Oil jacket mixed injection CO2 energizing fracturing composite transformation method

Abstract

The invention relates to an oil jacket mixed injection CO2 energizing fracturing composite transformation method, and belongs to the technical field of low-pressure oil and gas reservoir development. According to the oil jacket mixed injection CO2 energized fracturing composite transformation method provided by the invention, the problem that large-scale construction is difficult due to the fact that fracturing equipment and a transformation liquid system are limited in existing CO2 energized fracturing is solved. The CO2 large-scale energy increasing and fracturing transformation composite construction can be achieved by using existing conventional fracturing equipment and a common transformation liquid system, and technical reference is provided for CO2 to play a greater role in reservoir transformation and low-quality oil reservoir development.

Description

Technical Field

[0001] The present invention belongs to the technical field of development of low-pressure oil and gas reservoirs, and particularly relates to a method for combined injection of CO 2 for energy enhancement and fracturing composite transformation. Background Technique

[0002] CO 2 As an energy-enhancing gas, it has been used to improve the recovery rate of gas wells since the 1950s and has been widely applied in stimulation operations since the 1960s. Currently, the commonly used CO 2 energy-enhancing fracturing technology mainly includes CO 2 dry fracturing, CO 2 foam fracturing, and injecting CO 2 to the target reservoir through special tools for fracturing operations.

[0003] CO 2 Dry fracturing, also known as anhydrous fracturing, implements a fully enclosed operation, with CO 2 pumped simultaneously through the tubing and casing, and a special sand-carrying fluid viscosifier is used. Conventional fracturing fluids cannot be used for dry fracturing, and it has high requirements for fracturing equipment. CO 2 Foam fracturing is implemented more frequently. It has the dual functions of energy enhancement and fracturing. Mostly, it uses pre-injected CO 2 as the main component, and a certain amount of pure CO 2 is designed to be pumped as an energy-enhancing liquid before fracturing construction. Generally, it is pumped through the tubing. In the second half, conventional fracturing fluid is mainly used to carry sand, and a small amount of CO 2 is mixed in to achieve the foaming effect. After being injected into the formation, it has the function of energy enhancement. In the high sand ratio stage, in order to reduce the sand addition risk, generally, the injection of CO 2 is reduced and stopped. A large amount of CO 2 is mainly pumped in the preflush stage. Since the pure CO 2 has poor fracture-forming ability and is difficult to enter the deep part of the reservoir, the energy enhancement effect of this method is general. It is mainly used to increase the flowback energy and cannot significantly increase the energy of the reservoir in the far wellbore area. Currently, special downhole tools are also used to directly pump CO 2 to the target reservoir to achieve the purpose of energy-enhancing fracturing. However, this method mainly relies on special downhole string combination tools. The core of the process technology lies in the tools themselves, and the combined operation is relatively complex, making it difficult to meet the purpose of large-scale CO 2 fracturing energy enhancement and reservoir transformation.

[0004] Therefore, there is an urgent need to develop a method for implementing large-scale CO 2 energy-enhancing fracturing operations using existing fracturing pumping equipment and existing conventional fracturing fluid systems, so as to achieve the purpose of deep energy enhancement and large-scale transformation of low-pressure oil reservoirs. Summary of the Invention

[0005] The object of the present invention is to overcome the problems existing in the above-mentioned prior art, and to provide a method for combined injection of CO into the tubing-casing annulus for energy enhancement and fracturing composite transformation. 2

[0006] The present invention is realized by the following technical solutions:

[0007] The present invention provides a method for combined injection of CO into the tubing-casing annulus for energy enhancement and fracturing composite transformation, comprising the following steps: 2

[0008] (1) According to the well pattern and well spacing of the oil well, software simulates the total amount of CO required for single-well energy enhancement and the transformation scale, and optimizes the fracturing string according to the process of combined injection into the tubing-casing annulus; 2

[0009] (2) A gel sand-carrying fluid containing proppant is pumped into the casing to form a supported artificial fracture, and the tubing is filled with a mixture of CO and fracturing fluid; 2

