[0100] A viscosity-reducing foam system, which differs from Example 1 in that the anionic surfactant APEC-15Na is replaced by APEC-10Na.

[0101] Comparative Example 2

[0102] A viscosity-reducing foam system is provided, which differs from Example 1 in that the zwitterionic surfactant BS-12 is replaced by BS-14.

[0103] Comparative Example 3

[0104] A viscosity-reducing foam system, which differs from Example 1 in that only anionic surfactant APEC-15Na is added.

[0105] Comparative Example 4

[0106] A viscosity-reducing foam system, which differs from Example 1 in that only the zwitterionic surfactant BS-12 is added.

[0107] Experimental Example 4

[0108] The viscosity-reducing foam systems provided in Examples 1-3 and Comparative Examples 1-4 were aged at 150° C. for 7 days, and then taken out to measure their foaming volumes and liquid separation half-lives.

[0109] Table 4 Test results

[0110] Group Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Foaming volume / mL 565 535 555 245 155 415 515 Eluate half-life / s 739 703 712 127 80 669 645

[0111] From the data in Table 4, it can be seen that the foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs prepared in Example 1 can 4mg / L, exhibits excellent foaming and foam stabilization properties, effectively ensuring its plugging effectiveness in fracture-cavity reservoirs. However, in Comparative Examples 1 and 2, experiments conducted at 150°C using the anionic surfactant APEC-10Na and the zwitterionic surfactant BS-14, respectively, showed significantly poorer foaming performance and heat resistance than Example 1. Comparative Examples 3 and 4, conducted at 150°C using only one surfactant, showed lower foaming performance, lower foam volume, and lower liquid separation half-life compared to Example 1, demonstrating the synergistic effect of the combination of anionic and zwitterionic surfactants.

[0112] Experimental Example 5

[0113] Viscosity reduction performance test of heavy oil foam system in fracture-cavity reservoirs

[0114] The experimental temperature was maintained at 150°C, and the specific steps were as follows:

[0115] (1) According to the permeability conditions of fracture-cavity reservoirs, a sand core model was prepared by filling quartz sand in a certain ratio. The pore volume was calculated to be 69 mL. The model was fixed and placed in a saturated field crude oil at 150 °C.

[0116] (2) The sand core model was subjected to water flooding at 0.2 PV and 1 mL / min and gas flooding at 0.2 PV and 1 mL / min, and the viscosity of the produced oil was measured after cooling to room temperature;

[0117] (3) Based on the viscosity range of the produced oil and the data in Table 3, the surfactant ratio was selected. 0.05 wt% of surfactant was added to the water in the subsequent 0.2 PV water flooding to achieve a pre-viscosity reduction operation on the crude oil. The viscosity of the produced oil was measured after cooling to room temperature in conjunction with the subsequent 0.2 PV gas flooding.

[0118] (4) Repeat step (3) until the viscosity of the produced oil is less than 10,000 mPa·s after cooling to room temperature.

[0119] (5) Perform viscosity reduction foam flooding at 1.5 PV and 1 mL / min, and measure the viscosity of the produced oil after it cools to room temperature.

[0120] Table 5 Elastic viscosity reduction test of heavy oil viscosity reduction foam system in fracture-cavity reservoir

[0121]

[0122]

[0123] In the recovery factors in Table 5, 56% is the recovery factor after five water-gas alternations, and 21% is the recovery factor after viscosity-reducing foam flooding.

[0124] As shown in Table 5, during the water-gas alternating injection phase, the concurrent injection of foaming base fluid can, on the one hand, effectively reduce the viscosity of the formation crude oil in advance due to the early introduction of the foaming base fluid. On the other hand, the shearing between the injected gas and the foaming base fluid generates in situ viscosity-reducing foam, which locally blocks the high-permeability channels and enhances the effect of the water-gas alternating injection. After the displacement process, the crude oil viscosity is lower, resulting in better viscosity reduction and greater cost savings.

[0125] Experimental Example 6

[0126] Testing the plugging performance of a heavy oil viscosity reduction foam system in fracture-cavity reservoirs

[0127] A plugging experiment was conducted on the fracture-cavity reservoir heavy oil viscosity reducing foam system prepared in Example 1. The specific steps are as follows:

[0128] (1) Oil red was used to dye paraffin oil with a viscosity of 45 mPa·s, and brilliant blue was used to dye the experimental mineralized water to enhance the contrast between different fluids in the model.

