Research on Thread Seal Failure Mechanism of Casing Hanger in Shale Gas Wells and Prevention Measures

Abstract

The strength and sealing failure of the connecting thread of the casing head mandrel hanger causes huge economic losses. One of the major challenges is the thread seal failure mechanism of the casing hanger in the wellhead during pressure testing in shale gas wells. In order to analyze the failure causes of connecting threads and put forward improvement measures, a typical case of a well accompanied by a hanger seal failure is analyzed in this paper, and a series of material tests are carried out. The microstructure and mechanical properties of casing materials and hanger materials could meet the field requirements. It is concluded that both the hanger material and casing material are characterized with significant ductile fracture. A three-dimensional model of the hanger and casing system is established, and the mechanical behavior is calculated for the connecting thread under different working conditions. The results showed that the connection degree of the hanger–casing is insufficient at the torque recommended by the manufacturer because of the difference in wall thickness between the box thread of the hanger and the box thread of the joint according to the connection degree of the coupling casing. It is seen that the high contact pressure ring of zone three on the sealing surface plays an effective sealing role under the manufacturer’s recommended torque (20,465 N·m). Finally, when the torque is increased by 25%, the maximum contact pressure between the pin thread of the casing and the box thread of the hanger can fully meet the internal pressure from the wellbore pressure test and the internal pressure strength required for subsequent operations.

1. Introduction

The commonly used and typically lower end threads of the mandrel hanger mainly have two connection methods: pin and box. In the past, the casing of the mandrel type hanger mainly used male threads to connect the casing string, and the conventional male and female thread ends used API standard thread buckle types. However, for high-pressure and heavy-duty connectors, the male end below the mandrel is the most stressed part of the threaded connection. The conventional structure of the casing thread buckle cannot meet the on-site high-pressure and heavy-duty working requirements. During on-site use, fracture accidents are often found in the lower end threads of the mandrel [1,2]. Some scholars and experts have studied the bearing structure of the casing hanger. Mohammed et al. [3,4] found that the critical stress between the male and female buckles of the oil casing mainly depends on the preload of the axisymmetric model, while considering the initial make-up torque and the alternating bending load during the experiment. Zexin et al. [5,6] considered the effects of material properties, thread lead angle, and nonlinear contact on the casing joint and used a finite element model to analyze the internal stress distribution of the coupling under the bending moment load, elucidating the fatigue damage evolution process of the casing joint. Yosuke et al. [7,8,9] studied the thread bearing capacity through torque testing and the finite element method, and the maximum stress on the external thread approached the material yield limit. In the torque test, when the torque reached the upper limit, the torque of the threaded connection exceeded the standard torque value, resulting in excessive preloading of the threaded connection. Qinfeng et al. [10,11,12] studied the mechanical properties of threaded stabilizer connections,, decomposed vibration signals through Fourier analysis, and obtained fatigue frequency ratios and vibration spectra. On this basis, the fracture mechanism of the stabilizer connecting thread was revealed. Amir et al. [13,14] described the composite string design method, which can serve as a guide for selecting suitable candidates for this design and an operational guide for complex well section design. Yu et al. [15,16,17] explored the mechanical properties of hanger casings through corrosion and tensile tests and optimized different thread connection methods. A three-level specialized threaded joint was designed for hanger casings, and its feasibility was verified through comparative experiments. Maoxian et al. [18] developed a new type of 140 MPa mandrel-type casing head. Its sealing structure adopted the form of metal sealing at the upper end and rubber sealing at the lower end and had the characteristics of high pressure resistance and reliable sealing performance. Udaya et al. [19,20] proposed a semi-analytical method to analyze the mechanical response of multiple pipe strings, the wellhead, and mud line hangers in offshore wells with mud line suspension systems. This method was applied to calculate the forces on the surface and mud line during production and injection processes. Zhi et al. [21,22,23,24] analyzed the reasons for the failure of the casing head slip hanger using techniques such as optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy. Based on the finite element research results and theoretical equations, the stress distribution on the casing wall at the deepest point of slip engagement was derived, and suggestions for improving the casing material structure under this stress were proposed.

