A method for designing wellbore structure of salt cap gas storage

Abstract

The present invention discloses a method for designing a wellbore structure for a salt cap layer gas storage reservoir, comprising: step one, determining the size of the production casing according to the needs of gas injection and production in the gas storage reservoir; step two, determining the minimum thickness of the cement sheath based on the principle of wellbore integrity design and ensuring cementing quality; step three, determining the maximum thickness of the cement sheath based on the displacement limit of existing equipment capabilities; and step four, designing a wellbore structure scheme using a reverse design method based on the production needs of the salt cap layer gas storage reservoir and the cement sheath thickness. In this scheme, based on the principle of ensuring wellbore integrity and salt cap layer cementing quality, a reverse design method is used to first design a reasonable thickness of the production casing and the salt cap layer technical casing cement sheath, and then based on this and the production needs of the salt cap layer gas storage reservoir, the wellbore structure is designed. This solves the problem of unstable cementing quality in the salt layer section, ensures effective isolation of the target layer section, and thus effectively ensures the safety and stability of the entire gas storage reservoir construction and operation.

Description

Technical Field

[0001] The present invention relates to the technical field of engineering drilling for resources such as oil and natural gas, and in particular to a method for designing a wellbore structure for a salt cap layer gas storage reservoir. Background Art

[0002] As the nation's demand for clean energy grows, demand for natural gas, a clean energy source, is also increasing. However, my country's geological conditions dictate that gas sources are plentiful in the western and southwestern regions, while they are scarce in the eastern region. Due to uneven development, natural gas demand is high in the eastern region. Gas storage facilities can help meet peak gas demand in developed regions like Beijing, Shanghai, and Guangzhou. The Zhongyuan Oilfield is located in the natural gas peak-shaving center of North China. Its salt layer, 200 to 500 meters thick, serves as a caprock, effectively preventing the loss of stored gas. However, using salt layers as caprocks presents several technical challenges: First, the production layer must be permanently cemented, placing high demands on wellbore integrity. A leak in a single well could impact the safety of the entire gas storage facility. Second, high-quality cementing of the caprock is crucial, requiring continuous, high-quality cementing of over 25 meters in the salt layer, or a cumulative high-quality cementing of 50 meters. Cementing quality is crucial to gas storage construction. Therefore, wellbore structure design methods that prioritize cementing quality are crucial.

[0003] The commonly used drill bit size in China is 26"-171 / 2"-121 / 4"-95 / 8"-81 / 2", and the relative casing size is 20"-133 / 8"-95 / 8"-7"-5". This is suitable for drilling conventional wells, but not for gas storage wells with salt cap layers. For example, in conventional gas storage wellbore design, the first drilling uses a 171 / 2" diameter casing, with a 133 / 8"-135 / 8" casing run in. The second drilling uses a 121 / 4"-125 / 8" diameter casing, with a 95 / 8"-103 / 4" diameter run in. The third drilling uses a 95 / 8" drill bit, with a 7" casing run in. Because the second drilling encounters salt layers, the casing collapse strength safety factor needs to be greater than 1.25, requiring thicker casing. This results in an excessively small gap and a thin cement sheath, impacting cementing quality and wellbore integrity. Furthermore, excessively small gaps affect casing centering and can easily cause lost circulation, making cementing quality impossible to guarantee and impacting the gas storage's designed service life of 30 to 50 years. Summary of the Invention

[0004] In view of this, the present invention provides a method for designing the wellbore structure of a salt cap gas storage reservoir, which can solve the problem of unstable cementing quality in the salt layer section, ensure the effective isolation of the target layer section, and thus effectively ensure the safe and stable construction and operation of the entire gas storage reservoir.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for designing a wellbore structure for a salt cap gas storage reservoir comprises:

[0007] Step 1: Determine the production casing size based on the gas injection and production needs of the gas storage;

[0008] Step 2: Determine the minimum thickness of the cement sheath based on the principle of wellbore integrity design and cementing quality assurance;

[0009] Step 3: Determine the maximum thickness of the cement sheath based on the displacement limit of the existing equipment capability;

[0010] Step 4: Based on the production requirements of the salt cap gas storage and the thickness of the cement sheath, the wellbore structure scheme is designed using the reverse design method.