[0129] (2) After the two-dimensional visual fracture-cavity model was saturated with dyed paraffin oil, water flooding at 1PV, 1mL / min, gas flooding at 1PV, 1mL / min, and viscosity-reducing foam flooding at 1.5PV, 1mL / min were started. A graduated cylinder was connected to the model outlet to measure the amount of produced fluid at each stage.

[0130] The two-dimensional visual fracture-cavity model used in this experimental example has the following characteristics: the model is set up horizontally as a whole, with injection in the middle and production on both sides. The low-permeability channel is near the production well on the left, and the high-permeability channel is near the production well on the right.

[0131] Recovery factor curve Figure 4 shown by Figure 4 It can be seen that due to the significant control effect of gravity differentiation on fluid distribution within the reservoir, injected water preferentially reaches the bottom of the reservoir under the action of gravity, while injected gas can only displace the remaining oil at the top of the reservoir and cause gas channeling in the high-permeability channel near the right production well, resulting in no further oil production. However, foam, with its mobility control properties and density adjustability, can effectively block the dominant gas channeling channel, mobilize the remaining oil in the central reservoir, and significantly increase the oil recovery factor of the left production well. In addition, foam flooding exhibits the characteristics of both gas and surfactant flooding to a certain extent. After the foam collapses, a liquid phase and a gas phase are produced. The residual surfactant in the liquid phase can reduce the interfacial tension of the crude oil, promote oil-water emulsification and viscosity reduction, and further increase the oil washing efficiency. The separated gas phase can then replace the remaining oil in the high-lying area, significantly improving the recovery factor of the remaining oil.

[0132] In summary, the foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs prepared by the present invention has good plugging performance.

[0133] Experimental Example 7

[0134] Recovery factor verification of different mining methods

[0135] There are three mining methods:

[0136] a. Using the method provided by the present invention, alternate water and gas flooding is performed on a fracture-cavity reservoir. During the flooding process, the viscosity of the produced crude oil is monitored. Based on the viscosity of the produced crude oil, the ratio of anionic surfactant and zwitterionic surfactant in the foaming base fluid of the heavy oil viscosity reduction foam system for the fracture-cavity reservoir is determined. Formation water is used for the first water flooding round, and 0.05 wt% of the surfactant is added to the water for the subsequent 0.2 PV water flooding. When the viscosity of the produced crude oil is less than 10,000 mPa·s, viscosity reduction foam flooding is performed until production is terminated.

[0137] The fracture-cavity reservoir heavy oil viscosity reducing foam system is selected from the fracture-cavity reservoir heavy oil viscosity reducing foam system or the fracture-cavity reservoir heavy oil viscosity reducing foam system prepared by the above preparation method; the viscosity reducing foam flooding is to inject the fracture-cavity reservoir heavy oil viscosity reducing foam system into the fracture-cavity reservoir for displacement after foaming;

[0138] The water flooding was performed with an injection rate of 0.2 PV at each injection, and an injection rate of 1 mL / min; the water-gas alternation injection was repeated for a total of 5 rounds. The viscosity-reducing foam flooding was performed with an injection rate of 1 mL / min, and a total of 1.5 PV of viscosity-reducing foam was injected in the viscosity-reducing foam flooding.

[0139] b. Use a conventional tertiary recovery method, performing water flooding, gas flooding, and viscosity-reducing foam flooding in sequence. Water flooding uses a total of 1PV of water, gas flooding injects a total of 1PV of gas, and finally, viscosity-reducing foam flooding until the production end point of method a is reached.

[0140] c. Carry out the same rounds of water-gas alternating flooding as in method a, followed by viscosity-reducing foam flooding until the production end point of method a is reached.

[0141] Table 6 Crude oil recovery rate of different mining methods

[0142] Mining methods Recovery rate after water flooding and gas flooding Foam flooding recovery rate a 56％ 21％ b 33％ 20％ c 42％ 23％

[0143] Table 6 shows that the method using alternating water-gas injection and adding a small amount of foaming base fluid during the water flooding process ultimately achieves a higher crude oil recovery factor. This is because the addition of a small amount of foaming base fluid during the water flooding process can, on the one hand, pre-violating the crude oil in the reservoir, reducing its viscosity and increasing its fluidity, making it easier to recover the heavy oil. On the other hand, the small amount of foaming base fluid added during the water flooding process is then injected into the gas flooding process, causing in-situ foaming and localized plugging in the reservoir, enhancing the gas flooding effect and significantly increasing the crude oil recovery factor. Furthermore, method A achieves a significant increase in crude oil recovery factor with only a small amount of foaming base fluid, making it both economical and efficient.