[0010] (3) Reduce the displacement of the gel sand-carrying fluid containing proppant in the casing to ensure that the temperature in the artificial fracture within 50 m is not higher than 50% of the reservoir temperature, and the tubing pumps pure CO at the maximum displacement until 85% of the designed volume is reached; 2

[0011] (4) Gradually increase the displacement of the gel sand-carrying fluid containing proppant in the casing to the maximum displacement within the pressure limit range, and at the same time gradually reduce the displacement of CO in the tubing to the minimum displacement until the proppant, gel sand-carrying fluid, and CO reach the designed volume; 2 2

[0012] (5) Stop pumping CO, replace it with low-viscosity slickwater in the tubing, and replace it with slickwater and gel sand-carrying fluid in the casing to start displacement until the displacement is completed. 2

[0013] The present invention provides a method for combined injection of CO into the tubing-casing annulus for energy enhancement and fracturing composite transformation, which can solve the problem that conventional fracturing equipment and fracturing fluid systems are difficult to meet the requirements of large-scale energy enhancement and fracturing transformation. The method of the present invention mainly uses the casing to pump the gel sand-carrying fluid and proppant, and the tubing to pump pure CO. According to the gel fracturing fluid and CO liquid scale designed for energy enhancement and transformation, the tubing string is optimized to meet the requirements of simultaneous separate pumping of the gel sand-carrying fluid and CO in the tubing-casing annulus. At the same time, according to the simulation of the temperature field at the bottom of the well and in the artificial fracture during the fracturing process, the pumping sequence and pumping displacement of the gel sand-carrying fluid and CO are adjusted. The method of the present invention can be used in CO 2 2 2 2 2 2 2During the enhanced fracturing wellbore pumping process, the gel sand-carrying fluid and CO are forced to be separated 2 , to avoid their mixing in the wellbore and affecting their respective performances. In particular, avoid the influence of CO 2 on the gel sand-carrying fluid, which may cause sand plugging due to sand separation from the gel sand-carrying fluid and lead to construction failure. The technical method provided by the present invention enables the gel sand-carrying fluid and CO 2 to start mixing only before entering the formation at the bottom of the well. Since CO 2 has extremely strong penetration ability, it can quickly penetrate into the interior of the reservoir after entering the oil and gas reservoir, and can reduce the influence on the gel sand-carrying fluid. The present invention only uses conventional fracturing equipment and modified fluid systems, and adopts the method of co-injection through the production tubing and casing to avoid the mixing of CO 2 and the conventional modified fluid system in the wellbore, prevent the influence of CO 2 on the performance of the sand-carrying fluid, reduce construction risks such as sand plugging, and compared with the existing process technology, can significantly reduce the construction risk of enhanced sand fracturing with CO 2 . At the same time, the cost of the fracturing fluid is greatly reduced, and both the technical and economic advantages are relatively significant.

[0014] The method of the present invention is mainly aimed at implementing large-scale enhanced fracturing with CO 2 in low-pressure oil reservoirs, which can increase formation energy, raise reservoir pressure, and at the same time implement large-scale fracturing transformation to achieve high and stable production in low-pressure oil reservoirs.

[0015] Preferably, the specific operation of the step (1) includes: demarcating the single-well controlled reservoir range according to the well pattern and well spacing, using reservoir numerical simulation software to simulate the total amount of CO 2 required for single-well energy enhancement, then using fracturing software to simulate the artificial fracture length and transformation scale required for energy enhancement, and then designing the fracturing string of the co-injection process through the production tubing and casing according to the simulation results to meet the displacement requirements of the gel sand-carrying fluid, proppant and CO 2 .

[0016] In the step (1) of the present invention, in combination with the deployment of the well pattern and well spacing, reservoir numerical simulation software and fracturing simulation software are used to simulate the CO 2 pumping scale and fracturing transformation scale required for energy enhancement, and then the production tubing during the design of the co-injection process through the production tubing and casing is designed to meet the displacement requirements of the gel sand-carrying fluid, proppant and CO 2 .