The above research provides important engineering reference value for the structural optimization design, fatigue testing, and life analysis of oil casing threaded joints. The research results provide a good reference for the thread design of hanger casings and the application of casings under complex underground conditions. The above research mainly involves casing hangers in shallow or medium-depth wells. There is limited design and research data on the threaded structure of high-strength hangers, and higher requirements are put forward for the bearing capacity of casing hangers in shale gas wells. However, the threaded connection end of conventional casing hangers has lower strength. In order to meet the requirements of on-site use, this study designs a high-strength core shaft-type hanger casing threaded joint to ensure the safety of the bearings of casing hangers in oil and gas wells.

2. Introduction of Failure Case in the Field

A serious failure of wellhead equipment occurred in a shale gas well called Lu1 in southwest China. The Lu1 well is a horizontal well; its measured depth is 6218 m, its vertical depth is 4295 m, and its configuration is a three-hole-in well.

Table 1. Parameters of the casing string in the Lu1 well.

Type	Depth (m)	Outer Diameter (mm)	Thickness (mm)	Steel Grade	Thread Type
Surface casing	0~399	339.7	9.65	J55	BC
Intermediate casing	0~3148	244.5	11.99	P110	BC
Production casing	0~6218	139.7	12.7	P110	BX2

shows the parameters of the casing string in the Lu1 well. It is worth noting that the string (Φ139.7 × 12.7 − P110) is used as a production casing. Fortunately, no field accidents occurred during the production casing run in and cementing process. However, in the process of pressure testing, when the wellhead pressure rose to 20 MPa (the target pressure was 50 MPa, with 2.0 g/cm³ density drilling fluid in the wellbore), the wellhead pressure began to decline continuously. When the wellhead pressure dropped to 13 MPa, drilling mud oozed from the channel connecting the production casing annulus in the wellhead tree, as shown in Figure 1, and the wellhead pressure operation was stopped. It was initially suspected that there was a leak somewhere in the production casing annulus, resulting in the failure to suppress pressure in the wellbore. The valve was opened after the wellhead pressure was reduced to 0.7 MPa. A long strip of micro-cameras was inserted into the annulus through the channel and found that mud was leaking significantly from the hanger-casing connection thread. Finally, after 103 days, the order to abandon was given, and huge economic losses were incurred.

3. Experimental

3.1. Material and Specimen Preparation

The casing suspender material used in this work was UNS N07718; its chemical composition was measured by an HCS 140 high-frequency infrared ray carbon sulfur analyzer (Shanghai Dekai Instruments Co., Shanghai, China): (wt.%) C—0.03, Ni—52.50, Cr—18.00, Mo—3.00, Nb—5.00, Ti—0.90, AI—0.50, Ca—0.003, Cu—0.23, B—0.005, Co—0.9, Mn—0.32, Si—0.33, S—0.01, P—0.01, Mg—0.005, Fe—balance. Meanwhile, the casing material used in this work was P110; its chemical composition was also measured by an HCS 140 high-frequency infrared ray carbon sulfur analyzer (Shanghai Dekai Instruments Co., Shanghai, China): (wt.%) C—0.24, Si—0.25, Mn—1.75, Ti—0.018, V—0.011, Ni—0.013, Cr—0.06, Mo—0.012, Fe—balance.

From the perspective of chemical composition, both materials meet the requirements. Hence, in order to verify whether the problem of product quality of the hanger and casing caused the wellhead leakage, a series of experiments involving material mechanical properties of the used hanger and casing were carried out. Six types of specimens were prepared and characterized by metallography, XRD, tensile tests, and impact tests. The dimensions of all types of specimens are given in Figure 2. Meanwhile, the specimens were cut from the hanger and casing from the Lu1 well, which had a history of wellhead leaks during pressure testing, as shown in Figure 2.

3.2. Microstructure Characterization

In order to verify whether the hanger and casing materials have microstructure problems, metallographic observation of the hanger material and the casing material is carried out using an optical microscope Axio Scope A1 (Carl Zeiss, Oberkochen, Germany). The specimen is ground on 800-grit SiC paper, followed by mechanical polishing with a suspension of SiO₂ particles 50 nm in diameter. Subsequently, the polished surface is chemically etched in a 2:1:17 (mass ratio) HF–HNO₃–distilled water solution for 45 s.

