Dual oil production four-way cross concentric dual string Christmas tree and separate layer gas injection method thereof

Abstract

This invention discloses a concentric double-tube wellhead with dual oil production four-way connectors and its stratified gas injection method. The inner tubing is located inside the outer tubing, forming an annular space between the outer wall of the inner tubing and the inner wall of the outer tubing. Both ends of the inner tubing extend beyond the ends of the outer tubing. The bottom ends of both the inner and outer tubing are connected to the wellbore via two permanent packers, and both are sealed to the two permanent packers using a metal backfill string. The inner and outer tubing hangers are located at the top of the inner and outer tubing, respectively, with the wellhead connected to the top of the inner tubing hanger. The upper oil production four-way connector is connected to the bottom of the inner tubing hanger and the top of the outer tubing hanger, respectively. The lower oil production four-way connector is connected to the bottom of the outer tubing hanger. This design improves the overall sealing performance of the wellhead and enables long-term stable gas injection.

Description

Technical Field

[0001] This invention belongs to the field of petroleum exploration and development, and relates to a dual-oil recovery four-way concentric double-tube production tree and its stratified gas injection method. Background Technology

[0002] The Yaha high-pressure condensate gas reservoir in the Tarim Oilfield has been undergoing circulating gas injection development for 20 years, consistently employing a general dry gas injection model. Due to significant differences in the physical properties of the sand bodies above and below the same injection layer in the injection wells, including substantial variations in average permeability and porosity, analysis of gas intake profile test data shows a nine-fold difference in gas intake between the upper and lower sand bodies. This unevenness in the driving interface at the dry gas displacement front ultimately leads to uneven utilization of gas reservoir reserves in the effective production wells, highlighting the contradiction between high and low permeability layers. Therefore, research on stratified gas injection technology is crucial for better segmenting the gas injection system. Related research has been ongoing. Jing Hongtao published "Exploring the Application of Concentric Dual-Tube Stratified Gas Injection Technology" in Petrochemical Technology in September 2016, and Xue Chengwen published "Concentric Tube Stratified Gas Injection Technology in Yaha High-Pressure Condensate Gas Field" in Natural Gas Industry in April 2016, proposing hypotheses and theoretical calculations for stratified gas injection in the Yaha condensate gas field. However, no related products or stratified gas injection process methods have been developed.

[0003] Existing stratified gas injection tubing typically uses a single tube with an injection valve for stratification. This process is limited by the size of the injection valve and the bottom sliding sleeve, making it impossible to adjust the injection volume. If the injection valve malfunctions, it cannot be replaced, requiring restarting the process. This results in low reliability and an inability to provide stable gas injection over long periods. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a dual-oil-producing four-way concentric double-tube production tree and its stratified gas injection method, which improves the overall sealing performance of the production tree and achieves long-term stable gas injection.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] A dual-oil-production four-way concentric double-tube string oil production tree includes an oil production tree, an upper oil production four-way, a lower oil production four-way, an inner tubing hanger, an outer tubing hanger, inner tubing, and outer tubing.

[0007] The inner tubing is located inside the outer tubing, forming an annular space between the outer wall of the inner tubing and the inner wall of the outer tubing, and an annular space between the outer wall of the outer tubing and the inner wall of the wellbore. Both ends of the inner tubing extend beyond the ends of the outer tubing. The bottom ends of the inner tubing are connected to the wellbore via a first permanent packer, and the inner tubing and the first permanent packer are sealed together using a first insertion string. The bottom end of the outer tubing is connected to the wellbore via a second permanent packer, and the outer tubing and the second permanent packer are sealed together using a second insertion string. Metal seals are used between the first insertion string and the first permanent packer, and between the second insertion strings.

[0008] The inner and outer tubing hangers are located at the top of the inner and outer tubing, respectively. The wellhead is connected to the top of the inner tubing hanger and is connected to the inner tubing via the inner tubing hanger. A metal seal is installed between the wellhead and the top of the inner tubing hanger. The upper production tee is connected to the bottom of the inner tubing hanger and the top of the outer tubing hanger. The upper production tee is connected to the annular space formed between the inner and outer tubing hangers and the outer wall of the inner and outer tubing. A metal seal is installed between the upper production tee and the bottom of the inner tubing hanger and the top of the outer tubing hanger. The lower production tee is connected to the bottom of the outer tubing hanger and is connected to the annular space formed between the outer tubing hanger and the outer wall of the wellbore via the outer tubing hanger and the inner wall of the wellbore. A metal seal is installed between the lower production tee and the bottom of the outer tubing hanger.

