A method for reducing the miscible pressure of CO2

Abstract

The present invention relates to the technical field of CO2 flooding and improving oil recovery in oil field development, and specifically, to a method for reducing the miscible pressure of CO2. The method comprises step 1, preliminary preparation; step 2, injection; and step 3, technical assistance. In the method for reducing the miscible pressure of CO2, on the one hand, CO2 is used for oil recovery, which realizes the geological storage of CO2, reduces greenhouse gas emissions, and has positive significance for alleviating global climate change; on the other hand, by controlling injection parameters and adopting a variety of technical assistance, excessive damage to the oil reservoir is avoided, and the adverse impact on the environment is reduced. At the same time, the viscosity enhancer and the injected gas are selected to ensure good compatibility with crude oil and CO2, reducing the potential risk of environmental pollution. In addition, regular effect evaluation can timely adjust the strategy, further reduce the impact on the environment, realize the sustainable development of the oil field, and improve the protection of the environment while ensuring energy supply.

Description

Technical Field

[0001] The invention relates to the technical field of CO2 flooding and improving oil recovery in oil field development, and in particular to a method for reducing the miscible pressure of CO2. Background Art

[0002] With the continuous development of the global economy, the demand for energy continues to grow. As an important energy resource, the efficient development of oil is crucial to ensuring energy supply. However, traditional oil extraction methods face many challenges. On the one hand, with the continuous development of oil fields, reservoir pressure gradually decreases, and crude oil recovery rate decreases, and new technical means are needed to improve the recovery rate. On the other hand, the problem of global climate change is becoming increasingly serious, and reducing greenhouse gas emissions has become a top priority. As one of the main greenhouse gases, CO2 emissions have put tremendous pressure on the environment.

[0003] At present, the technology of using CO2 flooding in oil field development has been applied to a certain extent, but there are still some problems. In the process of CO2 miscible flooding, the high miscible pressure is a key problem.

[0004] Existing methods for reducing miscible pressure often have limited effects and lack systematicity and comprehensiveness. For example, although a single technical means can reduce miscible pressure to a certain extent, the effect achieved is not good. In addition, during the injection process, the conditions of the oil field may not be fully considered, which may easily lead to problems such as gas channeling and affect the oil recovery effect. In addition, there may be a lack of specificity for different reservoir types, which may lead to poor oil recovery effects in some reservoirs, and the advantages of CO2 oil recovery cannot be fully utilized. It may even cause damage to the reservoir due to improper operation, thereby affecting the overall development efficiency and sustainability of the oil field.

[0005] Therefore, there is an urgent need for a method with better effectiveness and adjustability to reduce the CO2 miscibility pressure, improve the efficiency and sustainability of oil field development, and improve the shortcomings of existing technologies. Summary of the invention

[0006] The object of the present invention is to provide a method for reducing the mixed phase pressure of CO2 to solve the problems raised in the above background technology.

[0007] To achieve the above object, the present invention provides a method for reducing the mixed phase pressure of CO2, comprising the following steps:

[0008] S1.1. Preliminary preparation: prepare CO2 and dissolved oil mainly composed of C3, and collect various data of the oil field, including reservoir, porosity, permeability, formation pressure, temperature, fracture network scale and inter-well heterogeneity;

[0009] S1.2, injection: adopt multi-point injection method, inject CO2 gas into the oil reservoir through injection equipment, control the injection rate, and then adopt one of the segmented injection method and synchronous injection method to inject C3 dissolved oil into the oil reservoir;

[0010] S1.3, Technical assistance: Combine various technologies including solubilization technology, permeation technology, viscosity enhancement technology, mixing reduction technology and gas-water alternating injection technology;

[0011] S1.4, Monitoring: During the injection process, monitoring equipment is used to monitor the reservoir's pressure, temperature, CO2 concentration, crude oil production and water content in real time;

[0012] S1.5. Effect evaluation: Evaluate the oil recovery effect and calculate the indicators of crude oil recovery rate and miscible pressure change.