Subsequently, to verify whether the hanger and casing materials have corrosion problems, an X-ray diffractometer Bruker D8 Advance (Bruker Corporation, Billerica, MA, USA) is used to identify the phase composition of the hanger material and the casing material. Diffractograms are acquired at a tube voltage and current of 40 kV and 40 mA, respectively, a scan range of 5~90°, and a scan speed of 5°/min. Finally, to further explore the changes in grain size quantitatively, the results of XRD are analyzed by data analysis software MDI Jade 5.0 (Materials Data Inc., Livermore, CA, USA).

3.3. Tensile and Impact Tests

In order to compare the static mechanical properties and impact behavior of the hanger alloy and the casing material, tensile tests and Charpy impact tests were conducted at a temperature of 25 ± 1 °C. An MTS-810 tensile machine (MTS System Corp., Eden Prairie, MN, USA) was employed to run stress–strain tests at a velocity of 1.5 mm/min. Meanwhile, a Charpy impact test machine (MTS System Corp., Eden Prairie, MN, USA) was used for impact energy measurements at an impact velocity of 5.4 m/s. The surfaces of the impact-fractured specimens were analyzed using a Phillips Quanta 200 SEM microscope (FEI Company, Hillsboro, OR, USA).

4. Results and Discussion

4.1. Microstructure

Figure 3 shows metallographic images, revealing the microstructure of both the hanger material and the casing material. In Figure 3a, the grain size of the hanger alloy material is uniform, the grain boundary is straight, there is no precipitated phase at the grain boundary, and there are no obvious inclusions in the grain group. Meanwhile, the alloy material of the P110 casing shows a typical tempered sorbite, the ferrite grains are dominant, the grain is uniform, and there is no significant inclusion, as shown in Figure 3b. Therefore, from the perspective of the metallographic image, there are no major defects in the microstructure of the two materials, and it is assumed that they should have good mechanical properties. Meanwhile, there is a good chance that the differences in the mechanical properties of two materials may be obviously present in the structure.

Based on Figure 3, there are clear changes in the grain size and shape. To further investigate whether corrosion factors cause thread seal failure, XRD patterns were acquired from the hanger material and the casing material. Figure 4 shows that there are no significant corrosive elements on the hanger material and the casing material, and the XRD results are all base material components.

4.2. Tensile and Impact Mechanical Properties

To further examine the mechanical properties of the two types, tensile and impact tests are carried out. Figure 5 shows the stress vs. strain curves of the two types of specimens. Key parameters are summarized in

Table 2. Static mechanical properties of the casing hanger and P110 casing, as extracted from the stress–strain curves.

Property	Casing Hanger	P110 Casing
	Average ± Standard Deviation (n = 3)	Average ± Standard Deviation (n = 3)
Yield strength σy, Rp 0.2 (MPa)	889.62 ± 8.67	826.21 ± 7.73
Tensile strength σu, (MPa)	980.27 ± 7.23	913.41 ± 7.71
Yield ratio σy/σu	0.91 ± 0.01	0.90 ± 0.01
Young’s modulus E (GPa)	216 ± 0.001	204 ± 0.001
Elongation δ (%)	22.47 ± 0.62	19.12 ± 1.12

. The yield strength, tensile strength, and modulus of elasticity of the casing hanger base material are higher than those of the P110 casing base material. The yield strength of the casing hanger base material is 889.62 MPa, the tensile strength is 980.27 MPa, and the modulus of elasticity is 216 GPa. Meanwhile, the yield strength of the P110 casing base material is 826.21 MPa, the tensile strength is 913.41 MPa, and the modulus of elasticity is 204 GPa. Hence, from the perspective of tensile mechanical properties, the two materials meet the quality requirements. It is worth noting that the casing hanger has higher modulus of elasticity, suggesting that the casing hanger will carry higher stresses than the P110 casing under the same strain caused by the same load.