[0017] Preferably, the specific operation of the step (2) includes: according to the artificial fracture length described in the step (1), using the casing to pump the gel sand-carrying fluid containing proppant at the maximum displacement within the pressure limit to form a supported artificial fracture. At the same time, a mixed fluid of fracturing fluid and CO 2 is pumped into the production tubing. The mixed fluid does not contain sand. After the production tubing wellbore is filled with the mixed fluid, the production tubing wellbore is closed.

[0018] Preferably, step (1) further includes: according to the design of limited pressure but unlimited displacement in fracturing, simulating the temperature at the bottom of the well and in the artificial fracture during the construction of the maximum casing displacement, and determining the phase characteristics of CO 2 before entering the formation.

[0019] Preferably, in step (2), the viscosity of the gel sand-carrying fluid is 200 mPa·s, the proppant is a ceramsite proppant, and the particle size of the proppant is 30 / 50 mesh.

[0020] Preferably, in step (2), the pumping volume of the gel sand-carrying fluid and the proppant is 20% of the total designed volume.

[0021] In step (2) of the present invention, first, a large displacement of the gel sand-carrying fluid is pumped through the casing, and ceramsite proppants with a particle size above medium are added to the sand-carrying fluid. A tubing volume of slickwater and CO 2 mixed liquid are injected through the tubing to maintain the pressure balance between the casing and the tubing. After the gel sand-carrying fluid with proppants pumped through the casing enters the formation, a long artificial fracture is formed. At the same time, the ceramsite proppants can support the artificial fracture after the displacement of the gel sand-carrying fluid decreases later, preventing the artificial fracture formed by the gel from completely closing, which is beneficial for the subsequent CO 2 to smoothly enter the deep reservoir. Pumping a large displacement of the gel sand-carrying fluid through the casing also has the effect of reducing the wellbore temperature, which can ensure that the subsequent CO 2 is in an environment where it remains in a liquid state. The tubing is filled with a mixed liquid of CO 2 and fracturing fluid to test whether the tubing can pump CO 2 and to maintain the pressure balance between the casing and the tubing. Therefore, the ratio of CO 2 to fracturing fluid in the tubing is not restricted. The main purpose is to test whether CO 2 can be smoothly pumped in the tubing. Generally, a small amount of fracturing fluid is mixed with CO 2 and the pumping displacement can be increased at any time.

[0022] Preferably, in step (2), the sand ratio of the gel sand-carrying fluid to the proppant is 15%.

[0023] Preferably, in step (3), the sand ratio of the gel sand-carrying fluid to the proppant is 10%.

[0024] Preferably, in step (4), the sand ratio of the gel sand-carrying fluid to the proppant is 20%.

[0025] Preferably, the specific operation of step (3) includes: gradually reducing the displacement of the gel sand-carrying fluid containing proppants in the casing, from the maximum displacement to a value that satisfies maintaining the temperature from the wellhead to the bottom of the well and in the artificial fracture not higher than 50% of the reservoir temperature within 50 m; at the same time, increasing the CO 2The pumping displacement of the pump, during this stage, pure CO is pumped in the tubing 2 , gradually increase the displacement to the maximum displacement under the pressure limit condition. When the casing pressure reaches the minimum value to maintain the balance of the oil-casing pressure, the displacement of the gel-carrying sand fluid in the casing annulus will no longer be reduced

[0026] When the gel-carrying sand fluid and ceramic proppant pumped in the casing reach about 30% of the designed volume of this well, step (3) can be carried out, reducing the displacement of the gel-carrying sand fluid containing proppant in the casing, from the maximum displacement to the extent that it meets the requirement of maintaining a relatively low temperature (about 50% of the reservoir temperature (within a range of 50 m)) from the wellhead to the bottom of the well and within the artificial fracture. At the same time, the tubing starts to pump CO 2 , increase the displacement to the maximum displacement under the pressure limit, and maintain the pumping of CO in the tubing 2 The maximum displacement until it reaches about 60% of the total designed pumping volume. During this period, keep pumping the gel-carrying sand fluid in the casing at the lowest displacement