[0013] As a further improvement of this technical solution, in S1.3,

[0014] The solubilization technology is carried out through multiple experiments to adjust the ratio of CO2 and C3 dissolved oil;

[0015] The permeability enhancement technology needs to control the permeability of the reservoir so that CO2 and C3 dissolved oil can flow and diffuse better in the reservoir;

[0016] The viscosity increasing technology is by adding a viscosity increasing agent;

[0017] The downmixing technology changes the injected gas composition by adding nitrogen, methane, ethane, propane and flue gas;

[0018] The gas-water alternating injection is performed during the CO2 injection process.

[0019] As a further improvement of this technical solution, in S1.1,

[0020] The CO2 injection volume accounts for 10%-30% of the reservoir pore volume;

[0021] The injection amount of the C3 dissolved oil is based on an injected CO2 molar fraction of 40%, and the added amount is 15%-25% of the CO2 molar amount.

[0022] As a further improvement of this technical solution, in S1.1,

[0023] The oil reservoir is one of a structural oil reservoir, a lithological oil reservoir, a stratigraphic oil reservoir and a composite oil reservoir.

[0024] As a further improvement of the present technical solution, in S1.2, the injection rate of CO2 is 4-16m³ / h, and the injection rate of C3 dissolved oil is 6-12m³ / h.

[0025] As a further improvement of the technical solution, in S1.5, the evaluation period is once every 1-4 months.

[0026] As a further improvement of the technical solution, the permeability is less than 50×10 -3 μm² oil reservoirs, using either hydraulic fracturing or chemical treatment.

[0027] As a further improvement of the technical solution, the permeability is greater than 500×10 -3 μm² oil reservoirs use plugging technology to reduce gas channeling.

[0028] As a further improvement of the technical solution, the usage amount of the viscosity enhancer is 0.4%-2.2% of the volume of the crude oil.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. In the method for reducing the miscible pressure of CO2, on the one hand, CO2 is used for oil recovery, which realizes the geological storage of CO2 and reduces greenhouse gas emissions, which has positive significance for alleviating global climate change; on the other hand, by controlling injection parameters and adopting a variety of technical assistance, excessive damage to the oil reservoir is avoided and the adverse impact on the environment is reduced. At the same time, the viscosity enhancer and injection gas are selected to ensure good compatibility with crude oil and CO2, reducing the potential risk of environmental pollution. In addition, regular effect evaluation can adjust the strategy in time, further reduce the impact on the environment, realize sustainable development of oil fields, and improve environmental protection while ensuring energy supply.

[0031] 2. In the method for reducing the miscible pressure of CO2, by preparing CO2 and C3 dissolved oil in advance and collecting comprehensive reservoir data, accurate basic information can be provided for subsequent operations. During the injection process, multi-point injection method is combined with segmented or synchronous injection method to make CO2 and C3 dissolved oil play a better role. Various technical assistances further enhance the oil recovery effect, such as solubilization technology to improve the solubility of CO2 in crude oil, permeability technology to improve reservoir permeability and promote fluid flow, viscosity enhancement technology to reduce gas channeling risk, demixing technology to reduce miscible pressure, alternating gas and water injection to improve recovery rate, real-time monitoring and regular effect evaluation to improve effectiveness and adjustability, thereby realizing efficient development of oil fields, increasing crude oil production, and increasing economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a flow chart of the method for reducing the CO2 miscible pressure of the present invention;

[0033] Figure 2 It is a bar graph of the recovery factor of the present invention;

[0034] Figure 3 It is a histogram of the pressure of the mixed phase according to the present invention;

[0035] Figure 4 is a bar graph of the effectiveness of the present invention;

[0036] Figure 5 This is a bar graph of the adjustability of the present invention. DETAILED DESCRIPTION

[0037] The following will be combined with the accompanying drawings in the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0038] according to Figure 1 As shown, the embodiment of the present invention also provides a method for preparing the above-mentioned method for reducing the mixed phase pressure of CO2, and the specific steps are as follows:

[0039] Step 1: Preliminary preparation: prepare CO2 and dissolved oil mainly composed of C3, with the CO2 injection amount accounting for 10%-30% of the reservoir pore volume; the C3 dissolved oil injection amount is based on the injected CO2 mole fraction of 40%, and the added amount is 15%-25% of the CO2 molar amount; and collect various oil field data, including reservoir, porosity, permeability, formation pressure, temperature, fracture network scale and inter-well heterogeneity. Among them, the reservoir adopts one of the structural reservoir, lithological reservoir, stratigraphic reservoir and composite reservoir;

[0040] Step 2, injection: adopt multi-point injection method, evenly distribute injection points, improve oil recovery effect, inject CO2 gas into the reservoir through injection equipment, CO2 injection rate is 4-16m³ / h, avoid excessive pressure shock to the reservoir, and then adopt one of the segmented injection method and the synchronous injection method to inject C3 dissolved oil into the reservoir, the injection rate of C3 dissolved oil is 6-12m³ / h. When the synchronous injection method is adopted, use mixing technology to fully mix the two substances during the injection process. The mixing technology can adopt one of static mixer and dynamic stirring device; when the segmented injection method is adopted, arrange the injection time and sequence to give full play to the solubilization effect of C3 dissolved oil on CO2 and improve oil recovery efficiency;

[0041] Step 3: Technical assistance: Combine various technologies including solubilization technology, permeation technology, viscosity enhancement technology, mixing reduction technology and gas-water alternating injection technology;

[0042] The solubilization technology adjusts the ratio of CO2 and C3 dissolved oil through multiple experiments, effectively increasing the solubility of CO2 in crude oil, thereby enhancing the oil recovery effect;

[0043] Permeability enhancement technology requires controlling the permeability of the reservoir and improving the permeability of the reservoir so that CO2 and C3 dissolved oil can flow and diffuse better in the reservoir. -3 μm² reservoirs, use either hydraulic fracturing or chemical treatment. It is necessary to control the scale of fracturing to avoid exacerbating the heterogeneity between wells. When the permeability is greater than 500×10 -3 μm² oil reservoirs use plugging technology to reduce gas channeling and improve oil recovery effects;

[0044] Viscosity enhancement technology uses 0.4%-2.2% of the crude oil volume by adding a viscosity enhancer. The appropriate type of viscosity enhancer is selected to ensure good compatibility with crude oil and CO2, reduce the risk of CO2 gas channeling, increase the viscosity of crude oil, and thus enhance the oil recovery effect.

[0045] Downmixing technology changes the injected gas composition by adding nitrogen, methane, ethane, propane and flue gas to reduce the partial pressure of CO2, thereby reducing the miscible pressure;

[0046] Alternating gas and water injection During the CO2 injection process, alternating gas and water injection not only improves the oil displacement effect, but also increases the crude oil recovery rate;

[0047] Step 4: Monitoring: During the injection process, use monitoring equipment to monitor the reservoir's pressure, temperature, CO2 concentration, crude oil production and water content in real time;

[0048] Step 5. Effect evaluation: Conduct a comprehensive evaluation of the oil recovery effect every 1-4 months, calculate the indicators of crude oil recovery rate and miscible pressure change, improve the oil recovery effect, reduce the miscible pressure, and achieve efficient development of the oil field.

[0049] In the present invention, firstly, comprehensive preliminary preparations are carried out to prepare CO2 and dissolved oil mainly composed of C3, and collect various oil field data including reservoirs, porosity, permeability, etc. This can not only provide accurate basic information for subsequent operations, but also prepare for reducing the CO2 miscibility pressure; such as accurately determining the proportion of CO2 injection volume to the reservoir pore volume and the injection rate, as well as the injection volume and injection speed of C3 dissolved oil, etc.; and adopting a multi-point injection method to evenly distribute the injection points to improve the oil recovery effect. On the one hand, it can give full play to the solubilization effect of C3 dissolved oil on CO2 and improve the oil recovery efficiency, while avoiding excessive pressure shock to the reservoir; on the other hand, it can effectively improve the overall oil recovery effect and be combined with different reservoir conditions for a second time;

[0050] The reaction mechanism works synergistically through a variety of technical means. The solubilization technology adjusts the ratio to increase the solubility of CO2 in crude oil; the permeability enhancement technology uses different methods according to the permeability conditions to increase the permeability of the reservoir to facilitate fluid flow and diffusion; the viscosity enhancement technology adds suitable viscosity enhancers to reduce the risk of CO2 gas channeling and increase the viscosity of crude oil; the demixing technology adds a variety of gases to change the composition of the injected gas and reduce the miscibility pressure; the alternating injection of gas and water improves the oil displacement effect and crude oil recovery rate, and jointly realizes the efficient development of the oil field.