SEM was used to characterize the fracture surfaces of the samples fractured by tensile testing, as shown in Figure 6. There are significant dimple groups on the fracture surface of the tensile specimen cut from the hanger base material, and no characteristics (such as cleavage, quasi-cleavage, etc.) are found to indicate brittleness of the material, as seen in Figure 6a. Similarly, there also are significant dimple groups on the fracture surface of tensile specimen cut from the casing base material, as seen in Figure 6b. Therefore, these surface characteristics indicate that hanger and casing materials have good mechanical toughness; combined with the mechanical parameters obtained by the tensile test, it can be preliminarily determined that the mechanical properties of the casing and suspension materials meet the needs of the field.

The related parameters from the Charpy impact tests are listed in

Table 3. Dynamic mechanical properties obtained from Charpy impact tests.

Property	Casing Hanger	P110 Casing
	Average ± Standard Deviation (n = 3)	Average ± Standard Deviation (n = 3)
Impact energy (J)	150.43 ± 1.62	154.36 ± 1.56
Crack initiation energy (J)	42.62 ± 0.15	45.91± 0.22
Crack propagation energy (J)	107.81 ± 0.16	108.45 ± 0.82

. The impact energy of the casing hanger base material reaches 150.43 J, and the impact energy of the P110 casing base material is 154.36 J. It is evident that the dynamic mechanical properties, such as impact energy, crack initiation energy, and crack propagation energy, are higher for the P110 casing than for the casing hanger. Significantly, from the perspective of impact mechanical properties, the two materials meet the quality requirements.

SEM was used to characterize the fracture surfaces of the samples fractured by impact, as shown in Figure 7. Two zones are noticed on the fracture surface: (I) shear lips zone and (II) crack growth (fibrous) zone. On the fracture of the hanger specimen, zone Ⅰ occupies almost half of the entire fracture area and has an approximately 45° angle (γ), as shown in Figure 7a. Moreover, according to the zoom-in image (Ⅲ) of zone Ⅱ, a large dimple morphology is the main feature on the hanger material fracture (Figure 7b,c). Uniformly, on the fracture of the casing specimen, zone Ⅰ occupies almost half of the entire fracture area and has an approximately 45° angle (γ), as shown in Figure 7d; dimple morphology is also the obvious characteristic on the casing material fracture (Figure 7e,f). Therefore, it can be concluded that both the hanger material zone and casing material zone are characterized by significant ductile fracture. These findings are in good agreement with the values obtained from the Charpy impact tests.

In summary, through a series of material tests, it can be seen that the microstructure and mechanical properties of casing materials and hanger materials could meet the field requirements, and no obvious corrosion products are found on the thread surface. Therefore, the analysis shows that the material quality and corrosion are not the root cause of wellhead leakage in the Lu1 well.

5. Cause Analysis of Wellhead Leakage

5.1. FE Modeling

According to the results of microscopic inspection and mechanical test, there is no defect in the base material quality of the hanger and casing, and the size of the pin thread of the casing and box thread of the hanger meets the requirements of the manufacturer. However, there is a great difference between the structural size of the hanger and the structural size of the coupling, especially in the wall thickness. Hence, it is doubtful that the actual torque (20,465 N·m, as recommended by casing coupling manufacturers) of the hanger–casing is the same as that of the coupling casing. Therefore, it is necessary to carry out research to demonstrate the rationality of the torque value of the hanger –casing in the field. In order to further study the torque rationality, a three-dimensional FE mechanical model of the hanger and casing system is established, as shown in Figure 8. In the model, the casing size is Φ139.7 + 12.7 mm for P110, the outside diameter of the hanger box thread is 220 mm, the type is a gas seal thread BX2, the shoulder face angle is 20°, the sealing face angle is 30°, the thread guide surface angle is 10°, the thread bearing surface angle is 4°, and the thread taper is 1/16. Meanwhile, in order to increase the accuracy of the calculation results, the elements near the step, sealing surface, and tooth are secondary encrypted (each element is 0.01 mm in size, which is optimized after grid sensitivity analysis). In the model, the material mechanical constitutive relationship of the hanger and the casing is set according to the mechanical test of the two materials (UNS N07718 and P110) in Section 4.2.

The ABAQUS V6.0/Standard solver is selected for solving analysis in the FE model. the application of the torque is obtained according to the integration of contact pressure, contact area, and other parameters. Figure 9 shows the illustration of the integration of contact mechanics parameters. In Figure 9, the contact position between the casing and hanger is zoomed in. In this area, four basic elements are zoomed in (two red elements belonging to the casing and two blue elements belonging to the hanger). Taking the casing as an example, there are three unit nodes on the casing: N₁, N₂, and N₃.