[0027] Preferably, the specific operation of the said step (4) includes: gradually increasing the displacement of the gel-carrying sand fluid in the casing to the maximum displacement under the pressure limit condition, adding proppant during the pumping process; at the same time, gradually reducing the displacement of CO in the tubing 2 until it reaches the lowest displacement to maintain the balance of the oil-casing pressure, and pump until the proppant, gel-carrying sand fluid, and CO 2 reach the designed volume

[0028] Preferably, the specific operation of the said step (5) includes: stop pumping CO 2 , replace the inside of the tubing with low-viscosity slickwater to start displacement, and the pumping volume is 100% of the tubing volume; pump 50% of the casing volume of the gel-carrying sand fluid without proppant and 50% of the casing volume of slickwater in the casing until the displacement is completed

[0029] In step (4), start to gradually reduce the displacement of CO pumped in the tubing 2 , and at the same time gradually increase the displacement of the gel-carrying sand fluid in the casing until all the designed gel-carrying sand fluid, proppant, and CO 2 are pumped. Then start to displace the casing with 50% gel-carrying sand fluid (without adding proppant) + 50% slickwater to complete the displacement, and the total pumping volume is the casing volume; the tubing is displaced with slickwater, and the total displacement is the total volume of the tubing

[0030] Preferably, the viscosity of the low-viscosity slickwater is 5 mPa·s; the viscosity of the slickwater pumped in the casing is 15 mPa·s

[0031] The present invention has the following beneficial effects

[0032] The present invention is a method that utilizes CO 2A composite transformation process method that implements large-scale energy enhancement and fracturing transformation simultaneously is implemented. This method only uses conventional fracturing equipment and transformation fluid system, adopts the method of oil-casing mixed injection, and avoids CO 2 Mixed with conventional reforming fluid system in the wellbore to prevent CO 2 The impact on the performance of the sand-carrying fluid can reduce the construction risks such as sand plugging. The energy-enhancing fracturing composite transformation method provided by the present invention overcomes the existing CO 2 Energy-enhanced fracturing is difficult to carry out on a large scale due to the limitations of fracturing equipment and transformation fluid systems. Using existing conventional fracturing equipment and ordinary transformation fluid systems, CO 2 Large-scale energy enhancement and fracturing reconstruction combined construction, CO 2 It plays a greater role in reservoir transformation and low-quality oil reservoir development and provides technical reference. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of the well pattern deployment, well spacing and artificial fracture extension direction of the oil reservoir in Example 1;

[0034] Figure 2 Schematic diagram of the wellbore, fracturing string, perforation layer, tubing and casing pumping in Example 1;

[0035] Figure 3 The gel-carrying sand liquid in the artificial fracture in Example 1 replaces CO 2 Schematic diagram of entering the deep reservoir. DETAILED DESCRIPTION

[0036] To better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be understood by those skilled in the art that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0037] Unless otherwise specified, the experimental methods used in the examples are all conventional methods; the materials, reagents, etc. used, unless otherwise specified, can be obtained from commercial channels.

[0038] Example 1

[0039] This embodiment provides a method of injecting CO into the oil-casing 2 The energy-enhanced fracturing method comprises the following steps:

[0040] (1) The main characteristics of the reservoir in this embodiment are as follows: the reservoir burial depth is 3000 m, the pressure coefficient is 0.8, the reservoir temperature is 80 °C, the permeability is 0.8 mD, the porosity is 12%, the reservoir thickness is 10 m, belonging to a typical low-permeability and low-pressure reservoir. Natural fractures are locally developed in the reservoir. Downhole rock samples are cored, the natural fracture density is 0.5 fractures / m, the comprehensive filling degree is 90%, and a small part is unfilled. The well pattern is 600 m × 400 m, and the well spacing in the direction of the maximum horizontal principal stress of the formation is 600 m;

[0041] According to the oil well pattern and well spacing of the above reservoir, through reservoir numerical simulation analysis, a single well controls a reservoir area of 300 m × 200 m. In order to achieve the goal of increasing energy and raising the reservoir pressure by 15%, about 2000 cubic meters of CO 2 need to be pumped per single well; through fracture simulation software analysis, for a single well to control oil and gas in the range of 300 m × 200 m, the artificial propped half fracture length should reach about 200 m. Considering that natural fractures open to form some complex fractures, it is analyzed that the total amount of liquid for the single well reconstruction scale is about 2000 cubic meters, and 300 cubic meters of sand are added. According to the reservoir well pattern, well spacing and the artificial fracture shape formed by simulation and its relationship with the well pattern and well spacing are as Figure 1 shown;

