A special threaded joint for casing used in multi-stage fracturing of unconventional oil and gas horizontal wells

Abstract

The invention belongs to the technical field of threaded joints, and discloses a special threaded joint for casing used in multi-stage fracturing of unconventional oil and gas horizontal wells, including an external threaded joint and an internal threaded joint connected by a coupling type; the external threaded joint includes an external threaded section, an external threaded sealing section and an external threaded shoulder section connected in sequence; the external threaded sealing section includes an external threaded main sealing surface and an external threaded sealing surface guide surface; the internal threaded joint includes an internal threaded section, a back-cutting groove section, an internal threaded sealing section and an internal threaded shoulder section connected in sequence; the internal threaded sealing section includes a conical sealing surface and an arc sealing surface, the conical sealing surface is connected to the back-cutting groove section through an arc transition surface, and the arc sealing surface is connected to the internal threaded shoulder section; the thread profiles of the external threaded section and the internal threaded section are both interchangeable with the API standard trapezoidal profile. The problem of insufficient working condition applicability of existing API standard threads, semi-special threads and high-end special threads is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of threaded joints, and in particular relates to a special threaded joint for casing used in multi-stage fracturing of unconventional oil and gas horizontal wells. Background Art

[0002] Shale gas and shale oil are unconventional oil and gas resources. Shale gas is stored in the cracks and pores of organic-rich mud shale or high-carbon mud shale formations in a free or adsorbed state. Shale oil is contained in the pores and cracks of mud shale, and in the adjacent layers and interlayers of dense carbonate or clastic rocks. This type of oil and gas resource has no natural production capacity, and special mining processes and technologies such as horizontal wells and multi-stage fracturing must be used. In this way, the contact area between the wellbore and the reservoir is increased, and oil and gas production gap channels are artificially created to obtain industrial output. The engineering technology is difficult and the cost control requirements are strict. At present, the average length of the horizontal well section for shale oil and gas development exceeds 2,000 meters, and the multi-stage fracturing sections exceed 20. The horizontal well production casing consumption is large and the service performance requirements are unique. During the entire service life, the production casing of the horizontal well section is subjected to the rotational bending and alternating tension and compression loads when being lowered into the well, the high internal pressure circulating load of the liquid during fracturing, and the long-term low internal pressure of the gas during the production process. This puts forward the requirements for the applicability of the special threads of the casing while taking into account the economy. The economic requirements are mainly manifested in: easy processing, simple detection, wide quality control, etc. The specific applicability of working conditions includes: anti-galling, high torsion resistance, high compression resistance, fatigue resistance, high-pressure liquid / low-pressure gas sealing performance, etc. The selection process of casing threads for shale oil and gas development has undergone continuous exploration and tortuous development. In the early stage, API standard threads were adopted based on the experience of North America. However, my country's shale oil and gas are buried deeper and the fracturing level is higher. The sealing, compression resistance, torsion resistance and fatigue resistance of API standard threads are insufficient. Then, on the basis of API standard threads, semi-special threads with added shoulders were upgraded. However, the large torque fluctuation range made it impossible to guarantee fatigue resistance, and the lack of main sealing structure made it impossible to meet high-pressure fracturing requirements. In recent years, in order to solve the above problems, high-end special threads with typical radial seals, cylindrical noses and shoulder structures are generally used. However, they show disadvantages such as excessive sealing, insufficient compression resistance and high cost, which cannot meet the engineering needs of safe and efficient large-scale development of shale oil and gas. Summary of the invention

[0003] The purpose of the present invention is to provide a special threaded joint for casing used in multi-stage fracturing of unconventional oil and gas horizontal wells, which solves the problem of insufficient working condition applicability of existing API standard threads, semi-special threads and high-end special threads.