The distance between N₁ and N₂ is L₁ + L₂, where L₁ = L₂, and the distance between N₂ and N₃ is L₃ + L₄, where L₃ = L₄. For the N₂ node, L₅ (L₅ = L₂ + L₃) can be considered as the influence range of the N₂ node. The distance from the N₂ node to the central axis of the casing is defined as R₂, so the contact area of the N₂ node control area can be calculated as

$$A_2=2\pi R_2{L}_5$$

(1)

After iterative calculation by the FE method, there will is contact force P₂ in the N₂ node. Therefore, the contact force in the control area of N₂ node can be calculated as

$$F_2=2\pi R_2{L}_5{P}_2$$

(2)

Then, the torque in the control area of the N₂ node can be calculated as

$$T_2=2\pi R_2^2{L}_5{P}_2$$

(3)

Therefore, the torque of each node control area can be defined as

$$T_i=2\pi R_i^2{L}_i{P}_i(\mathrm{i}=1,2,3,\dots \mathrm{n})$$

(4)

The A-B path is defined over the entire casing-to-hanger contact range, which includes the entire casing-to-hanger contact area. Then, on the A-B path, the torque of each contact node is integrated, as shown in Equation (5):

$$T_T=0.002\pi f_f{\displaystyle {\int }_B^A{R}_i^2{L}_i{P}_{i}d{L}_T}(\mathrm{i}=1,2,3,\dots \mathrm{n})$$

(5)

Through the secondary development of finite element software, through the automatic adjustment of the interference of the pin thread of the casing and the box thread of the hanger, the calculated torque value is matched with the torque value recommended by the manufacturer.

5.2. Analysis of Thread Contact Pressure

Figure 10 presents the contour of the von Mises stress distribution of the hanger–casing under the manufacturer’s recommended torque (20,465 N·m). It can be seen that the maximum von Mises stress is distributed near the step and sealing surface under the 20,465 N·m torque, and the maximum von Mises stress reaches 666.5 MPa. Therefore, it can be seen that (1) neither the hanger nor casing has plastic deformation according to the tensile test results of the two materials (UNS N07718 and P110) and (2) the maximum sealing contact force between A and B is distributed on the sealing surface; this stress distribution is consistent with the initial design of the thread structure.

Figure 11 presents the contour of contact pressure distribution of the hanger–casing under the manufacturer’s recommended torque (20,465 N·m). Axially, along the hanger–casing contact area, the A-B path is defined on the casing pin thread surface. The step, sealing surface, and bearing surface of each thread tooth have contact pressure on the A-B path, It can be found that (1) on the area of the step and sealing surface, there are three high-contact pressure positions, which are defined as zone 1 (on step), zone 2 (on sealing surface), and zone 3 (on sealing surface), and the contact pressure of these three regions is 687 MPa, 592 MPa, and 699 MPa, respectively; (2) on each thread tooth, the closer to the sealing surface, the higher the contact pressure on the bearing surface of threaded teeth; the sealing pressure range of each thread bearing surface is 89~572 MPa. It can be seen that the high contact pressure ring of zone 3 on the sealing surface plays an effective sealing role under the manufacturer’s recommended torque (20,465 N·m).

Figure 12a shows the contact pressure distribution on the casing thread surface under different loads: (1) only with torque, (2) with torque and tension force from the weight of casing string. It can be seen that the casing–hanger contact pressure is relatively high (699 MPa) on the sealing surface and step only under the manufacturer’s recommended torque; however, the contact pressure in the mentioned area decreases significantly (143 MPa) under the tensile force from the weight of the casing string (986 kN, according to the logging weight of the casing string at the wellhead). Concretely, on the surface of the step, the average contact pressure decreases from 198 MPa to 8 MPa, and the maximum contact pressure decreases from 687 MPa to 143 MPa. Meanwhile, on the sealing surface, the average contact pressure decreases from 465 MPa to 89 MPa, and the maximum contact pressure decreases from 699 MPa to 143 MPa. In order to further study the change of thread contact pressure after casing setting, the contact pressure contour of casing thread surface with torque and tension force from the weight of the casing string is presented (Figure 12b). It is shown that the contact pressure of the sealing surface decreases obviously, and the contact pressure of the bearing surface of the threaded teeth is higher.