[0042] According to the above simulation analysis, according to the technology of co-injection through the tubing-casing annulus, the completion and fracturing string are optimized. The casing size is 8-1 / 8 inches, and the tubing is a combined string of 5 inches and 4-1 / 2 inches. The energy-increasing reconstruction construction is carried out by the method of co-injection through the tubing-casing annulus. The schematic diagrams of the wellbore, fracturing string, perforated interval, tubing and casing pumping are as Figure 2 shown; the internal pressure resistance of the casing is 80 MPa, a 105 wellhead is used, the maximum allowable pressure for casing construction is 80 MPa, and the tubing pressure limit is 90 MPa. The maximum displacement of the simulated casing gel can reach 8 m 3 / min, and the CO 2 pumping displacement of the tubing can reach 10 m 3 / min. At a displacement of 8 m 3 / min of the gel-carrying sand fluid in the casing, the temperature in the bottom hole tubing-casing annulus can be reduced by about 50% to about 40 °C. In the later stage, when the CO 2 pumping displacement of the tubing can reach 10 m 3 / min, the temperature inside the tubing can be about 50%, ensuring that the CO 2 is still in the liquid state (<31 °C, >10 MPa is liquid state) in the wellbore, which is more conducive to pumping;

[0043] (2) According to the optimization results obtained in step (1), the casing pumps the gel-carrying sand fluid and proppant. The selected viscosity of the gel-carrying sand fluid is 200 mPa·s, which can meet the requirements of fracture formation and high-intensity sand addition. The construction displacement is quickly increased to 8 m 3 / min. At the same time, the blender truck adds 30 / 50 mesh ceramsite proppant, and the sand ratio is 15%. Inside the tubing, at 1 m3 Pump in slickwater (fracturing fluid with a viscosity of 5 mPa·s) and a small amount of CO 2 mixture at a displacement of about / min, mainly for testing the injection of CO 2 in the tubing can be smoothly implemented and the displacement can be increased at any time; Pump in 400 cubic meters of gel sand-carrying fluid and add 60 cubic meters of sand. The length of the artificially supported fracture can reach about 100 m. This supported artificial fracture can provide a flow channel for the subsequent injection of CO 2 so that CO 2 can quickly reach a position about 100 m deep in the reservoir;

[0044] (3) Gradually reduce the displacement of the gel sand-carrying fluid containing proppant in the casing. The adjustment sequence is 8 m 3 / min - 7 m 3 / min - 6 m 3 / min - 5 m 3 / min - 4 m 3 / min - 3 m 3 / min. Keep each stage for 10 minutes. During the stage of reducing displacement, the sand ratio of the sand-carrying fluid is reduced to 10%. At the same time, gradually increase the injection displacement of CO 2 in the tubing. Each 1 cubic meter is a stage and gradually increase to 10 m 3 / min. Keep each stage for 10 minutes and observe and evaluate whether there are any abnormalities in the pumping equipment and the wellbore; Inject CO 2 in the tubing and keep a displacement of 10 m 3 / min. Keep the displacement of the gel sand-carrying fluid injected in the casing at 3 m 3 / min and pump for 120 minutes. At this stage, the total injection of CO 2 is 1700 cubic meters, the gel sand-carrying fluid is 1200 cubic meters, and the proppant is 150 cubic meters;

[0045] The CO 2 injected in this step mainly enters the deep reservoir along the supported artificial fracture formed by the gel sand-carrying fluid in the previous step. At the same time, the CO 2 injected in the tubing and the gel sand-carrying fluid injected in the casing still have a certain viscosity in the reservoir after mixing, which plays a role in continuing to create fractures and is also beneficial for CO 2 to enter a deeper reservoir. According to the simulation of the temperature field in the artificial fracture, the temperature of the reservoir within 50 m of the wellbore range is still relatively low (<31 °C). In this range, CO 2 is mainly in a liquid state, and part of the CO 2 seeps into the interior of the reservoir along the high-permeability natural fractures. In the range beyond 50 m, as the temperature rises, CO 2 gradually becomes in a supercritical state, with extremely strong penetration ability, and can penetrate into the interior of relatively tight oil and gas reservoirs. Therefore, CO 2More enters the reservoir more than 50 m away from the wellbore to supplement the deep energy of the reservoir;