[0009] Preferably, a POP valve is provided below the first permanent packer of the inner tubing, and a retrievable ball seat is provided below the second permanent packer of the outer tubing.

[0010] Preferably, the inner tubing hanger's feed end is a female thread, and the outer tubing hanger's feed end is a male thread.

[0011] Preferably, the wellhead, upper wellhead cross-section, and lower wellhead cross-section are connected by flanges using double-ended bolts.

[0012] Preferably, a steel ring seal is provided between the upper oil production four-way and the lower oil production four-way.

[0013] Preferably, both the inner and outer oil pipes are gas-tight oil pipes.

[0014] Preferably, a hydraulic safety valve and a manual valve are provided on one side of the wellhead, the upper four-way valve, and the lower four-way valve, with the manual valve on the inside and the hydraulic safety valve on the outside. Two manual valves are provided on the other side of the wellhead, the upper four-way valve, and the lower four-way valve.

[0015] A stratified gas injection method based on the dual-well four-way concentric double-tube wellhead described in any one of the above, characterized by comprising the following steps:

[0016] Step 1: Install the dual-oil-production four-way concentric double tubing production tree to complete the well completion process;

[0017] Step 2: Connect the test injection pipeline to the Christmas tree and the upper production four-way valve respectively, and ensure the pressure test is successful; inject nitrogen to force the protective fluid in the annular space formed between the outer wall of the inner tubing and the inner wall of the outer tubing, as well as the annular space inside the inner tubing, into the formation.

[0018] Step 3: Connect the gas injection pipelines of the wellhead and the upper wellhead four-way valve to the main pipeline to implement stratified natural gas injection.

[0019] Preferably, the specific process of well completion is as follows:

[0020] Step 1.1: Install the lower oil production four-way valve, lower the first permanent packer matching the inner layer tubing, and release the first permanent packer seat after the ball is thrown and pressurized.

[0021] Step 1.2: After the ball is thrown and pressurized, the permanent packer seat of the outer tubing is released.

[0022] Step 1.3: Lower the outer tubing's second insertion string, raise the outer tubing to the designed tonnage, and confirm that the second insertion string of the outer tubing has successfully inserted the second permanent packer of the outer tubing.

[0023] Step 1.4: Connect the bottom of the outer tubing to the lower oil production cross-connector and the top of the outer tubing;

[0024] Step 1.5: Install the upper oil production four-way connector and connect the outer oil pipe to the upper oil production four-way connector at the top.

[0025] Step 1.6: Insert the first insertion string of the inner tubing, raise the inner tubing to the designed tonnage, and confirm that the first insertion string of the inner tubing has successfully inserted the first permanent packer of the inner tubing.

[0026] Step 1.7: Connect the bottom of the inner tubing to the inner oil production cross and the top of the inner tubing;

[0027] Step 1.8: Install the wellhead tree and connect the inner tubing to the wellhead tree by hanging it on top.

[0028] Furthermore, before proceeding to the next step in the well completion process, a sealing test is performed.

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

[0030] This invention improves downhole sealing performance by employing a permanent packer and installing a metal backfill string between the permanent packer and the tubing. This prevents issues like tubing creep and shortening that could cause seal failure during gas injection. The string configuration is simple, requiring only tubing and the packer, ensuring high reliability. Furthermore, the invention utilizes a dual-production four-way joint with a concentric dual-string structure, installing metal seals between the Christmas tree, upper production four-way joint, lower production four-way joint, and the inner and outer tubing layers to guarantee sealing. This well meets the gas injection requirements for mechanically hard stratification, enabling not only concentric dual-string stratified injection to avoid inter-layer interference but also adjustable injection volume at the surface for each layer, allowing for precise gas injection.

