Great economic benefit will be made if a kind of low-cost gas-tight thread can be developed on the basis of API round thread with a relatively lowest cost to replace the existing premium thread. In this paper, a kind of gas-tight threads was designed and manufactured based on the round threads according to the API Spec 5B standard. An elastic sealing ring was added in the middle of the collar, and gas tightness was realized by virtue of the interference matching between the elastic sealing ring and the casing thread. Then, its ultimate bearing capacity was analyzed by using the finite element analysis software MSC.Marc/Mentat. Finally, ISO 13679 B series and C series tests, and tension tests under internal gas pressure in the bending setting were carried out on Ø73.02 × 5.51 mm J55 gas-tight threads according to the ISO 13679 standard.
Zhang Yongqianga,b,*, Liu Lia,b, Lu Jinfuc, Yin Zhifua,b, Wang Kea,b, Liu Jiea,b, Ta Chuana,b
aResearch Institute of Shaanxi Yanchang Petroleum (Group) Co., Ltd., Xi’an, Shaanxi 710075, China. bShaanxi Key Laboratory of Lacustrine Shale Gas Accumulation and Exploitation (Under Planning), Xi’an, Shaanxi 710075, China. cBaoshan Iron & Steel Co., Ltd., Shanghai 201900, China.
Received 27 February 2017; accepted 25 May 2017
It is shown that the gas sealing performance of this gas-tight thread is good, and the results of ultimate internal pressure cycling and the thermal cycling test on sealing pipe ends meet the requirements of ISO 13679. Besides, its internal pressure strength, tensile strength and collapse strength are much higher than those required by GB/T 20657 standard. It is concluded that this type of gas-tight threads can be used in the development of low-pressure gas wells in the Ordos Basin. It has the potential to cut down the casing cost greatly and it can be used as the reference for the design of new gas-tight threads.
As international crude oil prices continued to go down over the past two years and the exploitation difficulty of remaining oil and gas blocks continues to increase, oil companies urgently need to reduce the cost of casing, especially in low-pressure gas field development and CO2 flooding process. In domestic and foreign development of gas-tight threads, seal ledge is usually adopted for sealing, so the cost cannot be significantly reduced [1–4]. API round thread is currently the thread type with the lowest relative cost. Therefore, great economic benefit will be made if a kind of low-cost gas-tight thread can be developed on the basis of API round threads to replace the existing premium thread. It will also be a revolutionary innovation.
1.Design and processing of gas-tight threads
1.1. Theoretical feasibility
Because of the structural design, after the API tubing thread is engaged, there is a spiral leakage channel between the tooth crest and the tooth bottom, which connects the inner space and the outer space of the casing [5–10]. Due to the existence of the leakage channel, the API round thread theoretically does not have fluid sealing capability, so it is generally not used as gas-tight thread. The sealing ability of API round threads can be greatly improved if the fluid leakage channel is plugged by technical means. Through the theoretical calculation and experimental study, Wang Li et al.  believed that the thread compound had a good fluid sealing performance, but the geometric parameter error of thread reduced the sealing ability of thread compound. Wang Jiandong et al.  successfully applied the special thread compound and long round thread string structure in the Sulige gas field. Although the long-term service performance of thread compound is debatable, it provides a theoretical basis for the development of gas-tight threads on the basis of API round threads. If the leakage channel can be plugged with an elastic material, the sealing performance and long-term service performance of API round threads can be significantly improved. In recent years, as the casing thread processing tools evolved from the original machine tools to CNC machine tools, casing thread quality and processing accuracy have been greatly improved. Casing thread processing companies can have a good control of the casing thread processing accuracy, making it possible to achieve elastic seal.
In addition, the development of materials science provides a technical basis for the development of gas-tight threads on the basis of API round threads. With the continuous development of materials science, advanced polymer materials continue to emerge. Under this background, the elastic sealing material applicable in the working conditions of oil and gas fields is not difficult to find. It is feasible to develop gas-tight threads on the basis of API round threads.
The proposed gas-tight thread has an elastic sealing ring added on the basis of an API standard round thread. The elastic sealing ring made of special polymer materials has high elasticity and high strength, and it is embedded in the middle of the collar to realize gas tightness by virtue of the interference matching between the elastic sealing ring and the casing thread. In the new thread, the sealing ring of special polymer materials is installed in the seal groove in the middle of the collar. After the external thread is engaged with the sealing ring, the sealing ring can completely fill and engage with the leakage channel between the tooth top and the tooth bottom through deformation, since the strength of the sealing ring is far lower than that of metal.
Fig. 1. Seal status of the gas-tight thread structure and a local detailed view.
In this way, the non-contact seal of the API round thread is converted to contact the seal, thus greatly improving the reliability of the seal. It is noted that this kind of sealing structure needs higher thread processing precision to make the J value (API 5B provides that the J value is “the distance from the end of the tube to the center of the collar after power-tight”) zero, so that the end of the tube is aligned with the roof and the inner wall after both ends of the casing are screwed on. The connection structure that makes J value zero can guarantee the sealing performance, and also enable the screwing to be conducted by directly using the J value in the oil field, which is easy for workers to operate. The detailed structure is shown in Fig. 1.