[0046] (4) Rapidly increase the displacement of the gel-carrying sand fluid in the casing to 8 m 3 / min, increase the sand ratio to 20%, and at the same time rapidly reduce the displacement of CO 2 in the tubing to 3 m 3 / min. Use the gel-carrying sand fluid pumped by the casing pump to displace the 1700 m³ of CO 2 pumped into the formation in the previous step into the deep part of the reservoir. The schematic diagram of the gel-carrying sand fluid in the artificial fracture displacing CO 2 into the deep part of the reservoir is as shown in Figure 3 . After the displacement of the high-displacement oil-casing is adjusted in place, continuously pump for 100 minutes; after completing this stage, a total of about 2000 m³ of CO 2 has been pumped into the reservoir, about 2000 m³ of gel-carrying sand fluid, and about 300 m³ of proppant pumped into the formation;

[0047] In this stage, the high-displacement gel-carrying sand fluid in the casing enters the formation to create fractures, so that the half fracture length of the artificial support fracture can reach 180 - 200 m, displacing a large amount of CO 2 pumped in the previous step into the deep part of the reservoir. At the same time, it also carries a small amount of CO 2 pumped by the tubing at the bottom of the well into the reservoir. This part of CO 2 can diffuse along the artificial fracture near the wellbore to supplement the energy of the formation near the wellbore;

[0048] (5) Keep the displacement in the casing at 8 m 3 / min, pump 50% of the casing volume of gel-carrying sand fluid and 50% of the casing volume of slickwater (viscosity 15 mPa·s); stop pumping CO 2 , increase the displacement in the tubing to 5 - 8 m 3 / min, pump slickwater (viscosity 5 mPa·s) equivalent to the entire tubing volume until the displacement is completed.

[0049] So far, this embodiment has completed the large-scale CO 2 energy-increasing fracturing reconstruction construction for a certain well in the reservoir, and completed the total amounts of CO 2 , gel-carrying sand fluid and proppant designed to achieve the dual purposes of energy increase and reconstruction.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. An oil-casing mixed injection CO 2 energy-increasing fracturing composite transformation method, It is characterized in that it includes the following steps: (1) According to the oil well pattern and well spacing, the software simulates the total amount of CO required for single well energy enhancement and the transformation scale, and optimizes the fracturing string according to the process of co-injection of oil and casing; 2 ​ (2) Inject a gelled proppant-carrying fluid through the casing to form a supported artificial fracture, and fill the tubing with a mixture of CO 2 and the fracturing fluid; (3) Reduce the displacement of the gelled proppant-carrying fluid in the casing to ensure that the temperature in the artificial fracture within a range of 50 m does not exceed 50% of the reservoir temperature. The tubing pumps pure CO at the maximum displacement 2 to reach 85% of the designed volume; (4) Gradually increase the displacement of the gel sand-carrying fluid containing proppant in the casing to the maximum displacement within the limited pressure range, and at the same time gradually reduce the displacement of CO 2 in the tubing to the minimum displacement until the proppant, gel sand-carrying fluid, and CO 2 reach the designed quantity; (5) Stop CO 2 Pump injection is stopped. The tubing is replaced with low-viscosity slickwater, and the casing is replaced with slickwater and gelled sand-carrying fluid to start displacement until the displacement is completed.