[0031] Furthermore, the second permanent packer for the outer tubing uses a retrievable ball seat for setting. After re-insertion of the outer tubing, it achieves a full-bore outer tubing passage, eliminating the narrowing effect that occurs with conventional packers after ball seat setting, effectively ensuring the safe insertion of the inner tubing. If the first permanent packer for the inner tubing uses a conventional ball seat for setting, removing the ball seat after setting would make it impossible to verify the seal after subsequent tubing re-insertion. Therefore, a POP valve is needed to replace the ball seat as the setting and seal verification tool. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the overall structure of the dual-oil-production four-way concentric double-tube oil well tree of the present invention;

[0033] Figure 2 This is a cross-sectional view of the inner tubing of the present invention;

[0034] Figure 3 This is a cross-sectional view of the outer oil pipe of the present invention;

[0035] Figure 4 This is a schematic diagram of the bottom structure of the wellbore of the present invention.

[0036] Among them: 1-Oil production tree; 2-Upper oil production four-way; 3-Lower oil production four-way; 4-Inner tubing hanger; 5-Outer tubing hanger; 6-Inner tubing; 7-Outer tubing; 8-First permanent packer; 9-First insertion string; 10-Second permanent packer; 11-Second insertion string. Detailed Implementation

[0037] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] It should be noted that the terms “front,” “back,” “left,” “right,” “up,” and “down” used in the following description refer to the directions shown in the attached diagram, while the terms “inside” and “outside” refer to the directions toward or away from the geometric center of a specific component, respectively.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0040] like Figure 1 As shown, the present invention is a dual-oil-production four-way concentric dual-tubing production tree, which consists of a layered gas injection wellhead and a wellbore tubing string, including a production tree 1, an upper production four-way 2, a lower production four-way 3, an inner tubing hanger 4, an outer tubing hanger 5, an inner tubing 6, and an outer tubing 7.

[0041] The wellhead 1 is connected to the top of the inner tubing hanger 4. The wellhead 1 is connected to the inner tubing 6 through the inner tubing hanger 4. A metal seal is provided between the wellhead 1 and the top of the inner tubing hanger 4. The upper four-way valve 2 is connected to the bottom of the inner tubing hanger 4 and the top of the outer tubing hanger 5. The upper four-way valve 2 is connected to the annular space formed between the outer wall of the inner tubing 6 and the inner wall of the outer tubing 7 through the inner tubing hanger 4 and the outer tubing hanger 5. A metal seal is provided between the upper four-way valve 2 and the bottom of the inner tubing hanger 4 and the top of the outer tubing hanger 5. The lower four-way valve 3 is connected to the bottom of the outer tubing hanger 5. The lower four-way valve 3 is connected to the annular space formed between the outer wall of the outer tubing 7 and the inner wall of the wellbore through the outer tubing hanger 5. A metal seal is provided between the lower four-way valve 3 and the bottom of the outer tubing hanger 5.

[0042] Layered gas injection wellhead: Composed of a tree trunk 1, an upper production cross-section 2, a lower production cross-section 3, and specially designed tubing hangers respectively seated in the upper production cross-section 2 and the lower production cross-section 3. The tree trunk 1, the upper production cross-section 2, and the lower production cross-section 3 are connected by flanges with double-ended bolts. The upper production cross-section 2 and the lower production cross-section 3 are sealed by a steel ring main seal. Each of the two wings of the tree trunk 1, the upper production cross-section 2, and the lower production cross-section 3 is equipped with two gate valves, which are connected by double-ended bolts. One side is equipped with a hydraulic safety valve and a manual valve, with the manual valve on the inside and the hydraulic safety valve on the outside. The other side of the tree trunk 1, the upper production cross-section 2, and the lower production cross-section 3 is equipped with two manual valves.

[0043] like Figure 2As shown, the inner tubing hanger 4 is equipped with a female thread at the inlet end. Both the upper and lower ends of the inner tubing hanger 4 are equipped with two O-rings and one metal seal to achieve sealing with the upper production tree 1 and the upper production four-way 2. There are pressure test holes extending to the body between the various levels of seals. After the sealing is completed, the reliability of the seal is checked through the pressure test holes.