5.3. Discussion

These surface characteristics of the tensile specimen indicate that hanger and casing materials have good mechanical toughness; combined with the mechanical parameters obtained by tensile test, it can be preliminarily determined that the mechanical properties of the casing and suspension materials meet the needs of the field. The impact energy of the casing hanger base material reaches 150.43 J, and the impact energy of the P110 casing base material is 154.36 J. It is evident that the dynamic mechanical properties, such as impact energy, crack initiation energy, and crack propagation energy, are higher for the P110 casing than for the casing hanger. Significantly, from the perspective of the impact mechanical properties, the two materials meet the quality requirements.

Figure 13 presents the contour of the contact pressure distribution on the casing pin thread surface after applying the manufacturer’s recommended torque (20,465 N·m) to the coupling and casing. Axially, along the hanger–casing contact area, the C-D path is defined on the casing pin thread surface. And the step, sealing surface, and bearing surface of each thread tooth have contact pressure on the C-D path; it can be found that (1) on the area of the step and sealing surface, there are three high-contact pressure positions, which are defined as zone 1 (on step) and zone 2 (on sealing surface), and the contact pressure of these two regions is 887 MPa and 966 MPa, respectively; (2) on each thread tooth, the closer the teeth are to the middle, the lower the contact pressure; the sealing pressure range of each thread bearing surface is 15~392 MPa. It can be seen that the high contact pressure ring of zone 2 on the sealing surface plays an effective sealing role under the manufacturer’s recommended torque (20,465 N·m).

Figure 14 shows the contact pressure comparison on the casing thread surface between the casing pin thread and the box thread of the hanger, and the casing pin thread and the box thread of the casing coupling, under the manufacturer’s recommended torque (20,465 N·m). It can be seen that under the same torque and the same thread type, the contact pressure distribution on the A-B path is significantly different from that on the C-D path: (1) in the step region, the contact pressure on the casing pin thread connected to the box thread in the hanger (average contact pressure 1 MPa, maximum contact pressure 1 MPa) is significantly less than that of the casing pin thread connected to the box thread in the coupling (average contact pressure 1 MPa, maximum contact pressure 1 MPa); (2) in the sealing surface region, the contact pressure on the casing pin thread connected to the box thread in the hanger (average contact pressure 1 MPa, maximum contact pressure 1 MPa) is significantly less than that of the casing pin thread connected to the box thread in the coupling (average contact pressure 1 MPa, maximum contact pressure 1 MPa); (3) in the tooth region, the contact pressure on the casing pin thread connected to the box thread in the hanger (average contact pressure 1 MPa, maximum contact pressure 1 MPa) is significantly greater than that of the casing pin thread connected to the box thread in the coupling (average contact pressure 1 MPa, maximum contact pressure 1 MPa).

It is well known that the distribution of the contact pressure determines the torque of the pin thread and box thread. Hence, it is seen that the connection degree of the hanger–casing is insufficient at the torque recommended by the manufacturer according to the connection degree of the coupling–casing. This is similar to the failure mode specified in the literature [5,10,11,12,13]. The contact pressure on the tooth region is significantly higher in the hanger–casing system, which results in a greater sum of torque shared in the thread teeth region based on Equation (5), and at the same torque, the contact degree in the sealing surface and the step is relatively insufficient. In other words, the threaded connection between the hanger and the casing was never completed in the field. Hence, the wellhead leakage during the pressure test is caused by insufficient contact pressure under the tension force from the weight of the casing string.

In order to further explain the phenomenon that the contact degree of the casing–hanger and casing–coupling varies greatly under the same torque and the same thread type, radial displacement comparison of the thread surface between the box thread of the hanger and the box thread of the casing coupling under the manufacturer’s recommended torque (20,465 N·m) is carried out (shown in Figure 15). The thread surface on the box thread of the hanger is defined as the E-F path, and the thread surface on the box thread of the casing coupling is defined as the G-H path.