[0051] On the one hand, the use of CO2 for oil recovery achieves the geological storage of CO2, reduces greenhouse gas emissions, and has positive significance for alleviating global climate change; on the other hand, by controlling injection parameters and adopting a variety of technical assistance, excessive damage to the oil reservoir is avoided and the adverse impact on the environment is reduced. At the same time, the selection of viscosity enhancers and injection gases ensures good compatibility with crude oil and CO2, reducing the potential risk of environmental pollution. In addition, regular effect evaluation can adjust strategies in a timely manner, further reduce the impact on the environment, and achieve sustainable development of oil fields. While ensuring energy supply, it also improves environmental protection.

[0052] By preparing CO2 and C3 dissolved oil in advance and collecting comprehensive reservoir data, accurate basic information can be provided for subsequent operations. During the injection process, multi-point injection is combined with segmented or synchronous injection methods to make CO2 and C3 dissolved oil play a better role. Various technical assistance further enhances the oil recovery effect, such as solubilization technology to increase the solubility of CO2 in crude oil, permeability technology to improve reservoir permeability and promote fluid flow, viscosity enhancement technology to reduce gas channeling risk, mixing reduction technology to reduce miscibility pressure, alternating gas and water injection to improve recovery, real-time monitoring and regular effect evaluation to improve effectiveness and adjustability, thereby achieving efficient development of oil fields, increasing crude oil production, and increasing economic benefits.

[0053] According to different raw material dosages, the CO2 flooding and oil field recovery enhancement provided by the present invention are further described through the following specific examples.

[0054] Example 1: Preliminary preparation: adopt structural reservoir, CO2 injection volume accounts for 20% of the reservoir pore volume; C3 dissolved oil addition amount is 20% of CO2 molar amount; collect relevant data, porosity 20%, permeability greater than 60mD, formation pressure 16MPa-24MPa, temperature 50℃-120℃, small fracture network scale, fracture length 60m-360m, fracture width 2cm-5cm, fracture density 60-460 / square kilometer, and weak inter-well heterogeneity.

[0055] Injection: Multi-point injection, first inject CO2 at a rate of 10m³ / h, then simultaneously inject C3 dissolved oil at a rate of 8m³ / h, and use a mixing device to mix;

[0056] Technical assistance: Adopt injection sequence solubilization technology to hydraulically fracture and increase the permeability of low permeability areas, add 1% viscosity enhancer by volume of crude oil, and inject gas and water alternately at appropriate times, with a cycle of one week of CO2 injection and three days of water injection;

[0057] Monitoring: Real-time monitoring of reservoir pressure, temperature, CO2 concentration, crude oil production, water content and other parameters;

[0058] Effect evaluation: Conduct a comprehensive evaluation of the oil recovery effect every two months and calculate the crude oil recovery factor and miscible pressure change indicators.

[0059] Example 2: Preliminary preparation: A lithologic reservoir is used, with the CO2 injection volume accounting for 25% of the reservoir pore volume; the amount of C3 dissolved oil added is 18% of the CO2 molar amount; the porosity is 18%, the permeability is 6mD-60mD, the formation pressure is 25MPa-35MPa, the temperature is 120℃-150℃, the fracture network is medium-sized, the fracture length is 360m-2000m, the fracture width is 5cm-15cm, the fracture density is 460-2000 / square kilometer, and the heterogeneity between wells is strong.