The thread of Ø73.02 × 5.51 mm J55 steel tubing was processed according to the design, as shown in Fig. 2.
Fig. 2. Photos of a gas-tight thread.
2. Finite element analysis
The ultimate bearing capacity of thread structure of Ø73.02 × 5.51 mm J55 steel tubing was analyzed by finite element method.
The material properties of the tubing and its collar were determined according to the API standard, with the elastic modulus of 2.1 × 105 MPa and the Poisson’s ratio of 0.3. In this study, the J55 steel tubing was selected for calculation, with the yield strength of 379 MPa and the tensile strength of 517 MPa. A bilinear reinforcement model was established as the material model, by using the stress–strain input data. The sealing material has an elastic modulus of 1420 MPa, a Poisson’s ratio of 0.4, a Moony coefficient C01 of 8 and C10 of 32, so it is a complete elastic material.
2.1. Finite element modeling
The large nonlinear finite element analysis software MSC.Marc/Mentat was used for modeling and analysis, where the axisymmetric three-node triangular solid element was selected. According to the structure and stress of the thread, it was treated as the axisymmetric problem, and the middle plane of the collar was treated as a symmetry plane. The points in the cross section only had the freedom in radial displacement [13–17]. For convenience, the following simplification and assumptions were introduced into the modeling in the premise that the problem concerned is not affected:
- Since the helix angle of the thread is small, its influence is ignored , and the connector is regarded as an axisymmetric structure.
- The connector is made of low alloy steel and is regarded as a homogeneous isotropic body.
- The friction coefficient of the contact surface is related to the type of thread compound. The friction coefficient of each contact surface (including thread and elastic sealing material) in the connector is 0.025 in the calculation [19,20].
The direct constraint method was used to simulate the contact of tubing–casing connector in this calculation.
2.2. Finite element analysis of ultimate bearing capacity
2.2.1. A tensile failure
A tensile load was applied to the engaged connector until tensile failure occurred. As a result, the failure mode of the connector is shown as slip-off failure. One or two engaging threads in the larger end firstly slipped off, and then other threads rapidly slipped off. The final failure load was 408.9 kN, which was greater than the minimum connection strength of the tubing given by the API standard (322.6 kN). The sealing ring still maintained good contact before the tensile failure, as shown in Fig. 3. It indicates that the auxiliary seal of the end face had failed and the thread leakage channel increased rapidly, but the sealing ring was still working at this time.
Fig. 3. Contact stress of the thread, the ring and the top end face surface before a tensile failure.
2.2.2. A compression failure
A compression load was applied to the engaged connector until compression failure happened. As a result, the failure mode of the connector is shown as buckling failure, which started from the first thread engaged with the external thread of the tube, as shown in Fig. 4.
Fig. 4. Equivalent stress distribution and deformation of the connector before a compression failure.
In the compression process, the contact stress on the sealing ring remained essentially unchanged at about −120 MPa. The contact stress on the top surface increased with the increase of the absolute value of the compression load, ranging from −440 MPa to −620 MPa.
2.2.3. An internal pressure failure
An internal pressure was applied to the engaged connector until failure happened. As a result, the failure mode of the connector is shown as failure of the tube blasting, as shown in Fig. 5. The internal pressure failure load is calculated to be 84.0 MPa, which is greater than the internal pressure strength of the tubing given by the API standard (50.06 MPa). In the process of internal pressure application until failure, with the increase of internal pressure, the contact stress on the seal ring increased steadily from −120 MPa to −280 MPa, which was the so-called self-sealing effect. The contact stress on the top surface remained essentially unchanged at about 440 MPa.
Fig. 5. Equivalent stress distribution and deformation of the connector before an internal pressure failure.
2.2.4. An external pressure failure
An external pressure was applied to the engaged connector until the failure. As a result, the failure mode of the connector is shown as failure of the tube collapse, as shown in Fig. 6. The external pressure failure load was calculated to be 87 MPa, which is greater than the external pressure strength of the tubing given by the API standard (52.95 MPa). In the process of external pressure application until failure, with the increase of external pressure, the contact stress on the seal ring increased slightly from −120 MPa to −140 MPa. The contact stress on the top surface slightly reduced from −440 MPa to −410 MPa. The external pressure is applied to the outer wall of the collar and the tube, which has a limited impact on the sealing ring and the contact stress of the top surface.
Fig. 6. Equivalent stress distribution and deformation of the connector before an external pressure failure.
2.3. Summary of ultimate bearing capacity analysis
According to the results of theoretical analysis, the failure load of each ultimate load is higher than the strength of equivalent round thread specified in the API standard, so the structure design is reasonable. With a good deformation ability, the elastic sealing ring can fill the leakage channel between the threads after deformation. In addition, the elastic sealing ring is slightly affected by the axial load, so it has a good sealing effect. It can also produce a lot of contact stress on the top surface, so it has an auxiliary sealing effect.