[0004] The present invention is achieved through the following technical solutions:

[0005] A special threaded joint for casing used in multi-stage fracturing of unconventional oil and gas horizontal wells, comprising an external threaded joint and an internal threaded joint that can be connected in a coupling manner;

[0006] The external thread joint comprises an external thread section, an external thread sealing section and an external thread shoulder section which are connected in sequence; the external thread sealing section comprises an external thread main sealing surface and an external thread sealing surface guide surface; both the external thread main sealing surface and the external thread sealing surface guide surface are arc surfaces;

[0007] The internal thread joint comprises an internal thread section, a back cut groove section, an internal thread sealing section and an internal thread shoulder section which are connected in sequence; the internal thread sealing section comprises a conical sealing surface and an arc sealing surface, the conical sealing surface is connected to the back cut groove section through an arc transition surface, and the arc sealing surface is connected to the internal thread shoulder section;

[0008] When the external thread joint and the internal thread joint are engaged, the external thread sealing section contacts the internal thread sealing section;

[0009] The thread profiles of both the external thread section and the internal thread section are interchangeable with the API standard trapezoidal profile.

[0010] Furthermore, the shoulder surfaces of the internal thread shoulder section and the external thread shoulder section are both right-angle shoulder surfaces or negative-angle shoulder surfaces.

[0011] Furthermore, the shoulder angle of the right-angle shoulder surface is 90° with respect to the axial direction of the sleeve, and the shoulder angle of the negative-angle shoulder surface is -3° with respect to the radial direction of the pipe.

[0012] Furthermore, the taper angle of the conical sealing surface is 10 to 25°, the axial length of the sealing surface of the internal thread sealing section is greater than or equal to 3.3 mm, and the roughness is controlled to be 0.5 to 0.8 um;

[0013] The radius of the arc sealing surface is 0.8 to 1.5 mm.

[0014] Further, the arc radius of the main sealing surface of the external thread is 30 to 80 mm, and the arc radius of the guide surface of the sealing surface of the external thread is 1 to 2 mm;

[0015] The axial length of the external thread sealing section is greater than or equal to 3.5 mm, and the roughness is 0.5 to 0.8 um.

[0016] Furthermore, the tool withdrawal groove is used for withdrawing the tool and storing thread compound, and the axial length of the tool withdrawal groove is 1 to 1.5 times of the thread pitch.

[0017] Furthermore, the taper of the external thread section is 1.59 mm in diameter increment for every 25.4 mm in axial length, and the tooth top and tooth bottom of the external thread section are parallel to the generatrix.

[0018] Further, the internal thread section adopts the same taper as the external thread section;

[0019] Or the internal thread section adopts a variable taper solution, specifically:

[0020] Taking the mid-diameter point as the axial boundary, an arc transition section within the pitch range is set, with different taper tolerance zones, a larger taper section on the side close to the coupling end face, and a smaller taper section on the side close to the shoulder face, and the tooth top and tooth bottom of the internal thread section are parallel to the generatrix.

[0021] Furthermore, the larger taper section adopts a taper tolerance zone with a diameter increment range of -0.015 to +0.114 mm per 25.4 mm axial length;

[0022] The smaller taper section is set to adopt a taper tolerance zone with a diameter increment range of -0.064 to +0.025 mm per 25.4 mm axial length.

[0023] Furthermore, the axial length of the complete thread of the external thread segment increases in the range of 20 to 30 mm; and the median diameter of the internal thread segment increases in the range of 0.1 to 0.2 mm.

[0024] Compared with the prior art, the present invention has the following beneficial technical effects:

[0025] The present invention discloses a special threaded joint for casing used in multi-stage fracturing of unconventional oil and gas horizontal wells. The sealing surface, shoulder and thread taper structure of the joint are designed, and different design parameter combinations are adopted according to the working condition parameter classification, aiming to solve the full service cycle requirements of horizontal well fracturing working conditions with a low-cost strategy; the external threaded joint adopts a compact connection and smooth transition of the thread section, the sealing section and the shoulder section, and omits the cylindrical nose. The internal threaded joint adopts the thread, sealing and shoulder parts corresponding to the external thread, and adopts the back-cut groove to take into account the functions of avoiding external thread screw-in interference, machining back-cutting, stress dispersion and storage of thread grease. The thread profile is interchangeable with the API standard trapezoidal thread profile. The above design ensures the connection performance, sealing performance and screw-in positioning functions, and greatly saves processing and maintenance costs. The internal thread sealing section includes a conical sealing surface and an arc sealing surface, and the external thread sealing section includes an external thread main sealing surface and an external thread sealing surface guide surface. The conical sealing surface and the external thread main sealing surface adopt a metal-metal interference fit sealing structure of arc surface to cone surface to achieve a normally distributed contact pressure distribution. The arc-conical metal sealing surface structure keeps the main sealing contact point away from the shoulder, reducing the risk of mechanical damage during the buckle process; greatly reducing the dependence of the seal on the support of the negative angle shoulder; the optimized external thread taper takes into account the sealing performance of anti-sticking and high tensile internal pressure composite load; the sealing length is sufficient to ensure that the integral value of the sealing contact pressure on the contact width is higher than the leakage threshold; the reliability of the sealing structure does not have additional high requirements for surface roughness control; the optimized arc radius takes into account the matching of contact width and peak contact stress, meeting the high sealing performance under the premise of anti-sticking.

[0026] Furthermore, the internal and external thread shoulder sections can adopt right angles and low negative angles, which reduces the dependence of the sealing structure on the supporting effect of the shoulder, makes the main seal position away from the shoulder to reduce the risk of mechanical damage to the main seal, and increases the structural strength of the shoulder section to improve the compression and torsion resistance of the threaded joint.

[0027] Furthermore, the shoulder surface of the internal thread shoulder section and the external thread shoulder section adopts a right-angle shoulder or a negative-angle shoulder. The right-angle shoulder controls the deformation of the nose end, reduces stress concentration, and maximizes the thread's ability to resist overtorque. The lack of support for the sealing surface can be selectively compensated by the internal thread variable taper scheme; at the same time, according to the comprehensive consideration of the working conditions and application occasions and the control of processing costs, a low negative angle shoulder (-3°) can be selected, which is much lower than the negative angle of the shoulder of the typical special thread (generally -10 to -15°), which significantly improves the sealing performance on the basis of limited reduction of the torsion resistance; both the right-angle and low negative angle shoulders are easy to clean. By omitting the cylindrical nose structure of the external thread, the rigidity of the external thread sealing structure is increased; by setting the stress dispersion of the backing groove, the stability of the internal thread sealing structure is increased. In this way, the full 90° right-angle shoulder structure is adopted to ensure high sealing and high compression resistance, and the optional -3° low negative angle shoulder is realized to achieve a reasonable match between high pressure sealing and compression resistance.

[0028] Furthermore, the optimization of the dimensions such as the length of the back-cut groove ensures sufficient length for the strength of the threaded connection, and realizes the functions of avoiding interference with the screw-in of the external thread, processing back-cut, stress dispersion and storage of thread compound.

[0029] Furthermore, the external thread sealing surface adopts a double arc surface, including a main sealing arc surface and an external thread sealing surface guide surface; the external thread sealing surface guide surface ensures smooth contact and sliding of the inner and outer sealing surfaces during the threading process, improves the stiffness of the main sealing surface, and cooperates with the internal thread nose arc surface to form a sealing section thread grease storage space.

[0030] Furthermore, the external thread adopts a single taper, and the internal thread adopts a selective variable taper technology to achieve flexibility in the design, manufacture and use of threaded joints. At the end of the pipe with smaller rigidity close to the external thread, the deformation of the external thread nose after threading is reduced by slowing down the taper of the internal thread, thereby reducing the risk of sticking, making up for the lack of excessive deformation of the sealing surface without cylindrical transition, and fully ensuring sufficient sealing surface contact pressure; at the end of the pipe with larger rigidity close to the disappearing end of the external thread, the taper of the internal thread is increased to increase the torque of the thread part during the threading process, ensuring that the thread is in place and has the ability to resist over-torque. By selectively adopting a combination of internal thread variable tapers, a reasonable match between torsion resistance and sealing performance is achieved. The combination of the change in the shoulder surface angle and the thread taper forms an economical sealing structure suitable for different internal pressure levels, achieving flexibility in the design and manufacture of threaded joints. In response to different sealing and torsion resistance working conditions, a variety of combination changes that comprehensively consider cost and performance are provided.

