Fractured-vuggy oil reservoir thick oil viscosity reduction foam system and method for improving recovery efficiency

Abstract

The invention belongs to the technical field of oil and gas field development, and particularly relates to a fracture-cavity reservoir thickened oil viscosity reduction foam system and a method for improving the recovery efficiency, the viscosity reduction foam system is composed of a foaming base solution and a gas phase in a high-salt environment, the mass concentration of the foaming base solution is 0.5%, and the mass concentration of the gas phase is 0.5%. The foaming base fluid is composed of an anionic surfactant and a zwitterionic surfactant according to the mass ratio of (1-4): (4-1), the specific component proportion needs to be determined according to the viscosity of crude oil in an oil field, and the balance is (18 * 10 < 4 >-22 * 10 < 4 >) mg / L of mineralized water. The viscosity-reducing foam system disclosed by the invention has relatively strong foaming performance and viscosity-reducing performance, after the viscosity-reducing foam system is aged for 7 days at 150 DEG C, the foaming multiple is 5.3-5.8, the liquid separation half-life period is 11 min-13 min, the viscosity-reducing rate of thick oil is 97.5%-99%, stable foaming and efficient viscosity reduction under high-temperature and high-salt conditions can be realized, and efficient exploitation of the thick oil / super thick oil of a fracture-cavity reservoir is ensured.

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-cave reservoirs have high geological reserves of heavy oil / super-heavy oil and great potential for exploitation. After multiple periods of tectonic karstification, the storage space of fracture-cave reservoirs is mainly composed of caves, cracks, and dissolution pores. This leads to the diverse distribution of fracture-cave reservoirs, complex types, and complex and uneven communication between caves, such as fault-karst bodies and residual hillocks.

[0003] Based on the temperature sensitivity of heavy oil viscosity, steam thermal recovery is one of the main development methods for high-viscosity heavy oil reservoirs. It mainly uses steam injection to reduce the viscosity of heavy oil at high temperature to increase its fluidity. However, for deeply buried and highly heterogeneous fracture-cavity reservoirs, on the one hand, steam has the problem of steam channeling in high-permeability layers and difficulty in injecting into low-permeability layers. On the other hand, fracture-cavity reservoirs are deeply buried and have high temperatures. It is generally believed that super-heavy oil has good fluidity in the formation, and steam thermal recovery is of little significance. At present, gas injection after water flooding is the most effective way to recover heavy oil in fracture-cavity reservoirs. Its principle mainly includes restoring formation pressure and activating loft oil through gravity differentiation. However, due to the low viscosity and high gas-oil mobility ratio of the gas, the affected area no longer increases after gas channeling, and a large amount of residual oil remains in the formation. As an effective way to block gas channeling channels, foam can expand the affected area of ​​subsequent gas flooding and further improve the recovery rate. At the same time, whether it is water drive or gas drive, 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 failure to produce smoothly. In order to improve the production efficiency of heavy oil, viscosity reduction production is of great significance.

[0004] Chinese patent document CN118291107A (application number CN202410217493.8) provides a preparation method and application of an in-situ self-emulsifying viscosity reducer. Sodium α-olefin sulfonate is added to the viscosity reducer, which is difficult to meet the harsh conditions of high temperature, high pressure and high mineralization of fracture-cavity reservoirs. In addition, viscosity reduction and foam plugging will greatly increase the development cost of oil fields. In this context, viscosity reduction foam provides a new idea for the development of heavy oil / super-heavy oil in fracture-cavity reservoirs. For example, the master's thesis "Experimental Study on Oil Displacement Mechanism of Viscosity Reduction Foam System in Block Z" (Qin Haoliang, Shandong, China University of Petroleum (East China), 2022) studied a KN-3 viscosity reduction foam system, which can reduce the viscosity of heavy oil while blocking the crossflow channel, but the system has good foaming performance only within 120°C, and the high viscosity reduction rate of the viscosity reducer is limited to 70-90°C and a water-oil ratio of 3:7, which cannot play a viscosity reduction effect in the complex fluid system in the wellbore.

[0005] Therefore, it is of great significance to prepare a temperature-resistant and salt-resistant viscosity-reducing foam system with the dual properties of stable foaming and effective viscosity reduction 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 a production method. The foam system achieves stable foaming and efficient viscosity reduction under high temperature and high salt 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 the viscosity of heavy oil in fracture-cavity reservoirs comprises 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 foam system for reducing the viscosity of heavy oil in fracture-cavity reservoirs was 5.3-5.8, the liquid separation half-life was 11min-13min, and the viscosity reduction rate of the heavy oil was 97.5%-99%.