[0011] Preferably, in step 1, determining the size of the production casing according to the gas injection and production needs of the gas storage includes:

[0012] The production casing size is determined based on the gas injection and production displacement of the gas storage reservoir and the size requirements of the wellbore tubing.

[0013] Preferably, in step 2, determining the minimum thickness of the cement sheath based on the principle of wellbore integrity design and ensuring cementing quality includes:

[0014] Calculate the deformation of the casing and cement sheath under stress, and calculate the minimum thickness of the cement sheath based on the principle that the deformation of the cement sheath must be greater than the deformation of the casing.

[0015] Preferably, the calculation of the deformation of the casing and cement sheath under stress includes:

[0016] The deformation of casing and cement sheath under stress is calculated using the modified formula of Lame equation and Hooke's law.

[0017] Preferably, the calculation of the deformation of the casing and cement sheath by using the modified formula of the Lame equation and Hooke's law includes:

[0018] Calculation of casing deformation:

[0019] Considering the casing is a completely symmetrical problem, it can be simplified to a plane stress and strain solution. According to the Lame equation:

[0020]

[0021] Where: s is the deformation of the outer wall of the casing, mm; a and b are the inner and outer radii of the casing, mm; E s is the elastic modulus of the casing, MPa; v s is the Poisson's ratio of the casing; p s is the actual internal pressure on the inner wall of the casing, MPa;

[0022] Considering that the outer wall of the casing is affected by the formation confining pressure, we have:

[0023] p s=κp (2)

[0024] Where: p is the maximum internal pressure, MPa; κ is the casing internal pressure correction coefficient, generally 0.91 to 0.99;

[0025] Considering cement as an elastic body, the deformation of the cement sheath is calculated according to Hooke's law:

[0026] p c =E c ·(Δl / l0) (3)

[0027] Where: p c is the internal pressure of the cement sheath, MPa; E c is the elastic modulus of cement, MPa; l0 is the original thickness of cement sheath, mm; Δl is the deformation of cement sheath, mm;

[0028] Considering the influence of the interface bonding strength and the confining pressure of the formation on the outer wall of the cement sheath, we have:

[0029] p c =εp s =εκp (4)

[0030] Where: ε is the cement compression correction coefficient, and its value range is 0.85~0.95;

[0031] Taking into account the actual wellbore diameter expansion rate, we have:

[0032] l0=ηl (5)

[0033] Where: l is the original annulus size, mm; η is the wellbore correction factor, and its value range is 1.01 to 1.1.

[0034] Preferably, while calculating the minimum thickness of the cement sheath, the method further includes:

[0035] The minimum thickness of the cement sheath should meet the requirements of the industry standard SY / T5374.1-2016 cementing operation procedures.

[0036] Preferably, in step 3, determining the maximum thickness of the cement sheath based on the displacement limit of existing equipment capabilities includes:

[0037] The calculation formula is:

[0038] Where: D is the thickness of the cement sheath, mm; d is the outer diameter of the casing, mm; L is the maximum displacement of the mud pump, L / s.

[0039] Preferably, in step three, before or simultaneously with determining the displacement limit based on existing equipment capabilities, the method further includes:

[0040] Based on ensuring cementing replacement efficiency.

[0041] Preferably, ensuring cementing displacement efficiency includes:

[0042] During cementing and injection, the annular return speed is required to be controlled at 0.9m / s-1.5m / s.

[0043] Preferably, in step 4, designing a wellbore structure solution includes:

[0044] Design the drill bit size and drilling depth for each opening, as well as the casing size, wall thickness and drilling depth for each opening.