[0044] like Figure 3 As shown, the inlet end of the outer tubing hanger 5 is set with a male thread. The outer tubing hanger 5 uses an external male thread as a lifting thread, and no female thread is set inside. The advantage of this is that it ensures that the inner wall of the outer tubing hanger 5 is smooth and that there will be no jamming when the inner tubing 6 is lowered in. The upper and lower ends of the outer tubing hanger 5 are equipped with two O-rings and one metal seal to achieve sealing with the upper oil production cross 2 and the lower oil production cross 3. There are pressure test holes extending to the body between each level of seal. After the sealing is completed, the sealing reliability is checked through the pressure test holes. The lower part of the outer tubing hanger 5 is suspended with an injection pipe string. The inner tubing 6 and the annulus of the inner tubing 6 and the outer tubing 7 each form an independent injection channel. This achieves the purpose of stratified injection of the upper and lower sand bodies.

[0045] like Figure 4 As shown, the inner tubing 6 is located inside the outer tubing 7. An annular space is formed between the outer wall of the inner tubing 6 and the inner wall of the outer tubing 7. An annular space is also formed between the outer wall of the outer tubing 7 and the inner wall of the wellbore. Both ends of the inner tubing 6 extend beyond both ends of the outer tubing 7. The inner tubing 6 is connected to the outer tubing via a gas-tight port and is sealed by a first permanent packer 8. The inner tubing 6 and the first permanent packer 8 are sealed together by a first return string 9. The outer tubing 7 is connected to the outer tubing via a gas-tight port and is sealed by a first permanent packer 8. The outer tubing 7 and the second permanent packer 10 are sealed together by a second return string 11. Metal seals are used between the first return string 9 and the first permanent packer 8, and between the second return string 11 and the second return string 11. The permanent packer with a return insertion string seal does not cause the problem of loss of seal due to string creep and shortening during the gas injection process; the string configuration is simple, requiring only tubing and packer, and has high reliability.

[0046] The second permanent packer 10 of the outer tubing 7 adopts a retrievable ball seat for sealing. After the outer tubing 7 is reinserted, it achieves a full-bore outer tubing 7 channel, eliminating the situation where conventional packers form a reduced diameter after ball seat sealing, effectively ensuring the safe insertion of the inner tubing 6.

[0047] The first permanent packer 8, which is matched with the inner tubing 6, replaces the setting ball seat with a POP valve. If the permanent packer 8 of the inner tubing 6 is set with a conventional ball seat, it will be impossible to verify the sealing performance after the tubing is re-inserted after the ball seat is removed. Therefore, a POP valve is needed to replace the ball seat as a setting and sealing verification tool.

[0048] The gas injection channel includes a lower gas injection channel and an upper gas injection channel: Lower gas injection channel: The pressurized high-pressure natural gas enters through the production tree 1, passes through the inner tubing hanger 4 connected to the upper production four-way valve 2 and the inner tubing 6, and enters the formation; In order to ensure sealing reliability, there is a metal seal between the top of the inner tubing hanger 4 and the production tree reducing flange to ensure sealing reliability, the inner tubing 6 adopts a gas-tight tubing to ensure reliability, and the inner tubing 6 and the first permanent packer 8 are sealed with a metal first return insertion string 9 to ensure reliability;

[0049] Upper gas injection channel: The pressurized high-pressure natural gas enters through the upper oil production four-way 2, and then enters the formation through the annular space between the outer layer of the inner layer tubing 4 connected to the upper oil production four-way 2 and the inner layer of the outer layer tubing 5 connected to the lower oil production four-way 3—the annular space between the inner layer tubing 6 and the outer layer tubing 7. To ensure sealing reliability, there is a metal seal between the bottom of the inner layer tubing 4 and the lower oil production four-way 3, a metal seal between the outer layer tubing 5 and the upper oil production four-way 2 to ensure sealing reliability, and the outer layer tubing 7 is a gas-tight tubing to ensure reliability.

[0050] This invention employs a dual-oil-production four-way structure with internal dual tubing hangers. The inner tubing hanger 4 and the outer tubing hanger 5 independently suspend the inner tubing 6 and the outer tubing 7, respectively. This allows the annulus between the inner tubing 6 and the outer tubing 7 to form independent gas injection channels, achieving the purpose of stratified gas injection into the upper and lower sand bodies. The well completion technology employs a method of first setting the inner tubing 6 and the outer tubing 7 with a first permanent packer 8 and a second permanent packer 10, and then reinstalling the inner tubing 6 and the outer tubing 7. After completion, the well meets the gas injection requirements for mechanically hard stratification, enabling not only concentric dual-tube stratified injection to avoid inter-layer interference, but also allowing for adjustable injection volume at the surface for each layer and precise gas injection.