[0012] The present invention also provides a method for preparing 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 condition is stirring at a speed of 500 rpm-1000 rpm for 30 min-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 generate foam.

[0017] In the foam system for reducing the viscosity of heavy oil in fracture-cavity reservoirs provided by the present invention, the anion (-COO - ) groups are ionized in the solution, giving the molecule a negative charge, which generates electrostatic repulsion when adsorbed at the gas-liquid interface, thereby inhibiting the molecules from being closely arranged, resulting in poor foaming performance. + The (CH3)3 group can combine with the anion end of APEC-15Na through electrostatic attraction to form an ion pair, significantly reducing the electrostatic repulsion between molecules. At the same time, the long-chain alkyl of BS-12 can fill the gaps between the hydrophobic chains of the benzene ring of APEC-15Na, enhance the hydrophobicity of the foam liquid film, and thus improve the strength and stability of the foam film. After the two are compounded, the molecules are arranged more closely at the gas-liquid interface, forming a dense and stable foam liquid film, which effectively improves the foaming performance and foam stabilization performance of the system in a high-salt environment.

[0018] In addition, the nonionic polyoxyethylene (EO) chain of the anionic surfactant APEC-15Na can combine with water through hydrogen bonds to form an oil-in-water emulsion, which has a good viscosity reduction effect. The long-chain alkyl hydrophobic tail of BS-12 can be inserted into the oil phase, while its zwitterionic head group is stabilized in the water phase through electrostatic action. The combination of the two forms a denser interfacial film, significantly improving the stability of the emulsion, thereby ensuring a good viscosity reduction effect in the wellbore and ensuring the smooth recovery of crude oil.

[0019] However, under high temperature conditions, the EO chain of APEC-15Na undergoes dehydration shrinkage, and excessive shrinkage will reduce its wettability to heavy oil. In contrast, the hydrophobic chain of BS-12 stretches at high temperatures and synergizes with the hydrophobic chain of APEC-15Na to form a dynamic interfacial film. This synergistic effect not only enhances the adsorption capacity, but also effectively maintains the wettability. After the two are compounded, they can show excellent foaming performance and good viscosity reduction effect in high temperature and high salt environment.

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

[0021] Water flooding and gas flooding are carried out alternately for fracture-cavity reservoirs. During the flooding process, the viscosity of the produced crude oil is monitored. The ratio of anionic surfactants and zwitterionic surfactants in the foaming base fluid of the heavy oil viscosity reduction foam system of fracture-cavity reservoirs is determined according to the viscosity of the produced crude oil. Formation water is used in the first round of water flooding, and the water used in subsequent rounds of water flooding 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 10000mPa·s, viscosity reduction foam flooding is carried out until the end of production.

[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 scheme; 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 reasonably proportioned according to the viscosity range of heavy oil, which can effectively reduce the viscosity of crude oil and maximize 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 crude oil is gradually reduced, thereby reducing the difficulty of mining. In the water-gas alternating injection stage, the surfactant carried in the foaming base liquid generates foam under the shearing action of the gas, and forms a local blockage in the fracture-cavity 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 reservoir on the one hand, and reduces the viscosity of crude oil on the other hand, maintaining its good fluidity, so that it can be smoothly extracted from the wellbore, significantly improving the crude oil recovery rate of the fracture-cavity 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 is in the range of 10000mPa·s-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 50000mPa·s-100000mPa·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 100000 mPa·s-150000 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 a foam generator to generate foam.

[0030] Preferably, water drive injects 0.1PV-0.2PV each time, and the injection rate is 1mL / min; gas drive injects 0.1PV-0.2PV each time, and the injection rate is 1mL / min; viscosity reduction foam drive injects 1.5PV, and the injection rate is 1mL / 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. In the fracture-hole reservoir heavy oil viscosity reduction foam system provided by the present invention, the anionic surfactant and the zwitterionic surfactant are used to exert their synergistic effect in salt resistance and temperature resistance, so that the fracture-hole reservoir heavy oil viscosity reduction foam system can be used at 150°C and a salinity of 18×10 4 -22×10 4 mg / L conditions, it still maintains good foaming performance and foam stabilization performance.