[0045] It can be seen from the above technical solution that the method for designing the wellbore structure of a salt cap layer gas storage reservoir provided by the present invention takes ensuring the integrity of the wellbore and the quality of the salt cap layer cementing as the principle, adopts a reverse design method, first designs a reasonable thickness of the production casing and the salt cap layer technical casing cement ring, and then designs the wellbore structure based on this and the production requirements of the salt cap layer gas storage reservoir, thereby solving the problem of unstable cementing quality in the salt layer section and ensuring the effective isolation of the target layer section, thereby effectively ensuring the safety and stability of the construction and operation of the entire gas storage reservoir. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0047] Figure 1 A flow chart of a method for designing a wellbore structure for a salt cap gas storage provided by an embodiment of the present invention;

[0048] Figure 2 A schematic diagram of a wellbore structure obtained by using a wellbore structure design method for a salt cap gas storage provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0049] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0050] The embodiment of the present invention provides a method for designing a wellbore structure of a salt cap gas storage reservoir, such as Figure 1 Said, including:

[0051] Step 1: Determine the production casing size based on the gas injection and production needs of the gas storage;

[0052] Step 2: Determine the minimum thickness of the cement sheath based on the principle of wellbore integrity design and cementing quality assurance;

[0053] Step 3: Determine the maximum thickness of the cement sheath based on the displacement limit of the existing equipment capability;

[0054] Step 4: Based on the production requirements of the salt cap gas storage and the thickness of the cement sheath, the wellbore structure scheme is designed using the reverse design method.

[0055] It can be seen from the above technical solution that in the wellbore structure design method for the salt cap layer gas storage provided in the embodiment of the present invention, based on the principle of ensuring the integrity of the wellbore and the cementing quality of the salt cap layer, a reverse design method is adopted to first design a reasonable thickness of the production casing and the salt cap layer technical casing cement ring, and then the wellbore structure is designed based on this and the production requirements of the salt cap layer gas storage. This solves the problem of unstable cementing quality in the salt layer section, ensures the effective isolation of the target layer section, and thus effectively ensures the safe and stable construction and operation of the entire gas storage.

[0056] In this solution, in step 1, determining the production casing size according to the gas injection and production needs of the gas storage includes:

[0057] The production casing size is determined based on the gas storage reservoir's injection and production capacity and the required wellbore tubing size. This solution is designed so that the determined production casing size can meet production requirements.

[0058] Furthermore, to ensure the integrity of the wellbore, the deformation of the casing under the action of internal pressure must be transmitted to the cement sheath. That is, the deformation of the cement sheath must be greater than the deformation of the casing to ensure that the cement sheath is not damaged. Accordingly, in step 2, based on the principles of wellbore integrity design and cementing quality assurance, the minimum thickness of the cement sheath is determined, including:

[0059] Calculate the deformation of the casing and cement sheath under stress, and calculate the minimum thickness of the cement sheath based on the principle that the deformation of the cement sheath must be greater than the deformation of the casing.

[0060] Furthermore, the calculation of the deformation of the casing and cement sheath under stress includes:

[0061] The deformation of the casing and cement sheath is calculated using the modified formula of Lamé's equation and Hooke's law. This design helps improve the accuracy of the calculation of the deformation of the casing and cement sheath, thereby further improving the accuracy of the calculation of the minimum thickness of the cement sheath.

[0062] Specifically, the calculation of the stress and deformation of the casing and cement sheath using the modified formula of the Lame equation and Hooke's law includes:

[0063] Calculation of casing deformation:

[0064] Considering the casing is a completely symmetrical problem, it can be simplified to a plane stress and strain solution. According to the Lame equation:

[0065]

[0066] Where: s is the deformation of the outer wall of the casing, mm; a and b are the inner and outer radii of the casing, mm; E s is the elastic modulus of the casing, MPa; ν s is the Poisson's ratio of the casing; p s is the actual internal pressure on the inner wall of the casing, MPa;

[0067] Considering that the outer wall of the casing is affected by the formation confining pressure, we have:

[0068] p s =κp (2)

[0069] Where: p is the maximum internal pressure, MPa; κ is the casing internal pressure correction coefficient, generally 0.91 to 0.99;

[0070] Considering cement as an elastic body, the deformation of the cement sheath is calculated according to Hooke's law:

[0071] p c =E c ·(Δl / l0) (3)

[0072] Where: p c is the internal pressure of the cement sheath, MPa; E c is the elastic modulus of cement, MPa; l0 is the original thickness of cement sheath, mm; Δl is the deformation of cement sheath, mm;

[0073] Considering the influence of the interface bonding strength and the confining pressure of the formation on the outer wall of the cement sheath, we have:

[0074] p c =εp s =εκp (4)

[0075] Where: ε is the cement compression correction coefficient, and its value range is 0.85~0.95;

[0076] Taking into account the actual wellbore diameter expansion rate, we have:

[0077] l0=ηl (5)

[0078] Where: l is the original annulus size, mm; η is the wellbore correction factor, and its value range is 1.01 to 1.1.