[0051] The main implementation process of this invention is as follows: replace the lower oil production four-way valve 3; lower the self-testing packer + inner tubing 6 ball seat connection to the inner tubing 6, seal the upper and lower sand bodies, verify the self-testing packer seal is qualified, remove the feed string, lower the outer tubing 7 + packer + drop-in plug, verify the outer tubing packer seal is qualified, lower the inner tubing 6 for the first time insert the tubing string 9, and install the upper oil production four-way valve 2 and the tree trunk 1.

[0052] The specific implementation steps are as follows:

[0053] a. After cleaning the well casing and logging the well, the well completion process begins;

[0054] b. Install the lower oil well four-way valve, and perform injection molding and pressure testing on the lower oil pipe head P-seal ring, the area between the P-seal and the metal seal, and the area between the main seal of the oil pipe head and the metal seal. The test results are qualified.

[0055] c. Run in the test string to verify that the casing cementing quality between the upper and lower gas injection layers is good and there is no external leakage, so as to ensure the integrity of the subsequent layered gas injection well structure and the implementation of the layered gas injection process.

[0056] d. Lower the first permanent packer 8, which is matched with the inner tubing 6, and replace the seated ball seat with a POP valve. After the ball is thrown and pressurized, the first permanent packer 8 is released from the seat but the POP valve is not removed. Then lower the test seal string to the top of the first permanent packer 8 for test seal. After the release, the release string is lifted.

[0057] e. The second permanent packer 10 drop string of the lower outer tubing 7 is set with a recyclable ball seat. The ball is thrown and pressurized to set the seal. The seal is qualified and the drop string is successfully released. The second permanent packer 10 drop string of the outer tubing 7 is then lifted.

[0058] f. The bottom of the second insertion string 11 of the lower outer tubing 7 is anchored and sealed. Adjust the outer tubing 7 and lift the outer tubing 7 to the designed tonnage. Confirm that the second insertion string 11 of the outer tubing 7 is successfully inserted into the second permanent packer 10 of the outer tubing 7.

[0059] g. Hang the outer tubing 5 to the lower oil production four-way 3, pressurize and seal to confirm that the second insertion string 11 of the outer tubing 7 is inserted into the second permanent packer 10 of the outer tubing 7 and the seal is qualified.

[0060] h. Install the upper oil well four-way valve, and perform injection molding and pressure testing on the upper oil pipe head P sealing ring, the area between the P seal and the metal seal, and the area between the main seal of the oil pipe head and the metal seal. The test results are qualified.

[0061] i. The bottom of the first insertion string 9 of the inner tubing 6 is anchored and sealed. Adjust the inner tubing 6 and lift the inner tubing 6 to the designed tonnage. Confirm that the first insertion string 9 of the inner tubing 6 is successfully inserted into the first permanent packer 8 of the inner tubing 6. After passing the test, pressurize and remove the POP valve.

[0062] j. Hang the inner tubing 4 to the upper oil production four-way 2, pressure test and seal, and confirm that the first insertion string 9 of the inner tubing 6 is inserted into the first permanent packer 8 of the inner tubing 6 and the seal is qualified.

[0063] k. Install the upper part of the oil well tree 1 and pass the pressure test.

[0064] 1. The nitrogen injection team connects the test injection pipeline to the wellhead 1 and the upper wellhead four-way 2 respectively, and the pressure test is qualified. Nitrogen gas is injected to squeeze the annular space formed between the outer wall of the inner tubing 6 and the inner wall of the outer tubing 7, as well as the protective fluid in the annular space inside the inner tubing 6, into the formation, thus opening a channel for subsequent stratified natural gas injection.

[0065] m. Complete the ground process transformation project, connect the gas injection pipeline between the Christmas tree 1 and the upper production cross-over 2 to the main pipeline, and implement stratified gas injection.

[0066] After installing the lower production cross-over 3 on site, injection molding and pressure testing are carried out on the lower P seal ring of the lower production cross-over 3, between the P seal and the metal seal, and between the main seal and the metal seal of the lower production cross-over 3 at 35 MPa, and the pressure is stabilized for 15 minutes without dropping, which is qualified.