[0034] 2. The present invention utilizes the synergistic effect of anionic surfactants and zwitterionic surfactants in emulsification and viscosity reduction, and the obtained viscosity reduction foam system still has a viscosity reduction rate of 97.5%-99% after aging for 7 days at 150°C, which can effectively improve the fluidity of high-viscosity crude oil and achieve long-term sustainable viscosity reduction, so that the heavy oil in the wellbore can be smoothly produced.

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

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

[0037] Second, during the water-gas alternation stage, the foaming base fluid is injected. First, the early intervention of the viscosity reduction system can effectively reduce the viscosity of the formation crude oil in advance; second, the injected gas and the viscosity reduction foam system are sheared to achieve the in-situ generation of viscosity reduction foam, improve gas mobility, and enhance the gas drive effect; third, the water-gas alternation injection method can make the viscosity reducer contact and dissolve with the formation crude oil multiple times to improve the distribution balance of the viscosity reducer in the reservoir; fourth, the excellent temperature resistance, salt resistance, and aging resistance of the viscosity reduction foam system enable it to have a long-term sustainable viscosity reduction ability, so as to ensure that its viscosity reduction effect can still be maintained in the wellbore and ensure the smooth recovery of crude oil;

[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 This is the viscosity reduction effect diagram of the foam system for reducing the viscosity of heavy oil in fracture-cavity reservoirs with different surfactant ratios. The original viscosity of the crude oil is 8030 mPa·s.

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

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

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

[0043] Figure 5 To visualize the recovery curves of each stage after water flooding, gas flooding and foam flooding of the fracture-cavity model. DETAILED DESCRIPTION

[0044] The present invention will be further described below in conjunction with embodiments and drawings.

[0045] The raw materials or instruments used in the embodiments, comparative examples and experimental examples are all existing commercially available raw materials or instruments, such as APEC-15Na purchased from Zibo Yonghong New Materials Co., Ltd., and the structural formula is as follows:

[0046]

[0047] In formula I, n=15;

[0048] BS-12 was purchased from Beijing Inokai 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 In the mineralized water with a concentration of 100 mg / L, the dosage of NaCl is 102 g / L, the dosage of CaCl2 is 84.8 g / L, and the dosage of MgCl2·6H2O is 71.1 g / 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 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 temperature of 800 mg / L.

[0055] (2) The foaming base liquid was stirred at 8000 r / min for 3 min in a high-speed stirrer 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 measuring cylinder and the timing was started to record the volume of foam generated and the time required for half of the liquid in the foam to precipitate.

[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 effusion half-life under mg / L conditions are shown in Table 1;

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

[0058]

[0059]

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

[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, 0.4 wt% APEC-15Na and 0.1 wt% BS-12 was prepared in 100 mL of mineralized water with a concentration of 1 mg / L;

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

[0065] (3) The foaming base liquid is stirred at 8000 r / min for 3 min in a high-speed stirrer to form foam. All the foam is poured into a 500 mL measuring cylinder and the timing is started. The volume of foam generated and the time required for half of the liquid in the foam to precipitate are 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 temperature resistance is good, which can adapt to the conditions of high temperature and high mineralization fracture-cavity oil reservoirs.

[0069] Experimental Example 3

[0070] Test on the aging viscosity reduction performance of the foam system for heavy oil viscosity reduction in fracture-cavity reservoirs

[0071] (1) At a mineralization of 22×10 4 A foaming base solution 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, 0.4wt% APEC-15Na and 0.1wt% BS-12 was prepared in mineral water with a concentration of 100 mg / L;

[0072] (2) placing the foaming base liquid in an oven at 150°C and taking 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 reducing the viscosity of heavy oil in fracture-cavity reservoirs complies with the Chinese petroleum and natural gas industry standard SY 17118-2018. Add 210.00 g ± 1.00 g of on-site crude oil and 90.42 g ± 1.01 g of foaming base liquid sample into a 500 ml low-profile beaker. Place a two-blade stirring paddle on a constant-speed electric stirrer into the low-profile beaker containing the above mixture, with the stirring paddle blade 2.5-3 cm from the bottom of the beaker. Stir the mixture in the low-profile beaker at a speed of 400 r / min for 1 min to obtain a water-in-oil system;