2. The method for combined enhanced fracturing and transformation by co-injecting into the tubing-casing annulus with CO 2 as claimed in claim 1 It is characterized in that The specific operations of the step (1) include: demarcating the single-well controlled reserve range according to the well pattern and well spacing, using reservoir numerical simulation software to simulate the total amount of CO required for single-well energy enhancement, then using fracturing software to simulate the artificial fracture length and stimulation scale required for energy enhancement, and then designing the fracturing string for the oil-casing mixed injection process according to the simulation results to meet the requirements of the gel-carrying sand fluid, proppant and CO displacement. 2 Then, use fracturing software to simulate the artificial fracture length and stimulation scale required for energy enhancement, and then design the fracturing string for the oil-casing mixed injection process according to the simulation results to meet the requirements of the gel-carrying sand fluid, proppant and CO displacement. 2 displacement requirements.

3. The method for combined transformation of casing and tubing by co-injecting CO 2 to enhance energy and perform fracturing It is characterized in that The specific operation of the step (2) includes: according to the artificial fracture length described in the step (1), using a casing to pump a gelled sand-carrying fluid containing proppant at the maximum displacement within the pressure limit to form a supported artificial fracture, and simultaneously pumping a mixed fluid of fracturing fluid and CO 2 in the tubing. The mixed fluid does not contain sand. After the tubing wellbore is filled with the mixed fluid, the tubing wellbore is closed.

4. The method for combined transformation of casing and tubing by co-injecting CO 2 with energy enhancement and fracturing It is characterized in that The said step (1) further includes: according to the design of limited pressure but unlimited displacement in fracturing, simulating the temperature at the bottom of the well and in the artificial fracture during the construction of the maximum casing displacement, and determining the phase state characteristics of CO 2 before entering the formation.

5. The method for combined enhanced fracturing and transformation by co-injecting into the tubing-casing annulus with CO 2 as claimed in claim 1 It is characterized in that In the step (2), the viscosity of the gel sand-carrying fluid is 200 mPa·s, the proppant is a ceramsite proppant, and the particle size of the proppant is 30 / 50 mesh.

6. The method for combined enhanced fracturing and transformation by co-injecting into the tubing-casing annulus with CO 2 as claimed in claim 1 It is characterized in that In the step (2), the pumping volume of the gel sand-carrying fluid and the proppant is 20% of the total designed volume.

7. The method for combined enhanced fracturing and stimulation by co-injecting into the tubing-casing annulus as claimed in claim 1, 2 wherein CO is co-injected It is characterized in that The specific operations of step (3) include: gradually reducing the displacement of the gel carrying proppant fluid in the casing, from the maximum displacement to a value that can maintain the temperature from the wellhead to the bottom of the well and within the artificial fracture not higher than 50% of the reservoir temperature within 50 m; meanwhile, increasing the pumping displacement of CO 2 in the tubing. During this stage, pure CO 2 is pumped in the tubing, and the displacement is gradually increased to the maximum displacement under the pressure limit condition. When the casing pressure reaches the minimum value to maintain the balance of the oil-casing pressure, the displacement of the gel carrying proppant fluid in the casing annulus is no longer reduced.

8. The method for combined enhanced energy fracturing and compound transformation by co-injecting CO into the tubing-casing annulus according to claim 1 2 ​ It is characterized in that The specific operations of step (4) include: gradually increasing the displacement of the gel sand-carrying fluid in the casing to the maximum displacement under the pressure limit condition, and adding proppant during the pumping process; at the same time, gradually reducing the displacement of CO 2 in the tubing until the minimum displacement for maintaining the pressure balance between the casing and the tubing is reached, and pumping until the proppant, gel sand-carrying fluid and CO 2 reach the designed quantity.

9. The method for combined enhanced fracturing and transformation by co-injecting into the tubing-casing annulus with CO 2 as claimed in claim 1 It is characterized in that The specific operation of the step (5) includes: stopping the CO 2 pump injection, replacing the inside of the tubing with low-viscosity slickwater to start displacement, with a pumping volume of 100% of the tubing volume; pumping 50% of the casing volume of a gelled proppant-free fluid and 50% of the casing volume of slickwater in the casing until the displacement is completed.

10. The method for combined transformation of casing and tubing by injecting CO 2 to enhance energy and perform fracturing It is characterized in that The viscosity of the low-viscosity slickwater is 5 mPa·s; the viscosity of the slickwater pumped into the casing is 15 mPa·s.

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