[0079] In this way, the casing deformation δ is calculated according to the maximum internal pressure p of the casing. s , according to the maximum stress on the cement sheath, calculate the deformation of the cement sheath Δl, the design requirement Δl ≥ δ s , that is, the deformation of the casing under the action of internal pressure p. The pressure can be transmitted to the cement sheath. The deformation of the cement sheath must be greater than the deformation of the casing to ensure that the cement sheath is not damaged and the integrity of the wellbore is guaranteed. Then, based on the principle that the deformation of the cement sheath must be greater than the deformation of the casing, the minimum thickness of the cement sheath is calculated.

[0080] Furthermore, while calculating the minimum thickness of the cement sheath, the method further includes:

[0081] The minimum cement sheath thickness must meet the requirements of the industry standard SY / T 5374.1-2016 Cementing Operations Specification. This means the minimum cement sheath thickness must be ≥19mm. This design ensures the rationality of the minimum cement sheath thickness design, thereby facilitating the achievement of a reasonable cement sheath thickness, thereby effectively ensuring safe production throughout the gas storage facility.

[0082] Specifically, in step 3, based on the displacement limit of existing equipment capabilities, determining the maximum thickness of the cement sheath includes:

[0083] The calculation formula is:

[0084] Where: D is the thickness of the cement sheath, mm; d is the outer diameter of the casing, mm; L is the maximum displacement of the mud pump, L / s.

[0085] Furthermore, in order to improve the replacement efficiency, accordingly, in step 3, before or at the same time as the displacement limit based on the existing equipment capability, the following steps are further included:

[0086] This solution is designed to ensure cementing displacement efficiency. Cementing displacement efficiency requires that the annular return velocity during cementing injection be controlled within a range of 0.9m / s to 1.5m / s. This design helps improve cementing efficiency.

[0087] Furthermore, in step 4, designing a wellbore structure solution includes:

[0088] The drill bit size and drilling depth for each drill pass, as well as the casing size, wall thickness, and run depth for each drill pass, are designed to meet the drilling requirements of salt cap gas storage reservoirs.

[0089] The following is a further explanation of this plan:

[0090] This invention addresses the challenges in the prior art. Ensuring the quality of production casing and salt capping casing cementing is key to design. Therefore, it is necessary to design a wellbore structure for a salt capping gas storage facility to ensure safe and stable construction and operation.

[0091] The present invention adopts a reverse design approach and proposes a wellbore structure design method for a salt cap rock gas storage reservoir, the method comprising:

[0092] (1) Determine the production casing size based on the gas injection and production volume of the gas storage reservoir and the size of the wellbore string;

[0093] (2) The deformation of casing and cement sheath is calculated by using the modified formula of Lame equation and Hooke's law. The casing deformation is required to be less than the cement sheath deformation to ensure the integrity of the wellbore. The minimum thickness of the production casing and the cement sheath of the salt cap layer casing is designed.

[0094] Among them, the modified formula of Lame equation and Hooke's law is used to calculate the stress and deformation of casing and cement sheath:

[0095] 1. Calculation of casing deformation:

[0096] Considering the casing is a completely symmetrical problem, it can be simplified to a plane stress and strain solution. According to the Lame equation:

[0097]

[0098] Where: s is the deformation of the outer wall of the casing, mm; a and b are the inner and outer radii of the casing, mm; E s is the elastic modulus of the casing, MPa; ν s is the Poisson's ratio of the casing; p s is the actual internal pressure on the inner wall of the casing, MPa;

[0099] Considering that the outer wall of the casing is affected by the confining pressure of the formation, we have:

[0100] p s =κp (2)