[0060] Injection: Select the injection point according to the heterogeneous distribution, first inject CO2 in stages at a rate of 8m³ / h, then inject C3 dissolved oil after a period of time at a rate of 7m³ / h;

[0061] Technical assistance: adjust the ratio of CO2 and C3 dissolved oil for solubilization, plug and increase permeability in high permeability areas, select appropriate viscosity enhancers to increase crude oil viscosity, and the gas-water alternation cycle is five days of CO2 injection and two days of water injection;

[0062] Monitoring: Pay close attention to parameter changes;

[0063] Effect evaluation: Evaluate every 3 months.

[0064] Example 3: Preliminary preparation: Using a formation oil reservoir, the CO2 injection volume accounts for 15% of the reservoir pore volume; the amount of C3 dissolved oil added is 15% of the CO2 molar amount; the porosity is 15%, the permeability is higher than 0.1mD-6mD, the formation pressure is 8MPa-15MPa, the temperature is 50℃-120℃, the fracture network is small, the fracture network is small, the fracture length is 60m-360m, the fracture width is 2cm-5cm, the fracture density is 60-460 / square kilometer, and the heterogeneity is weak.

[0065] Injection: Evenly select injection points, inject CO2 and C3 dissolved oil simultaneously, CO2 injection rate is 5m³ / h, C3 dissolved oil injection rate is 6m³ / h, and mixing device is used for mixing;

[0066] Technical assistance: chemical treatment to increase permeability, adding 0.6% viscosity enhancer, gas-water alternation cycle is four days of CO2 injection and two days of water injection;

[0067] Monitoring: Real-time analysis of monitoring data;

[0068] Effect evaluation: Evaluate every 4 months.

[0069] Example 4: Preliminary preparation: A composite reservoir is used, with the CO2 injection volume accounting for 28% of the reservoir pore volume; the amount of C3 dissolved oil added is 22% of the CO2 molar amount; the porosity is 22%, the permeability is greater than 60mD, the formation pressure is 25MPa-35MPa, the temperature is 50℃-120℃, the fracture network is large, the fracture length is 2000m-5200m, the fracture width is 15cm-50cm, the fracture density is 2000-5000 / square kilometer, and the heterogeneity is strong.

[0070] Injection: Multi-point injection, first inject part of CO2 at a rate of 12m³ / h, then inject C3 dissolved oil at a rate of 10m³ / h, and then continue to inject CO 2；

[0071] Technical assistance: Hydraulic fracturing in low permeability areas is carried out at the same time with controlled scale, 1.8% viscosity enhancer is added, and the gas-water alternation cycle is six days of CO2 injection followed by three days of water injection;

[0072] Monitoring: timely identify problems and adjust strategies;

[0073] Effect evaluation: Evaluate once every month.

[0074] Table 1 Amounts of raw materials used in Examples 1-4

[0075]

[0076] In order to verify that the CO2 flooding for oil field development and enhanced oil recovery prepared in the embodiment of the present invention has good effectiveness and adjustability, the CO2 flooding for oil field development and enhanced oil recovery provided in the embodiment of the present invention is described through the following test examples.

[0077] Test example

[0078] The purpose of this test group is to explore the effect of different component ratios on the recovery factor.

[0079] Test objective: To test the recovery rate, miscibility reduction pressure, effectiveness and adjustability of the CO2 flooding for oil field development of the present invention.

[0080] Test method: Test group A, test group B, test group C and test group D respectively use the composition ratios of the wastewater treatment materials provided in Examples 1-4;

[0081] The control group A, control group B, control group C, control group D, control group E, control group F, control group G and control group H were used as the control examples, among which:

[0082] Control group A used a similar reservoir as in Example 1, but injected directly without detailed evaluation, randomly determined the CO2 injection amount and C3 dissolved oil ratio; randomly selected the injection point, and had no planning for the injection process; did not use technical assistance; and did not carry out effective monitoring and effective evaluation measures;

[0083] Control group B used a similar reservoir as in Example 2, which was wrongly assessed as unsuitable for CO2 miscible flooding and abandoned without other conditions;

[0084] Control group C uses the same reservoir as in Example 3, and the assessment is inaccurate and it is considered unsuitable due to strong heterogeneity, and there are no other conditions;