Claims

1. A foam system for reducing viscosity of heavy oil in fracture-hole reservoirs, characterized in that: It includes a foaming base liquid and a gas phase, wherein the foaming base liquid is a mineralized aqueous solution of a surfactant. The mass concentration of surfactant is 0.5%, and the surfactant is composed of anionic surfactant and zwitterionic surfactant in a mass ratio of (1-4): (4-1). The mineralization degree of mineralized water is (18×10 4 -22×10 4 )mg / L, The anionic surfactant is APEC-15Na, the zwitterionic surfactant is BS-12, and the gas phase is air or nitrogen. After aging at 150°C for 7 days, the foaming multiple of the heavy oil viscosity reduction foam system for fracture-cavity reservoirs is 5.3-5.8, the liquid separation half-life is 11min-13min, and the viscosity reduction rate of heavy oil is 97.5%-99%.

2. A method for preparing a foam system for reducing the viscosity of heavy oil in fracture-hole reservoirs, characterized in that: Add surfactant into mineralized water and stir to obtain foaming base liquid. Foam the foaming base liquid to obtain the viscosity reduction foam system for heavy oil in fracture-hole reservoir. In the foaming base liquid, the mass concentration of the surfactant is 0.5%, and the surfactant is composed of anionic surfactant and zwitterionic surfactant in a mass ratio of (1-4): (4-1). The mineralization degree of the mineralized water is (18×10 4 -22×10 4 )mg / L, The anionic surfactant is APEC-15Na, the zwitterionic surfactant is BS-12, and the mineralization degree of the mineralized water is (18×10 4 -22×10 4 )mg / L.

3. The method for preparing the foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs according to claim 2, wherein: The stirring and mixing conditions are stirring at a speed of 500 rpm-1000 rpm for 30 min-60 min; The foaming condition is to pass the foaming base liquid and air or nitrogen into a foam generator to generate foam, or the foaming condition is to stir the foaming base liquid at a speed of 8000 rpm for 3-5 minutes under air or nitrogen conditions to foam.

4. A method for improving crude oil recovery, characterized in that: The steps include: Fracture-cavity reservoirs are subjected to alternating water and gas flooding. The viscosity of the produced crude oil is monitored during the flooding process. The ratio of anionic surfactants to zwitterionic surfactants in the foaming base fluid of the heavy oil viscosity reduction foam system for fracture-cavity reservoirs is determined based on the viscosity of the produced crude oil. Formation water is used in the first water flooding round, and the water used in subsequent water flooding rounds is composed of foaming base fluid and formation water in a mass ratio of 1:

9. When the viscosity of the produced crude oil is less than 10,000 mPa·s, viscosity reduction foam flooding is performed until production is terminated. The viscosity-reducing foam system for heavy oil in fracture-cavity reservoirs and the foaming base liquid are both the viscosity-reducing foam system for heavy oil in fracture-cavity reservoirs and the foaming base liquid provided in claim 1 or any one of claims 2-3; and the viscosity-reducing foam flooding is to inject the viscosity-reducing foam system for heavy oil in fracture-cavity reservoirs into the fracture-cavity reservoir for displacement.

5. The method for improving crude oil recovery according to claim 4, wherein: When the viscosity of crude oil is less than 10000 mPa·s, the mass ratio of anionic surfactant to zwitterionic surfactant in the foaming base liquid is 1:4; When the viscosity of crude oil ranges from 10000mPa·s to 50000mPa·s, the mass ratio of anionic surfactant to zwitterionic surfactant in the foaming base liquid is 2:3; When the viscosity of crude oil is in the range of 50,000 mPa·s-100,000 mPa·s, the mass ratio of anionic surfactant to zwitterionic surfactant in the foaming base liquid is 3:2; When the viscosity of crude oil is in the range of 100,000 mPa·s-150,000 mPa·s, the mass ratio of anionic surfactant to zwitterionic surfactant in the foaming base liquid is 4:

1.

6. The method for improving crude oil recovery according to claim 4, wherein: The foaming conditions of the foam system for viscosity reduction of heavy oil in fracture-cavity reservoirs are as follows: a foaming base liquid and nitrogen or air are introduced into a foam generator to generate foam.

7. The method for enhancing crude oil recovery according to claim 4, wherein: Each time water drive is used, 0.1PV-0.2PV of formation water is injected; each time gas drive is used, 0.1PV-0.2PV of nitrogen or air is injected.

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