[0101] Where: p is the maximum internal pressure, MPa; κ is the casing internal pressure correction coefficient, generally 0.91 to 0.99;

[0102] 2. Considering cement as an elastic body, calculate the deformation of the cement sheath according to Hooke's law:

[0103] p c =E c ·(Δl / l0) (3)

[0104] Where: p c is the internal pressure of the cement sheath, MPa; E cis the elastic modulus of cement, MPa; l0 is the original thickness of cement sheath, mm; Δl is the deformation of cement sheath, mm;

[0105] Considering the influence of the interface bonding strength and the confining pressure of the formation on the outer wall of the cement sheath, we have:

[0106] p c =εp s =εκp (4)

[0107] Where: ε is the cement compression correction coefficient, and its value range is 0.85~0.95;

[0108] Taking into account the actual wellbore diameter expansion rate, we have:

[0109] l0=ηl (5)

[0110] Where: l is the original annulus size, mm; η is the wellbore correction factor, and its value range is 1.01 to 1.1;

[0111] According to the maximum internal pressure p of the casing, calculate the casing deformation δ s , according to the maximum stress on the cement sheath, calculate the deformation of the cement sheath Δl, the design requirement Δl ≥ δ s , that is, the deformation of the casing under the action of internal pressure p, the pressure can be transmitted to the cement sheath, and the deformation of the cement sheath must be greater than the deformation of the casing to ensure that the cement sheath is not damaged and the integrity of the wellbore is guaranteed;

[0112] The minimum thickness of the cement sheath is calculated based on the principle that the deformation of the cement sheath must be greater than the deformation of the casing. At the same time, the thickness of the cement sheath should meet the requirements of the industry standard SY / T5374.1-2016 cementing operation regulations, that is, the cement sheath thickness ≥ 19mm;

[0113] (3) Based on ensuring cementing displacement efficiency and combining existing equipment capabilities to meet construction requirements, the maximum thickness of the cement sheath is designed;

[0114] (4) Based on the production requirements of the salt cap gas storage and the cement sheath thickness, the reverse design method is used to design the wellbore structure scheme. Among them, the reverse design method is to first design the cement sheath thickness of the production casing and the salt cap technical casing based on the gas storage wellbore integrity requirements, and then design the wellbore structure in combination with other requirements of the wellbore structure design.

[0115] Compared with the prior art, the present invention has the following beneficial effects:

[0116] This invention utilizes a reverse engineering approach to design the wellbore structure for a salt-capped gas storage facility, focusing on ensuring wellbore integrity and cementing quality in the salt cap layer. A reasonable cement sheath thickness resolves the issue of unstable cementing quality in the salt layer, ensuring effective isolation of the target layer and thus ensuring safe production throughout the gas storage facility.

[0117] In order to make the present invention easier to understand, the present invention is further described below with reference to specific embodiments:

[0118] First embodiment:

[0119] Reference Figure 2 A salt cap gas storage wellbore structure is provided, wherein the wellbore structure is a three-opening vertical well, specifically:

[0120] Salt cap gas storage, designed gas injection capacity 50×10 4 m / day. Based on the use of an 88.9mm diameter tubing string, a 177.8mm diameter production casing is required to achieve production. The estimated maximum gas injection pressure p is 38.6 MPa, and a casing wall thickness of 10.4 mm is selected. An elastic-ductile cement slurry system is used, with a cement stone elastic modulus of 7.5 GPa, a casing Poisson's ratio of 0.3, a casing elastic modulus of 20,600 MPa, a casing internal pressure correction factor κ of 0.99, and a wellbore diameter correction factor η of 1.01. Based on the deformation of the cement sheath and casing, the minimum cement sheath thickness required to ensure wellbore integrity is 19.2 mm, which exceeds the standard requirement of 19.0 mm. Therefore, the minimum cement sheath thickness is 19.2 mm. In order to improve the displacement efficiency, the annulus return speed is controlled at 0.9m / s-1.5m / s during cementing and replacement. The thicker the cement sheath, the greater the injection displacement. Based on the existing equipment capacity of 60L / s, the cement sheath thickness should not exceed 75.5mm. The calculation result meets the requirement that the compression expansion deformation of the casing is less than the deformation of the cement sheath after stress. Combined with the necessary sealing point, the design is to use a Ф241.3mm drill bit to drill to the completion depth of the well and install a Ф177.8mm casing. Combined with the drill bit size and salt layer creep, the second opening salt cap layer casing is selected as a Ф282.6mm thickened casing. The minimum cement sheath thickness of the casing is calculated to be 29.3mm and the maximum is 56.4mm. Combined with the necessary sealing points, the second opening is designed to use a Ф340mm drill bit to drill to 30m below the salt bottom boundary, and a Ф273.1mm casing is lowered to seal the conventional formation, and a Ф282.6mm thickened casing is lowered to seal the salt layer; the first opening is designed to use a Ф508mm drill bit to drill to a well depth of 500m for the surface layer, and a Ф406.4mm surface casing is lowered; the designed guide tube is designed to use a Ф660.4mm drill bit to drill to a well depth of 50m, and a Ф609.6mm guide tube is lowered.