[0067] After the outer tubing hanger 5 is suspended and seated in the lower production cross-over 3, the pressure test is carried out at 15 MPa, and the pressure is stabilized for 30 minutes without dropping, which is qualified.

[0068] After installing the upper production cross-over 2 on site, injection molding and pressure testing are carried out separately on the lower P seal ring of the upper production cross-over 2, between the P seal and the metal seal, and between the main seal and the metal seal of the upper production cross-over 2 at 56 MPa, and the pressure is stabilized for 15 minutes without dropping, which is qualified.

[0069] Install the Christmas tree 1; injection molding and pressure testing of the P seal of the Christmas tree 1 are carried out at 70 MPa / 15 min, which is qualified; the pressure test of the connection between the cover flange and the No. 1 main valve is carried out at 70.96 MPa, and the pressure is stabilized for 15 minutes, and the pressure drop is 0.46 MPa, which is qualified; the pressure test of the seal of the inner tubing hanger 4 of the inner tubing 6 is carried out at 70.46 MPa, and the pressure is stabilized for 15 minutes, and the pressure drop is 0.35 MPa, which is qualified; the pressure test of the auxiliary seal of the Christmas tree 1 is carried out at 56.99 MPa, and the pressure is stabilized for 15 minutes, and the pressure drop is 0.31 MPa, which is qualified.

[0070] From the pressure test conditions of each level of seal, it can be seen that the device of this invention has been successfully applied in Well YA23-1-11: the pump pressure is 34 MPa, the oil pressure is 34 MPa, the casing pressure is 27 MPa, the wellhead temperature is 29 °C, stratified gas injection is carried out, 140,000 cubic meters of sand are injected into the upper sand body, and 25,000 cubic meters of sand are injected into the lower sand body; the sealing effects of each level fully meet the design requirements, and independent gas injection channels are formed in the inner tubing 6 and the annulus between the inner tubing 6 and the outer tubing 7 respectively, successfully achieving the purpose of stratified gas injection for the upper and lower sand bodies, and the implementation of the stratified gas injection method is also successful.

[0071] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0072] It should be understood that the above description is for illustrative purposes and not for limitation. Many embodiments and applications beyond the provided examples will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of this teaching should not be determined by reference to the above description, but rather by reference to the foregoing claims and the full scope of their equivalents. For purposes of completeness, all articles and references, including patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended as a waiver of that subject matter, nor should it be construed as an indication that the applicant has not considered that subject matter as part of the disclosed inventive subject matter.

Claims

1. A dual-oil recovery four-way concentric double-tube wellhead, characterized in that, It includes a wellhead (1), an upper four-way valve (2), a lower four-way valve (3), an inner tubing hanger (4), an outer tubing hanger (5), an inner tubing (6), and an outer tubing (7); The inner tubing (6) is located inside the outer tubing (7). An annular space is formed between the outer wall of the inner tubing (6) and the inner wall of the outer tubing (7). An annular space is formed between the outer wall of the outer tubing (7) and the inner wall of the wellbore. The two ends of the inner tubing (6) extend out of the two ends of the outer tubing (7). The bottom end of the inner tubing (6) is connected to the wellbore through the first permanent packer (8). The inner tubing (6) and the first permanent packer (8) are sealed together by the first return string (9). The bottom end of the outer tubing (7) is connected to the wellbore through the second permanent packer (10). The outer tubing (7) and the second permanent packer (10) are sealed together by the second return string (11). Metal seals are used between the first return string (9) and the first permanent packer (8) and between the second return string (11) and the second return string (11). The inner tubing hanger (4) and the outer tubing hanger (5) are located at the top of the inner tubing (6) and the outer tubing (7), respectively. The tree (1) is connected to the top of the inner tubing hanger (4). The tree (1) is connected to the inner tubing (6) through the inner tubing hanger (4). A metal seal is provided between the tree (1) and the top of the inner tubing hanger (4). The upper oil production cross-connector (2) is connected to the bottom of the inner tubing hanger (4) and the top of the outer tubing hanger (5), respectively. The upper oil production cross-connector (2) is connected to the inner tubing hanger (4) and the outer tubing (7), respectively. The annular space formed between the hanger (5) and the outer wall of the inner tubing (6) and the inner wall of the outer tubing (7) is connected. The upper oil production tee (2) is connected to the bottom of the inner tubing hanger (4) and the top of the outer tubing hanger (5) with metal seals respectively. The lower oil production tee (3) is connected to the bottom of the outer tubing hanger (5). The lower oil production tee (3) is connected to the annular space formed by the outer wall of the outer tubing hanger (5) and the outer wall of the wellbore with the inner wall of the wellbore. The lower oil production tee (3) and the bottom of the outer tubing hanger (5) are connected with metal seals. A POP valve is provided below the first permanent packer (8) of the inner tubing (6), and a retrievable ball seat is provided below the second permanent packer (10) of the outer tubing (7). The inner tubing hanger (4) has a female thread at the inlet and the outer tubing hanger (5) has a male thread at the inlet.