[0031] Furthermore, on the basis of adopting the API standard thread profile, the effective length of the complete thread profile is increased, the interference fit of the thread middle diameter is reduced, and a flexible design is achieved through the change of taper, which not only improves the applicability to horizontal well multi-stage fracturing conditions, but also achieves the requirements of processing versatility, operational interchangeability and manufacturing economy; the thread bearing area is increased by increasing the complete thread axial length of the external thread; the middle diameter dimensions of the internal and external threads are optimized, and the anti-galling performance is significantly improved by reducing the interference fit of the thread, considering that the shoulder bears 90% of the total torque, and the risk of failure to buckle into place according to the final torque on site is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the cross-sectional structure of the special threaded joint of the present invention;

[0033] Figure 2 It is a schematic diagram of the right-angle shoulder thread and sealing form of the special threaded joint of the present invention;

[0034] Figure 3 It is a schematic diagram of the negative angle shoulder thread and sealing form of the special threaded joint of the present invention;

[0035] Figure 4 It is a schematic diagram of the segmented variable taper form of the internal thread of the special threaded joint of the present invention.

[0036] In the figure: 1 is an internal thread joint, 11 is an internal thread section, 12 is a cutter groove section, 13 is an internal thread sealing section, and 14 is an internal thread shoulder section;

[0037] 131 is a conical sealing surface, and 132 is an arc sealing surface;

[0038] 14-1 is the right-angle shoulder surface of the internal thread shoulder section, and 14-2 is the negative-angle shoulder surface of the internal thread shoulder section;

[0039] 2 is an external thread joint, 21 is an external thread section, 22 is an external thread sealing section, 23 is an external thread shoulder section; 221 is an external thread main sealing surface, and 222 is an external thread sealing surface guide surface;

[0040] 23-1 is the right-angle shoulder surface of the external thread shoulder section, and 23-2 is the negative-angle shoulder surface of the external thread shoulder section;

[0041] 15 is a relatively large taper section, 16 is an arc transition section, and 17 is a relatively small taper section. DETAILED DESCRIPTION

[0042] The present invention is further described in detail below in conjunction with specific embodiments, which are intended to explain the present invention rather than to limit it.

[0043] like Figure 1As shown, the present invention discloses a special threaded joint for casing for multi-stage fracturing of unconventional oil and gas horizontal wells, comprising an external threaded joint 2 and an internal threaded joint 1 connected by a coupling type; the external threaded joint 2 comprises an external threaded section 21, an external threaded sealing section 22 and an external threaded shoulder section 23 connected in sequence; the external threaded sealing section 22 comprises an external threaded main sealing surface 221 and an external threaded sealing surface guide surface 222; the internal threaded joint 1 comprises an internal threaded section 11, a back cutter groove section 12, an internal threaded sealing section 13 and an internal threaded shoulder section 14 connected in sequence; the internal threaded sealing section 13 comprises a conical sealing surface 131 and an arc sealing surface 132, the conical sealing surface 131 is connected to the back cutter groove section 12 through an arc transition surface, and the arc sealing surface 132 is connected to the internal threaded shoulder section 14; the thread profiles of the external threaded section 21 and the internal threaded section 11 both adopt a profile interchangeable with the API standard trapezoidal profile.

[0044] Both the internal thread joint 1 and the external thread joint 2 are steel casing joints, the outer diameter of the external thread joint 2 can be 139.7 mm, and the outer diameter of the internal thread joint 1 can be 160 mm, a tapered thread is used between the internal thread joint 1 and the external thread joint 2, the internal thread joint 1 and the external thread joint 2 adopt a metal interference sealing structure in the form of an arc surface-conical surface, and the internal thread joint 1 and the external thread joint 2 adopt a right-angle or low negative angle shoulder structure.

[0045] The external thread main sealing surface 221 and the external thread sealing surface guide surface 222 are both arc surfaces. The arc radius of the external thread main sealing surface 221 is 30-80 mm, and the arc radius of the external thread sealing surface guide surface 222 is 1-2 mm.