[0121] Second embodiment:

[0122] Reference Figure 2 A salt cap gas storage wellbore structure is provided, wherein the wellbore structure is a three-opening vertical well, specifically:

[0123] The gas storage production uses 73mm pipe string, with a designed gas injection displacement of 10×10 4m3 / day. The estimated maximum gas injection pressure p is 30 MPa, requiring 139.7 mm production casing for production, with a selected casing wall thickness of 9.2 mm. An elastic-ductile cement slurry system is used, with a cement stone elastic modulus of 7.5 GPa, a casing Poisson's ratio of 0.3, and a casing elastic modulus of 20,600 MPa. The casing internal pressure correction factor κ is 0.99, and the wellbore diameter correction factor η is 1.01. Calculations indicate a minimum cement sheath thickness of 19.0 mm and a maximum of 85.0 mm. Based on the required sealing points, the wellbore structure design is as follows: Drill to completion depth using a 215.9 mm diameter drill bit in the third opening, followed by the installation of 139.7 mm diameter casing. Taking into account the drill bit size and salt layer creep, the second opening salt caprock casing is thickened 250.8 mm diameter. The cement sheath thickness is a minimum of 33.5 mm and a maximum of 61.2 mm. The wellbore structure was designed based on the required sealing points: The third opening was drilled to the final depth using a 215.9mm drill bit, followed by the installation of 139.7mm casing. A 660.4mm drill bit was used for the casing to a depth of 50m, followed by the installation of a 508mm casing. The second opening was drilled to 30m below the salt bottom boundary using a 311.2mm drill bit, followed by the installation of a 244.5mm casing with a 12.0mm wall thickness to seal the conventional formation and a 250.8mm thickened casing with a 15.88mm wall thickness to isolate the salt formation. The first opening was drilled to a depth of 500m using a 444.5mm drill bit, followed by the installation of a 339.7mm surface casing.

[0124] In the early stages of gas storage construction, complex drilling processes frequently encountered problems such as lost circulation, casing run-in obstructions, cementing losses, and inadequate cementing quality assurance. However, the present invention effectively ensures cementing quality, meeting the drilling engineering requirements of gas storage construction and effectively resolving the challenges of salt-capped gas storage. This approach offers significant economic benefits and enables the long-term, safe, and efficient development of salt-capped gas storage. The present invention is widely applicable to gas storage well development.

[0125] In summary, the present invention discloses a wellbore structure design method for a salt cap gas storage reservoir, which belongs to the field of engineering drilling for resources such as oil and natural gas. The method comprises the following steps: (1) determining the size of the production casing according to the gas injection and production displacement of the gas storage reservoir and the size of the wellbore string; (2) based on the principle of wellbore integrity design and ensuring cementing quality, the highest pressure acts on the casing and cement ring during wellbore injection and production, and the Lame equation and Hooke's law correction formula are used to calculate the stress deformation of the casing and cement ring, requiring that the casing deformation is less than the cement ring deformation, and determining the minimum thickness of the cement ring; (3) based on ensuring cementing displacement efficiency and combining the existing equipment capabilities to meet construction requirements, designing the maximum thickness of the cement ring; (4) based on the production requirements of the salt cap gas storage reservoir and the cement ring thickness, using the reverse design method to design the wellbore structure scheme.