2. The dual-oil recovery four-way concentric double-tube wellhead according to claim 1, characterized in that, The wellhead (1), the upper wellhead cross-section (2) and the lower wellhead cross-section (3) are connected by flanges with double-headed bolts.

3. The dual-oil recovery four-way concentric double-tube wellhead according to claim 1, characterized in that, A steel ring seal is installed between the upper oil production cross-connector (2) and the lower oil production cross-connector (3).

4. The dual-oil recovery four-way concentric double-tube wellhead according to claim 1, characterized in that, Both the inner oil pipe (6) and the outer oil pipe (7) are gas-tight oil pipes.

5. The dual-oil recovery four-way concentric double-tube wellhead according to claim 1, characterized in that, Hydraulic safety valves and manual valves are installed on one side of the oil production tree (1), the upper oil production four-way (2), and the lower oil production four-way (3). The manual valves are on the inside and the hydraulic safety valves are on the outside. Two manual valves are installed on the other side of the oil production tree (1), the upper oil production four-way (2), and the lower oil production four-way (3).

6. A stratified gas injection method based on the dual-wellbore concentric double-tube wellhead as described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Install the dual-oil-production four-way concentric double tubing production tree to complete the well completion process; Step 2: Connect the test injection pipeline to the wellhead (1) and the upper wellhead cross-connector (2) respectively; the pressure test is qualified; inject nitrogen to force the protective fluid in the annular space formed between the outer wall of the inner tubing (6) and the inner wall of the outer tubing (7) and the annulus inside the inner tubing (6) into the formation. Step 3: Connect the gas injection pipelines of the wellhead (1) and the upper wellhead four-way (2) to the main pipeline and implement stratified natural gas injection.

7. The stratified gas injection method for a dual-well oil recovery system with concentric double tubing strings as described in claim 6, characterized in that, The specific process of well completion is as follows: Step 1.1, install the lower oil production four-way (3), lower the inner layer oil pipe (6) and the matching first permanent packer (8), after the ball is thrown and pressure is applied, the first permanent packer (8) is seated and released; Step 1.2, the second permanent packer (10) of the lower outer tubing (7) is released after the ball is thrown and pressure is applied; Step 1.3, the second insertion string (11) of the lower outer tubing (7) is raised to the designed tonnage, and it is confirmed that the second insertion string (11) of the outer tubing (7) is successfully inserted back into the second permanent packer (10) of the outer tubing (7); Step 1.4: Connect the bottom of the outer tubing hanger (5) to the lower oil production cross-connector (3) and the top of the outer tubing (7); Step 1.5, install the upper oil production four-way connector (2), and connect the top of the outer oil pipe hanger (5) to the upper oil production four-way connector (2); Step 1.6, lower the inner tubing (6) and insert the first insertion string (9) back into the inner tubing (6) to the designed tonnage, and confirm that the first insertion string (9) back into the inner tubing (6) has been successfully inserted into the first permanent packer (8). Step 1.7: Connect the bottom of the inner tubing hanger (4) to the inner oil extraction cross and the top of the inner tubing (6); Step 1.8, install the tree (1), and connect the top of the inner tubing hanger (4) to the tree (1).

8. The stratified gas injection method for a dual-oil recovery four-way concentric double-tube wellhead as described in claim 7, characterized in that, Before proceeding to the next step in the well completion process, a sealing test is performed.

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