[0046] Specifically, starting from the end face of the outer thread of the pipe body, a radial arc sealing surface composed of a small radius arc (radius 1-2mm) and a large radius arc (radius 30-80mm) is constructed, the small radius arc is the outer thread sealing surface guide surface 222, the large radius arc is the outer thread main sealing surface 221, and the outer thread main sealing surface 221 naturally disappears in the threaded part. The outer thread sealing section 22 is completed by a simple nose chamfering process to ensure that the axial length of the outer thread sealing section 22 is not less than 3.5mm, and the roughness is controlled to be 0.5-0.8um.

[0047] The external thread sealing section 22 adopts a small radius arc (radius 1-2mm) as the guide surface for threading, which disperses the contact pressure when the sealing surfaces start to contact during the threading process, greatly reducing the risk of sticking on the sealing surfaces; the external thread sealing section 22 adopts a large radius arc (radius 30-80mm) as the radial arc main sealing surface, which increases the sealing contact width and improves the robustness of the sealing structure.

[0048] The internal thread sealing section 13 includes a conical sealing surface 131 and an arc sealing surface 132. The conical sealing surface 131 has a taper angle of 10 to 25°, and the radius of the arc sealing surface 132 is 0.8 to 1.5 mm. Specifically, starting from the shoulder surface of the internal thread of the coupling, a radial conical sealing surface 131 composed of a small radius arc (0.8 to 1.5 mm) and a conical surface (taper angle 10 to 25°) is constructed, which disappears into the thread backing groove through chamfering. The axial length of the sealing surface is not less than 3.3 mm, and the roughness is controlled to be 0.5 to 0.8 um.

[0049] The internal thread sealing section 13 starts from the shoulder surface and constructs a radial conical sealing surface 131 composed of a small radius arc sealing surface 132 (0.8-1.5mm) and a conical sealing surface 131 (taper angle 10-25°). The axial length of the sealing surface of the internal thread sealing section 13 is not less than 3.3mm, which optimizes the total sliding distance after the sealing surface contacts, ensures the integrity of the sealing surface and realizes the high-pressure sealing capability under high tensile state. The conical sealing surface 131 can be completed by two processes of rough turning and fine turning. The measurement value change caused by the axial change of the sealing diameter measurement control point is in a simple linear relationship, which is beneficial to the measurement control accuracy and generally realizes easy processing and quality control.

[0050] like Figure 2 As shown, the sealing surface of the internal threaded joint 1 is a conical surface with a taper angle of 15°. It is connected to the shoulder surface with a 1mm radius arc and disappears in the thread backing groove through chamfering. The axial length of the sealing surface is 3.4mm. The sealing surface of the external threaded joint 2 is an arc surface. A 45mm radius arc surface is used to form the main sealing surface. A 2mm radius arc surface is used as a guide surface to connect to the shoulder surface and naturally disappears in the threaded part. The axial length of the sealing surface is 3.6mm.

[0051] Preferably, a back-cut groove is provided at the final tooth position of the internal thread of the casing near the sealing surface, which has both back-cut and thread grease storage functions, and does not interfere with the nose of the external thread after being made up. The internal thread back-cut groove is specifically provided with an axial length of 1 to 1.5 times the pitch (i.e., 5.08 to 7.62 mm).

[0052] like Figure 2 and Figure 3 As shown, the shoulder surface of the internal thread shoulder section 14 and the external thread shoulder section 23 can be selected as a right angle (0°) or a negative angle shoulder surface (-3°), and the inner surface of the pipe body and the coupling connection is flat. Figure 2 When the right-angle shoulder surface 14-1 of the internal thread shoulder section is shown, the right-angle shoulder surface 23-1 of the external thread shoulder section also matches the right-angle shoulder surface 14-1 of the internal thread shoulder section; when Figure 3When the negative angle shoulder surface 14-2 of the internal thread shoulder section is shown, the negative angle shoulder surface 23-2 of the external thread shoulder section also matches the negative angle shoulder surface 14-2 of the internal thread shoulder section accordingly.

[0053] like Figure 4 As shown, the thread profiles of the external and internal threads of the casing are interchangeable with the API standard trapezoidal profile. The thread bearing surface angle of the API standard trapezoidal profile is 3°, and the thread guide surface angle is 10°.