[0075] (5) The viscosity of the water-in-oil 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 the foaming base liquid is as follows: Figure 1-Figure 4 As shown in the figure, the polyoxyethylene (EO) chain of APEC-15Na combines with water through hydrogen bonds to form a hydration layer to wrap the oil droplets, thereby reducing the viscosity of the heavy oil; while BS-12 basically has no viscosity reduction ability, but the long-chain alkyl of BS-12 and the alkylphenol hydrophobic chain (rigid benzene ring) of APEC-15Na fill each other, which can stabilize the emulsification ability of APEC-15Na and maintain 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 reservoir

[0078]

[0079]

[0080] According to the viscosity reduction evaluation experiment, the elastic viscosity reduction range of the fracture-hole reservoir heavy oil viscosity reduction foam system is summarized as shown in Table 3. According to the viscosity of crude oil, the appropriate surfactant ratio is selected for compounding, which can not only achieve a good viscosity reduction effect, but also save raw material costs. The excellent high-salt and high-temperature aging resistance of the fracture-hole reservoir heavy oil viscosity reduction foam system enables it to have a long-term sustainable viscosity reduction ability to ensure that the emulsion still has good fluidity in the wellbore and ensure the smooth recovery of crude oil;

[0081] Example 1

[0082] A foam system for reducing 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 the foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs.

[0087] Example 2

[0088] A foam system for reducing 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 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 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-cavity reservoirs.

[0099] Comparative Example 1

[0100] A viscosity reducing foam system, which is different 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, which is different 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 is different from Example 1 in that only anionic surfactant APEC-15Na is added.

[0105] Comparative Example 4

[0106] A viscosity reducing foam system, which is different 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, 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 Half-life of dialysis / s 739 703 712 127 80 669 645

[0111] It can be seen from the data in Table 4 that the foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs prepared in Example 1 can be used in a high temperature environment of 150°C and a temperature of 22×10 4Under the condition of high mineralization of 1000 mg / L, it has good foaming performance and foam stabilization performance, which can effectively ensure its plugging effect in fracture-cavity reservoirs. In Comparative Examples 1 and 2, anionic surfactant APEC-10Na and zwitterionic surfactant BS-14 were used to conduct experiments at 150°C, respectively. Compared with Example 1, the foaming effect is much worse and the temperature resistance is poor. In Comparative Examples 3 and 4, only one surfactant was used to conduct experiments at 150°C. The foaming effect is lower than that of Example 1 in terms of foaming volume and liquid separation half-life, indicating that the compound of anionic surfactant and zwitterionic surfactant has a synergistic effect.

[0112] Experimental Example 5

[0113] Test on viscosity reduction performance of heavy oil foam system in fracture-cavity reservoir

[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, quartz sand was mixed in a certain ratio to prepare a sand core model. Its 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) According to the viscosity range of produced oil and the data in Table 3, the surfactant ratio was selected. 0.05 wt% of surfactant was added to the water for the subsequent 0.2 PV water flooding to achieve pre-viscosity reduction of 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 10000 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 factor in Table 5, 56% is the recovery factor after five water-gas alternations, and 21% is the recovery factor of the viscosity-reducing foam flooding.

[0124] As shown in Table 5, during the water-gas alternation stage, the foaming base fluid is injected. On the one hand, the early intervention of the foaming base fluid can effectively reduce the viscosity of the formation crude oil. On the other hand, the injected gas and the foaming base fluid shear to achieve the in-situ generation of viscosity-reducing foam, locally block the high-permeability channel, and enhance the effect of water-gas alternation injection. After the displacement process, the viscosity of the crude oil is lower, the viscosity reduction effect is better and the cost is saved.

[0125] Experimental Example 6

[0126] Plugging performance test of 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, and the specific steps were as follows:

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

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

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

[0131] The recovery factor curve is as follows Figure 4 shown by Figure 4 It can be seen that due to the significant control effect of gravity differentiation on fluid distribution in the reservoir, the injected water preferentially spreads to the bottom of the reservoir under the action of gravity, while the injected gas can only displace the remaining oil at the top of the reservoir and produce gas channeling in the high permeability channel near the right production well, and the gas drive no longer produces oil. However, the foam, with its mobility control characteristics and density adjustability, can effectively block the gas channeling advantage channel, mobilize the remaining oil in the middle of the reservoir, and thus greatly improve the crude oil recovery of the left production well. In addition, foam drive exhibits the characteristics of gas drive and surfactant drive to a certain extent. After the foam bursts, liquid phase and gas phase will be generated. The residual surfactant in the liquid phase can reduce the interfacial tension of crude oil, promote oil-water emulsification and viscosity reduction, and further increase the oil washing efficiency. The separated gas phase can replace the remaining oil in the high position again, greatly improving the recovery 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, water drive and gas drive are alternately carried out on the fracture-cavity oil reservoir, and the viscosity of the produced crude oil is monitored during the drive process. The ratio of anionic surfactant and zwitterionic surfactant in the foaming base liquid of the heavy oil viscosity reduction foam system of the fracture-cavity oil reservoir is determined according to the viscosity of the produced crude oil, wherein the first round of water drive uses formation water, and 0.05wt% of surfactant is added to the water of the subsequent 0.2PV water drive. When the viscosity of the produced crude oil is less than 10000mPa·s, the viscosity reduction foam drive is carried out until the production is completed;