[0085] Control group D uses a reservoir similar to that of Example 4, without considering the scale and heterogeneity of the fracture network; conventional injection without considering special circumstances; CO2 injection rate and C3 dissolved oil injection rate are the same as those of Example 4, but are not adjusted according to actual conditions; technical assistance is adopted but not systematically; detection is not timely and evaluation is not timely;

[0086] Control group E uses a similar reservoir as in Example 1, with the CO2 injection volume far exceeding the reservoir pore volume by 30%; the injection method is the same as in Example 1, with a CO2 injection rate of 15 m³ / h and a C3 dissolved oil injection rate of 14 m³ / h; the technical assistance, detection and effect evaluation are the same as in Example 1;

[0087] Control group F uses a similar reservoir as in Example 2, with the proportion of C3 dissolved oil far below 15%; the injection method is the same as in Example 2. The CO2 injection rate is 7m³ / h, and the C3 dissolved oil injection rate is 5m³ / h; the technical assistance, detection and effect evaluation are the same as in Example 2;

[0088] The control group G uses the same reservoir as in Example 3, and focuses on injection in the fracture development area; the injection method is the same as in Example 3, the CO2 injection rate is 4m³ / h, and the C3 dissolved oil injection rate is 5m³ / h; the technical assistance, detection and effect evaluation are the same as in Example 3;

[0089] The control group H uses a reservoir similar to that of Example 4, only using the solubilization technology, and no other technologies are used; the injection method is the same as that of Example 4, the CO2 injection rate is 10m³ / h, and the C3 dissolved oil injection rate is 8m³ / h; the monitoring method and effect evaluation are the same as those of Example 4, but the limited effect may affect the evaluation results.

[0090] Specific detection indicators are shown in Table 2.

[0091] Table 2 Test indicators of each sample

[0092]

[0093] according to Figure 2-Figure 5 As shown in Table 2, the above comparative data include the following summary:

[0094] 1. Recovery rate

[0095] The overall recovery rate of the test group is high, among which the recovery rate of test group D (Example 4 using composite oil reservoir) is the highest at 86%, the recovery rate of test group A (Example 1 using structural oil reservoir) is 80%, and the recovery rates of test group B (Example 2 using lithologic oil reservoir) and test group C (Example 3 using stratigraphic oil reservoir) are 76% and 72%, respectively. This shows that through comprehensive preliminary preparation, reasonable injection methods and the combination of multiple technical auxiliary means, the recovery rate of the oil field can be effectively improved.

[0096] Among the control groups, except for the control group E (similar to the reservoir of Example 1 with injection volume far exceeding the prescribed amount) with a recovery rate of 50%, the control group G (similar to the reservoir of Example 3 with injection concentrated in the fracture development area) with a recovery rate of 58%, and the control group H (similar to the reservoir of Example 4 with only solubilization technology) with a recovery rate of 65%, the recovery rates of other control groups were low or even 0%. This highlights that without scientific planning and reasonable technical assistance, the recovery rate of oil fields will be greatly affected.

[0097] 2. Reducing the mixed phase pressure

[0098] The experimental group had a significant effect in reducing the miscible pressure, with the miscible pressure reduction in experimental group D reaching 60%, in experimental groups A and C by 52% and 48% respectively, and in experimental group B by 45%. This shows that the synergistic effect of multiple technical means can effectively reduce the miscible pressure and create conditions for the efficient development of oil fields.

[0099] The effect of reducing the mixed phase pressure in the control group was generally poor. The mixed phase pressure reduction in control group B and control group C was 0% because no operation was performed. The mixed phase pressure reduction in other control groups was also much lower than that in the experimental group, with the highest control group E being 27% and control group H being 36%.

[0100] 3. Effectiveness

[0101] The effectiveness of the test groups was high, with the effectiveness of test group D reaching 92%, test group A reaching 88%, and test groups B and C reaching 76% and 75%, respectively. This reflects the significant advantages of the method of the present invention in improving the efficiency of oilfield development.

[0102] The effectiveness of the control group was low. The effectiveness of control group B and control group C was 0%. Among the other control groups, except for control group H, which had a relatively high effectiveness of 65%, the effectiveness of other control groups was below 50%, and the lowest control group A was only 30%.