[0054] The taper of the external thread section 21 is 1.59 mm in diameter increment per 25.4 mm axial length (the diameter increment range per 25.4 mm axial length is: -0.038 to +0.089 mm for the complete thread section and -0.038 to +0.114 mm for the incomplete thread section), and the tooth top and tooth bottom of the external thread section 21 are parallel to the generatrix.

[0055] The internal thread can choose to use the same taper as the external thread, or a variable taper solution can be selected. The variable taper of the internal thread uses the mid-diameter point at the axial distance of 40mm from the shoulder as the axial boundary, and sets the arc transition in the axial range of one pitch (5.08mm) according to the pitch of 5 teeth per inch. The arc radius can be 95-110mm. A larger taper is actually set on the side close to the end face of the coupling, that is, the taper tolerance range of the diameter increment is -0.015-+0.114mm for every 25.4mm axial length; a smaller taper is actually set on the side close to the shoulder surface of the coupling, that is, the diameter increment range is -0.064-+0.025mm for every 25.4mm axial length.

[0056] The internal thread section 11 can selectively adopt a thread variable taper solution. Under the premise of ensuring the anti-torsion performance of the thread, the circumferential deformation of the coupling can be controlled, and the radial contact pressure of the main seal can be selectively increased to further reduce the dependence on the negative angle of the shoulder, optimize the thread sealing and anti-torsion performance, and realize the flexible design of the thread joint structure; the thread variable taper achieves a smooth connection through the arc transition section to avoid local stress concentration. The thread variable taper solution is realized by controlling the change of the thread taper tolerance zone, and the optimized taper tolerance parameters ensure the performance and machinability of the flexible design; the large tolerance zone setting of the tapered thread is adopted, and the thread cone surface parallel generatrix structure that is easier to process and control is adopted, which reduces the processing cost under the premise of ensuring the thread connection performance.

[0057] The complete axial length of the external thread can be optimized to increase by 20 to 30 mm, and the median diameter of the internal thread can be increased by 0.1 to 0.2 mm. The complete axial length of the external thread and the median diameter of the internal thread are optimized to ensure the smoothness of the make-up and the connection performance after make-up. The above design achieves the requirements of processing versatility, operational interchangeability and manufacturing economy under the premise of meeting the working condition parameters.

[0058] The above design elements reflect the manufacturing economy and the applicability of working conditions. For example, the use of a thread profile that is interchangeable with the standard thread profile can save more than 50% of the thread processing cost. At the same time, the thread bearing area is increased by increasing the axial length of the complete thread, and the thread interference is optimized by changing the thread median diameter, thereby improving the anti-sticking performance of the make-up. The axial connection efficiency after make-up can achieve 100% pipe body strength, achieving processing versatility, operational interchangeability and manufacturing economy requirements under the premise of meeting the working condition parameters.

[0059] The arc-cone metal interference sealing structure is adopted. The sealing surface structure uses a small radius arc as the guide surface for the external thread. When the sealing surface starts to contact during the process of making the thread, the contact pressure is dispersed, which greatly reduces the risk of sticking of the sealing surface. The external thread uses a large radius arc as the main radial arc sealing surface to increase the sealing contact width. After the thread is engaged, the contact pressure is normally distributed, which improves the robustness of the sealing structure. At the same time, the main sealing contact point keeps a distance from the shoulder to reduce the risk of mechanical damage during the process of making the thread. The external thread sealing surface is completed by a simple nose chamfering process to ensure that the axial length of the sealing surface is not less than 3.5mm. The external thread sealing surface disappears naturally in the thread part, and the general design of transition from the cylindrical surface is not adopted. On the one hand, the processing cost of the thread sealing surface is greatly reduced, and on the other hand, the compression resistance of the thread nose is greatly improved. The internal thread sealing surface starts from the shoulder surface and constructs a radial cone sealing surface 131 composed of a small radius arc and a small angle cone surface. This not only optimizes the total sliding distance after the sealing surfaces contact and thus improves the anti-sticking property of the sealing surfaces, but also realizes high-pressure sealing capability under high tensile state. The main sealing cone surface is easy to process and control.