[0126] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0127] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for designing a wellbore structure for a salt cap gas storage reservoir, characterized in that: include: Step 1: Determine the production casing size based on the gas injection and production needs of the gas storage; Step 2: Determine the minimum thickness of the cement sheath based on the principle of wellbore integrity design and cementing quality assurance; Step 3: Determine the maximum thickness of the cement sheath based on the displacement limit of the existing equipment capability; Step 4: Based on the production requirements of the salt cap gas storage and the thickness of the cement sheath, the wellbore structure is designed using the reverse design method; In step 2, the minimum thickness of the cement sheath is determined based on the principles of wellbore integrity design and cementing quality assurance, including: Calculate the deformation of the casing and cement sheath under stress, and calculate the minimum thickness of the cement sheath based on the principle that the deformation of the cement sheath must be greater than the deformation of the casing; The calculation of the deformation of the casing and the cement sheath under stress includes: using the Lame equation and the modified formula of Hooke's law to calculate the deformation of the casing and the cement sheath under stress, including: Calculation of casing deformation: Considering the casing is a completely symmetrical problem, it can be simplified to a plane stress and strain solution. According to the Lame equation: （1） in: is the deformation of the outer wall of the casing, mm; a and b are the inner and outer radii of the casing, mm; is the elastic modulus of the casing, MPa; is the casing Poisson’s ratio; is the actual internal pressure on the inner wall of the casing, MPa; Considering that the outer wall of the casing is affected by the formation confining pressure, we have: （2） in: is the maximum internal pressure, MPa; is the casing internal pressure correction coefficient, generally ranging from 0.91 to 0.99; Considering cement as an elastic body, the deformation of the cement sheath is calculated according to Hooke's law: （3） in: is the internal pressure of the cement sheath, MPa; is the elastic modulus of cement, MPa; is the original thickness of the cement sheath, mm; is the deformation of the cement sheath, mm; Considering the influence of the interface bonding strength and the confining pressure of the formation on the outer wall of the cement sheath, we have: （4） in: is the cement compression correction coefficient, and its value range is 0.85~0.95; Taking into account the actual wellbore diameter expansion rate, we have: （5） in: is the original annulus size, mm; is the well diameter correction coefficient, and its value range is 1.01~1.

1.

2. The method for designing a wellbore structure for a salt cap rock gas storage reservoir according to claim 1, characterized in that: In step 1, determining the production casing size according to the gas injection and production needs of the gas storage includes: The production casing size is determined based on the gas injection and production displacement of the gas storage reservoir and the size requirements of the wellbore tubing.

3. The method for designing a wellbore structure for a salt cap rock gas storage according to claim 1, characterized in that: While calculating the minimum thickness of the cement sheath, the following steps are also included: The minimum thickness of the cement sheath should meet the requirements of the industry standard SY / T 5374.1-2016 Cementing Operation Procedures.

4. The method for designing a wellbore structure for a salt cap rock gas storage reservoir according to claim 1, wherein: In step 3, based on the displacement limit of existing equipment capabilities, determining the maximum thickness of the cement sheath includes: The calculation formula is: in: is the cement sheath thickness, mm; d is the outer diameter of the casing, mm; L is the maximum displacement of the mud pump, L / s.

5. The method for designing a wellbore structure for a salt cap rock gas storage according to claim 1, characterized in that: In step 3, before or simultaneously with the displacement limit based on existing equipment capabilities, the following is also included: Based on ensuring cementing replacement efficiency.

6. The method for designing a wellbore structure for a salt cap rock gas storage according to claim 5, characterized in that: Ensuring cementing displacement efficiency includes: During cementing and injection, the annular return speed is required to be controlled at 0.9m / s-1.5m / s.

7. The method for designing a wellbore structure for a salt cap rock gas storage according to claim 1, characterized in that: In step 4, designing a wellbore structure solution includes: Design the drill bit size and drilling depth for each opening, as well as the casing size, wall thickness and drilling depth for each opening.

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