[0103] 4. Adjustability

[0104] The test groups have good adjustability. The adjustability of test group D is 86%, test group A is 82%, test groups B and C are 75% and 72% respectively. This is due to real-time monitoring and regular effect evaluation, which can adjust strategies in time and improve the adaptability of oilfield development.

[0105] The control groups had large differences in adjustability. The adjustability of control groups B and C was 0%, and the other control groups were generally lower, such as control group A was only 26%, control group E was 45%, and control group G was 40%.

[0106] In summary, the distinguishing technical features of the present invention are effective in improving recovery, reducing miscible phase pressure, and enhancing effectiveness and adjustability; through comprehensive preliminary preparation, reasonable injection methods, a combination of multiple technical auxiliary means, and real-time monitoring and regular evaluation, efficient development and sustainable development of oil fields can be achieved.

[0107] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions are only preferred examples of the present invention and are not intended to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A method for reducing the miscible pressure of CO2, characterized in that: The following steps are involved: S1.

1. Preliminary preparation: prepare CO2 and dissolved oil mainly composed of C3, and collect various data of the oil field, including reservoir, porosity, permeability, formation pressure, temperature, fracture network scale and inter-well heterogeneity; S1.2, injection: adopt multi-point injection method, inject CO2 gas into the oil reservoir through injection equipment, control the injection rate, and then adopt one of the segmented injection method or synchronous injection method to inject C3 dissolved oil into the oil reservoir; S1.3, Technical assistance: Combine various technologies including solubilization technology, permeation technology, viscosity enhancement technology, mixing reduction technology and gas-water alternating injection technology; The solubilization technology is used to increase the solubility of CO2 in crude oil by adjusting the ratio of CO2 and C3 dissolved oil through multiple experiments; The permeability enhancement technology needs to control the permeability of the oil reservoir to increase the permeability of the oil reservoir so that CO2 and C3 dissolved oil can flow and diffuse better in the oil reservoir; The viscosity increasing technology ensures good compatibility with crude oil and CO2 by adding viscosity increasing agent; The downmixing technology changes the injected gas composition by adding nitrogen, methane, ethane, propane and flue gas to reduce the partial pressure of CO2; The gas-water alternating injection is carried out during the CO2 injection process to improve the oil displacement effect and the crude oil recovery rate; S1.4, Monitoring: During the injection process, monitoring equipment is used to monitor the reservoir's pressure, temperature, CO2 concentration, crude oil production and water content in real time; S1.

5. Effect evaluation: Evaluate the oil recovery effect and calculate the indicators of crude oil recovery rate and miscible pressure change.

2. The method for reducing the CO2 miscible pressure according to claim 1, characterized in that: In S1.2, the injection amount of CO2 accounts for 10%-30% of the pore volume of the reservoir; The injection amount of the dissolved oil of C3 is based on the injected CO2 molar fraction of 40%, and the added amount is 15%-25% of the CO2 molar amount.

3. The method for reducing the CO2 miscible pressure according to claim 1, characterized in that: In S1.1, the oil reservoir is one of a structural oil reservoir, a lithological oil reservoir, a stratigraphic oil reservoir and a composite oil reservoir.

4. The method for reducing the CO2 miscible pressure according to claim 1, characterized in that: In S1.2, the injection rate of CO2 is 4-16m³ / h, and the injection rate of C3 dissolved oil is 6-12m³ / h.

5. The method for reducing the CO2 miscible pressure according to claim 1, characterized in that: In S1.5, the oil displacement effect is evaluated once every 1-4 months.

6. The method for reducing the CO2 miscible pressure according to claim 2, characterized in that: The permeability is less than 50×10 -3 μm² oil reservoirs, using either hydraulic fracturing or chemical treatment.

7. The method for reducing the CO2 miscible pressure according to claim 2, characterized in that: The permeability is greater than 500×10 -3 μm² oil reservoirs use plugging technology to reduce gas channeling.

8. The method for reducing the CO2 miscible pressure according to claim 2, characterized in that: The usage amount of the viscosity enhancer is 0.4%-2.2% of the volume of the crude oil.

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