[0060] The thread design adopts a shoulder structure with an angle of -3° and 0°, which significantly improves the torsion and compression resistance of the thread. By reducing the stress concentration effect at the nose of the thread, the thread's anti-cyclic load fatigue performance is improved, the machining cost is significantly reduced, and it is convenient for on-site cleaning and other maintenance operations. Taking into account the contradictory relationship between the increase in the negative angle of the shoulder to improve the sealing performance and reduce the compressibility, two shoulder angle combinations are adopted to meet the requirements of different working parameters. At the same time, the internal thread of the butt collar can be treated with two schemes: fixed taper and variable taper. The variable taper scheme is to reduce the taper of the internal thread near the end of the external thread pipe to reduce the risk of sticking and increase the contact pressure of the sealing surface; the taper of the internal thread is increased near the end where the external thread disappears to ensure that the thread is buckled in place and has the ability to resist over-torque. By selectively adopting a combination of internal thread variable tapers, a reasonable match between torsion resistance and sealing performance is achieved. In addition, the combination of the change in the shoulder surface angle and the thread taper further strengthens the overall characteristics of the thread, forming an economical sealing structure suitable for different internal pressures and compression performance levels, and realizing the flexibility of thread joint design and manufacturing.

[0061] This special threaded joint can adapt to the selection of casings with different outer diameters, wall thicknesses and steel grades. It is processed by CNC machine tools, and the thickness of the internal thread coating is controlled in the range of 15 to 25um. When making up, apply universal casing thread grease evenly on both ends, clamp the thread close to the pipe body side and the coupling close to the factory end side, and use the best control torque for making up on site.

[0062] After assembly is completed, an expanded conical surface is provided at the interface of the inner wall of the casing external thread end, which is flush with the junction of the casing internal thread end, thereby improving the fluid erosion resistance of the inner surface of the joint.

[0063] After assembly, a radial sealing structure of arc surface and cone surface is formed between the external thread and the internal thread of the casing. Since this sealing structure does not require a transition between the cylindrical surface and the thread part, a simple radial seal is achieved, and an economical sealing structure suitable for different internal pressure levels is formed by the combination of the change in the shoulder surface angle and the thread taper.

[0064] According to the different needs of working parameters, especially the different requirements for the comprehensive performance of threaded joints in resisting gas internal pressure and torsion, different characteristic performances are subdivided through the combination of internal thread variable taper and different shoulder angles with a simple sealing structure. That is, starting from different internal pressure resistance and torsion resistance working conditions, different matching schemes of internal thread variable taper and shoulder angle are selected to reflect the flexible design, manufacturing process and working condition applicability of the product, as shown in the following table.

[0065]

[0066] The above design ensures the smoothness of the buckle and the tensile and compressive performance after buckle. This invention is suitable for the lifting, pressing, and rotating bending process of the casing in the long-distance horizontal and large dogleg wells of shale gas and shale oil wells, improving the water sealing performance of shale gas and shale oil wells under the fracturing load cycle conditions, and meeting the gas sealing performance requirements of the shale gas well production stage. In particular, the general design of the transition from the cylindrical surface between the external thread sealing surface and the threaded part is changed, and the external thread sealing surface directly starts from the end of the pipe body and disappears at the threaded part.

[0067] The present invention adopts the above technical scheme, and the threaded joint is easy to process and easy to thread, and the axial tension / compression and internal pressure / external pressure performance can be as strong as the pipe body. It is suitable for working conditions such as deep burial of shale oil and gas reservoirs in my country, high fracturing pressure, large displacement, and long production target cycle.

[0068] The above content is a further detailed description of the present invention. It cannot be determined that the specific implementation methods of the present invention are limited to these. For ordinary technicians in the technical field to which the present invention belongs, they can make several simple deductions or substitutions without departing from the concept of the present invention, which should be regarded as belonging to the scope of patent protection determined by the submitted claims of the present invention.

