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Development of gas-tight threads based on API round threads and its evaluation

Fig. 2. Photos of a gas-tight thread.

3. Full-scale test

3.1. Test procedures

A full-scale physical test was performed on the gas-tight thread of Ø73.02 × 5.51 mm J55 steel grade tubing according to GB/T 21267/ISO 13679 [21] and other standards. First of all, the non-destructive testing, size measurement, physical and chemical testing and make-and-break test of the tubing were conducted to ensure that the quality of the connector were in conformity with API 5CT and other relevant standards.

3.2. Gas sealing tests

In the test, the gas sealing performance of the tubing thread connector under the conditions of tension, compression and temperature was mainly evaluated, the load condition of the downhole tubing was simulated, and the test machine was the composite load test system. Bubble internal pressure attenuation detection method was adopted.

3.2.1. B series tests (tensile/compression + internal pressure cycle test)

The sample did not leak during the test. The test results met the requirements of GB/T 21267-2007/ISO 13679: 2002 Petroleum and natural gas industries – Procedures for testing casing and tubing connections. The test load is shown in Table 1. The thread did not leak in the whole process of loading.

Table 1. Load of B series tests.

aThe composite stress of VME was calculated according to the nominal outer diameter (73.02 mm), the minimum nominal wall thickness (5.23 mm) and the nominal yield strength (379 MPa) of the pipe as well as 60% of the tensile efficiency and 55% of the compression efficiency of the connector.

Table 1. Load of B series tests.

3.2.2. C series tests (thermal cycling test under tensile and internal pressure conditions)

The sample did not leak during the test. The test results met the requirements of GB/T 21267-2007/ISO 13679: 2002 Petroleum and natural gas industries – Procedures for testing casing and tubing connections. The test load is shown in Table 2. The thread did not leak in the whole process of loading.

Table 2. Load of C series tests.

aThe equivalent VME composite stress in the inner surface of the tube was calculated according to the nominal outer diameter (73.02 mm), the minimum nominal wall thickness (5.23 mm), the nominal yield strength at room temperature (379 MPa) and 90% of the nominal yield strength of the pipe (341 MPa) at 100 °C as well as 60% of the tensile connection efficiency of the joint.

Table 2. Load of C series tests.

3.2.3. Tension tests under internal gas pressure in the bending setting

The sample did not leak during the test. The test results met the requirements of GB/T 21267-2007/ISO 13679: 2002 Petroleum and natural gas industries – Procedures for testing casing and tubing connections. The test load is shown in Table 3. The thread did not leak in the whole process of loading.

Table 3. Loading step of tension tests under internal gas pressure in the bending setting.

aThe equivalent VME composite stress in the inner surface of the tube was calculated according to the nominal outer diameter (73.02 mm), minimum nominal wall thickness (4.82 mm), nominal yield strength (379 MPa) and 60% of connection efficiency of the connector.

Table 3. Loading step of tension tests under internal gas pressure in the bending setting.

3.2.4. Ultimate internal pressure cycling and thermal cycling test at the sealing pipe end

The sample did not leak during the test. The test results met the requirements of GB/T 21267-2007/ISO 13679: 2002 Petroleum and natural gas industries – Procedures for testing casing and tubing connections. The test conditions are shown in Table 4. The thread did not leak in the whole process of loading.

Table 4. Ultimate internal pressure cycling and thermal cycling test conditions.

aThe equivalent VME composite stress in the inner surface of the tube was calculated according to the nominal yield strength at room temperature (379 MPa), 90% of the nominal yield strength (341 MPa) at 100 °C, the nominal outer diameter (73.02 mm) and the minimum nominal wall thickness (4.82 mm).

Table 4. Ultimate internal pressure cycling and thermal cycling test conditions.

3.3. Ultimate load tests

The ultimate load test includes three kinds of failure paths (i.e., internal pressure failure, tensile failure, and external pressure failure), which respectively verify the tensile strength, bursting strength and collapse strength of the tubing. The tensile failure test and the internal pressure failure test were conducted on a composite loading test machine.