[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 injected at 0.2 PV each time, and the injection rate was 1 mL / min; 0.2 PV, and the injection rate was 1 mL / min; a total of 5 rounds of water-gas alternation were injected, and the injection rate of the viscosity-reducing foam flooding was 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 the conventional tertiary recovery method, and carry out water drive, gas drive and viscosity reduction foam drive in sequence, wherein the water drive uses a total of 1PV of water, the gas drive injects a total of 1PV of gas, and finally the viscosity reduction foam drive is carried out until the production end point of method a is reached;

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

[0141] Table 6 Oil recovery rate of different mining methods

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

[0143] As shown in Table 6, the final crude oil recovery rate of the mining method using alternating water and gas injection and adding a small amount of foaming base fluid during the water flooding process is higher. This is because adding a small amount of foaming base fluid during the water flooding process can, on the one hand, pre-reduce the viscosity of the crude oil in the reservoir, reduce the viscosity of the crude oil, increase the fluidity of the crude oil, and make the heavy oil easier to be recovered; on the other hand, the small amount of foaming base fluid added during the water flooding process is injected into the gas shear in-situ foaming during the subsequent gas flooding, and local plugging is performed in the reservoir, which improves the gas flooding effect and greatly improves the crude oil recovery rate. In addition, method a uses a small amount of foaming base fluid to achieve the effect of greatly improving the crude oil recovery rate, which is both economical and efficient.

Claims

1. A foam system for reducing viscosity of heavy oil in fracture-cavity 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 the surfactant is 0.5%. 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 gas phase is air or nitrogen. After aging at 150°C for 7 days, the foaming multiple of the foam system for reducing the viscosity of heavy oil in fracture-cavity reservoirs was 5.3-5.8, the liquid separation half-life was 11min-13min, and the viscosity reduction rate of the heavy oil was 97.5%-99%.

2. A method for preparing a foam system for reducing the viscosity of heavy oil in fracture-cavity reservoirs, characterized in that: Add surfactant into mineral water and stir to obtain foam base liquid. Foam the foam base liquid to obtain the foam system for reducing the viscosity of heavy oil in fracture-cavity 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, characterized in that: 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 generate foam.

4. A method for improving crude oil recovery, characterized in that: The steps include: Water flooding and gas flooding are carried out alternately for fracture-cavity reservoirs. During the flooding process, the viscosity of the produced crude oil is monitored. The ratio of anionic surfactants and zwitterionic surfactants in the foaming base fluid of the heavy oil viscosity reduction foam system of fracture-cavity reservoirs is determined according to the viscosity of the produced crude oil. Formation water is used in the first round of water flooding, and the water used in subsequent rounds of water flooding 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 10000mPa·s, viscosity reduction foam flooding is carried out until the end of production. The viscosity reducing foam system and foaming base liquid for heavy oil in fracture-cavity reservoirs are both the viscosity reducing foam system and foaming base liquid for heavy oil in fracture-cavity reservoirs provided in claim 1 or any one of claims 2-3; 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.

5. The method for improving crude oil recovery as claimed in claim 4, characterized in that: 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 is in the range of 10000mPa·s-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 50000mPa·s-100000mPa·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 100000 mPa·s-150000 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 as claimed in claim 3, characterized in that: The foaming conditions of the foam system for reducing viscosity of heavy oil in fracture-cavity reservoirs are that the foaming base liquid and nitrogen or air are introduced into the foam generator to generate foam.

7. The method for improving oil recovery as claimed in claim 3, characterized in that: Each time water drive is performed, 0.1PV-0.2PV of formation water is injected; each time gas drive is performed, 0.1PV-0.2PV of nitrogen or air is injected.

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