Claims

1. A special threaded joint for casing used in multi-stage fracturing of unconventional oil and gas horizontal wells, characterized in that: It comprises an external thread joint (2) and an internal thread joint (1) that can be connected in a coupling manner; The external thread joint (2) comprises an external thread section (21), an external thread sealing section (22) and an external thread shoulder section (23) which are connected in sequence; the external thread sealing section (22) comprises an external thread main sealing surface (221) and an external thread sealing surface guide surface (222); the external thread main sealing surface (221) and the external thread sealing surface guide surface (222) are both arc surfaces; The internal thread joint (1) comprises an internal thread section (11), a back-cut groove section (12), an internal thread sealing section (13) and an internal thread shoulder section (14) which are connected in sequence; the internal thread sealing section (13) comprises a conical sealing surface (131) and an arc sealing surface (132); the conical sealing surface (131) is connected to the back-cut groove section (12) via an arc transition surface, and the arc sealing surface (132) is connected to the internal thread shoulder section (14); When the external thread joint (2) and the internal thread joint (1) are engaged, the external thread sealing section (22) contacts the internal thread sealing section (13); The thread profiles of the external thread section (21) and the internal thread section (11) are both interchangeable with the API standard trapezoidal thread profile; The backing groove is used for backing out the tool and storing thread compound. The axial length of the backing groove is 1 to 1.5 times the thread pitch, specifically 5.08 to 7.62 mm. The taper angle of the conical sealing surface (131) is 10-25°, the axial length of the sealing surface of the internal thread sealing section (13) is greater than or equal to 3.3 mm, and the roughness is controlled to be 0.5-0.8 um; The radius of the arc sealing surface (132) is 0.8 to 1.5 mm; The arc radius of the external thread main sealing surface (221) is 30-80 mm, and the arc radius of the external thread sealing surface guide surface (222) is 1-2 mm; The axial length of the external thread sealing section (22) is greater than or equal to 3.5 mm, and the roughness is 0.5-0.8 um.

2. The special threaded joint for casing used for multi-stage fracturing of unconventional oil and gas horizontal wells according to claim 1, characterized in that: The shoulder surfaces of the internal thread shoulder section (14) and the external thread shoulder section (23) are both right-angle shoulder surfaces or negative-angle shoulder surfaces.

3. A special threaded joint for casing for multi-stage fracturing of unconventional oil and gas horizontal wells according to claim 2, characterized in that: The shoulder angle of the right-angle shoulder surface is 90° to the axial direction of the casing, and the shoulder angle of the negative-angle shoulder surface is -3° to the radial direction of the pipe.

4. The special threaded joint for casing used for multi-stage fracturing of unconventional oil and gas horizontal wells according to claim 1, characterized in that: The taper of the external thread section (21) is 1.59 mm in diameter increment for every 25.4 mm in axial length, and the tooth top and tooth bottom of the external thread section (21) are parallel to the generatrix.

5. The special threaded joint for casing used for multi-stage fracturing of unconventional oil and gas horizontal wells according to claim 1, characterized in that: The internal thread section (11) has the same taper as the external thread section (21); Or the internal thread section (11) adopts a variable taper scheme, specifically: With the mid-diameter point as the axial boundary, an arc transition section (16) within the pitch range is set, with different taper tolerance zones, a larger taper section (15) on the side close to the coupling end face, and a smaller taper section (17) on the side close to the shoulder face, and the tooth top and tooth bottom of the internal thread section (11) are parallel to the generatrix.

6. A special threaded joint for casing used in multi-stage fracturing of unconventional oil and gas horizontal wells according to claim 5, characterized in that: The larger taper section (15) adopts a taper tolerance zone with a diameter increment range of -0.015 to +0.114 mm per 25.4 mm axial length; The smaller taper section (17) is set to adopt a taper tolerance zone with a diameter increment range of -0.064 to +0.025 mm per 25.4 mm axial length.

7. The special threaded joint for casing used for multi-stage fracturing of unconventional oil and gas horizontal wells according to claim 1, characterized in that: The complete thread axial length of the external thread section (21) increases in the range of 20 to 30 mm; the median diameter of the internal thread section (11) increases in the range of 0.1 to 0.2 mm.

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