For the hanger, after connecting the thread according to the recommended torque, the radial displacement in the 38 mm~118 mm region along the E-F path is greater than 0 mm, which means that the inner diameter of the thread cavity in this region is extended by the casing thread; however, the radial displacement in the 0 mm~38 mm region along the E-F path is less than 0 mm, which means that the inner diameter of the cavity in this region is reduced. Obviously, the distribution of radial displacement for the casing coupling is similar to that for the hanger; the radial displacement in the 38 mm~118 mm region along the G-H path is greater than 0 mm, which means that the inner diameter of the thread cavity in this region is extended by the casing thread. However, the radial displacement in the 0 mm~38 mm region along the G-H path is less than 0 mm, which means that the inner diameter of the cavity in this region is reduced. But, on the two defined paths, although the trend of axial distribution of radial displacement is the same, the degree of radial deformation is different. In the region of 0~38 mm (near the sealing surface), the inner diameter reduction degree of the box thread cavity of the coupling is significantly greater than that of the box thread cavity of the hanger. Meanwhile, in the region of 38~118 mm (tooth area), the inner diameter expansion degree of the box thread cavity of the coupling is significantly less than that of the box thread cavity of the hanger.

It is necessary to ensure the tightness of various valves during the installation of the wellhead casing head and hanger. If the seal is not good, the high-pressure air flow at the lower part will form a high-pressure load on the upper hanger and wellhead casing head. In this paper, the stress state of the hanger and casing head is obtained through test analysis and finite element simulation calculation. These studies are helpful to guide the field installation and tests.

Under the same torque, due to the large wall thickness of the hanger box thread (average wall thickness is 40 mm, average wall thickness of casing coupling box thread is 14 mm), a small radial deformation can cause a large contact pressure in the tooth area, and a large contact pressure causes a large accumulation of torque in the tooth area. For the recommended torque (20,465 N·m) in Figure 15, the proportion is larger, and correspondingly, the torque accumulation near the sealing surface region is reduced. Finally, the threaded connection is not completed, and the thread leaks during the pressure test. Therefore, referring to the manufacturers recommended torque of the coupling–casing could result in insufficient torque of the hanger–casing.

Based on the above research results, it is necessary to give the recommended hanger–casing torque, which can reasonably increase the safety of the wellhead. Through extensive calculations, a suggested torque value was derived, and Figure 16 shows the contact pressure distribution. It can be seen that when the torque is increased by 25% (25,570 N·m), the contact force between the pin thread and the sealing surface of the box thread is only slightly reduced under axial tension force from the casing weight, but the maximum contact pressure can fully meet the internal pressure of the wellbore pressure test and the internal pressure strength required for subsequent operations.

6. Conclusions and Discussion

(1)

Through a series of material tests, it can be seen that the microstructure and mechanical properties of casing materials and hanger materials could meet the field requirements.

(2)

The connection degree of the hanger–casing is insufficient at the torque recommended by the manufacturer according to the connection degree of the coupling–casing. The contact pressure on the tooth region is significantly higher, which results in a greater sum of torque shared in the thread teeth region in the hanger–casing system; at the same torque, the contact degree in the sealing surface and the step is relatively insufficient.

(3)

Neither the hanger nor casing has plastic deformation according to the tensile test results of the two materials (UNS N07718 and P110), and the maximum sealing contact force between A and B should be distributed on the sealing surface; this stress distribution is consistent with the initial design of the thread structure.

(4)

The closer the teeth are to the middle, the lower the contact pressure on each thread tooth; the sealing pressure range of each thread bearing surface is 15~392 MPa. It can be seen that the high contact pressure ring of zone 2 on the sealing surface plays an effective sealing role under the manufacturer’s recommended torque (20,465 N·m).

(5)

Under the same torque, due to the large wall thickness of the hanger box thread (average wall thickness is 40 mm, average wall thickness of casing coupling box thread is 14 mm), a small radial deformation can cause a large contact pressure in the tooth area, and a large contact pressure causes a large accumulation of torque in the tooth area.

(6)

The maximum contact pressure between the pin and box thread of the hanger can fully meet the internal pressure test requirements when the torque is increased by 25%.