3.3.1. Internal pressure failure test

When the internal pressure of the sample was increased to 94.8 MPa, the leakage failure occurred in the thread at both ends A and B at the same time. The test result is higher than the required value (50.1 MPa) of GB/T 20657-2011 Petroleum and natural gas industries – formulae and calculation for casing, tubing, drill pipe and line pipe properties. The macroscopic appearance of the sample after failure is shown in Fig. 7.

Fig. 7 Download high-res image (307KB)Download full-size image.

Fig. 7. Macroscopic appearance of the sample after an internal pressure failure.

3.3.2. Tensile failure test

When the tensile load was 665 kN, the thread at end B slipped off. According to the test result, the ultimate bearing capacity of the sample is higher than the required value (322 kN) of GB/T 20657-2011 Petroleum and natural gas industries – formulae and calculation for casing, tubing, drill pipe and line pipe properties. The macroscopic appearance of the sample after failure is shown in Fig. 8.

Fig. 8. Macroscopic appearance of the sample after a tensile failure.

Fig. 8. Macroscopic appearance of the sample after a tensile failure.

3.3.3. External pressure failure test

When the external pressure was 70.1 MPa, the pipe at end B collapsed. According to the test result, the ultimate bearing capacity of the sample is higher than the required value (53.0 MPa) of GB/T 20657-2011 Petroleum and natural gas industries – formulae and calculation for casing, tubing, drill pipe and line pipe properties. The macroscopic appearance of the sample after failure is shown in Fig. 9.

Fig. 9. Macroscopic appearance of the sample after an external pressure failure.

Fig. 9. Macroscopic appearance of the sample after an external pressure failure.

3.4. Test result analysis

The test basically reached the J55 steel material limit. The test results show that the thread structure can pass through the ISO13679 B and C series tests, and has reliable gas sealing performance and stable sealing capacity. There is a certain difference between the full-scale test results and the finite element calculation results, which may be associated with the manufacturing tolerances and the material properties of the samples.

4. Application prospect

The above analysis shows that the thread structure has a good sealing performance and stability, suitable for use in low-pressure conditions. In recent years, there has been an increasing interest in CO2 storage and flooding projects at home and abroad, where cost is a key issue restricting such projects [22–25]. The proposed thread structure can effectively prevent the leakage of the wellbore in the CO2 storage and flooding projects and can reduce the casing cost. In addition, many large gas fields have been found in the Ordos Basin, such as Sulige, Wushenqi, Jingbian, Yulin, and Daniudi [26–28]. These gas fields are characterized by low porosity, low permeability, low pressure and low yield, so there is an urgent need for low-cost development technology. This thread sealing structure can completely satisfy the requirements of most gas wells in the Sulige gas field and the Yanchang gas field. If high-grade steel pipes are used, the sealing capacity of such thread sealing structure will be further improved to meet higher-pressure working conditions.

According to the current market price, the price of specially-threaded casing is at least twice that of API round thread casing with the same steel grade. The cost of the gas-tight thread developed on the basis of the API round thread is about 1.2 times that of the API round thread. Therefore, at least 40% of the casing cost can be saved and considerable economic benefits can be made if this gas-tight thread is used to replace the current widely-used premium thread.

5. Conclusions

  1. It is feasible to develop gas-tight threads on the basis of API round threads by relying on high molecular elastic seal material to improve the gas sealing performance of API round threads.
  2. The gas-tight threads can be used in the low-pressure gas wells, and CO2 storage and flooding projects of the Ordos Basin due to their great potential of cost reduction.

 

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© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Corresponding author. Research Institute of Shaanxi Yanchang Petroleum

(Group) Co., Ltd., Xi’an, Shaanxi 710075, China.

E-mail address: [email protected] (Zhang YQ.).

Peer review under responsibility of Sichuan Petroleum Administration.

 

 

 

Emanuel Martin
Emanuel Martin is a Petroleum Engineer graduate from the Faculty of Engineering and a musician educate in the Arts Faculty at National University of Cuyo. In an independent way he’s researching about shale gas & tight oil and building this website to spread the scientist knowledge of the shale industry.
http://www.allaboutshale.com

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