Abstract
In order to clarify the impact of cyclic loading on the mechanical properties and sealing performance of threaded connections on pipe string under the injection-production conditions of gas storage and the airtight threaded connections suitable for the actual working conditions can be preferred, the sealing failure of injection and production tubular strings of several domestic gas storages was investigated comprehensively. It was found that the airtight threaded connections with different performance may leak during the service of tubular strings. Existing standards and testing methods have limitations, which are not fully applicable to the working environment of gas storage wells and the production mode of forced injection-production and cannot reflect the operation characteristics of multiround injection and production. Based on the stress analysis of the tubular string during injection and production, a full-scale simulation testing of 30 cycles of gas seal cycle under alternating load was proposed. The cyclic loading test results of two P110·13Cr tubing in domestic gas storage showed that after 30 cycles of cyclic loading, the yield strength, internal pressure strength, collapse strength, and compressive bearing capacity of tubing were all decreased; the compression efficiency of the joints should be fully considered when selecting the gas-sealed thread.
1. Introduction
Similar to the gas production wells in the gas field, the injection-production string of gas storage is fixed in layers of casing, which is a two-way channel for natural gas injection and production. In the reservoir up to several kilometers deep, several pipe strings are connected by special threaded connections to bear most of the load from itself and the outside world. Different from other threaded connections, special threaded connectors achieve sealability through the interference fit between the sealing surfaces, which are less dependent on thread compound, and the sealing ability is far better than traditional API connections [1–3]. However, due to the influence of geoenvironment, fluid medium, and cyclical variation of temperature and pressure, the load conditions borne by the tubular string are complex. Failure accidents of injection-production tubular strings occur from time to time, and the statistics show that more than 50% of them occur at the threaded connection position [4–6].
In order to alleviate the potential safety risks and economic losses caused by the sealing failure of tubular string, domestic and foreign scholars have conducted a number of studies on the mechanical properties and sealing properties of special threaded connections by means of analytical method, numerical simulation, and physical test. Among them, the full-scale physical test has won favor for its intuitive phenomena and reliable results. In terms of mechanical properties of threaded connections, Yuan et al. [7] used foil strain gauges to measure the axial strain, hoop strain, and shearing strain of tubing threaded connections under different make-up and break-out torques and analyzed that the uneven contact stress distribution of the sealing structure and rigidity of the thread surface were the main factors contributing to the failure of thread galling. It was also found that torque, velocity, and friction heat of the make-up and break-out also affected the stress distribution of threaded connections. Through make-up and break-out test, internal pressure + tensile test under bending condition, and tensile to failure test, Lv et al. [8] proved that the coupling position with different strengths of internal and external threads was the weak part of special threaded connections casing. Mo et al. [9] combined the finite element method with the full-scale test and comprehensively analyzed the mechanical properties of threaded connections under five working conditions, including making-up, tension, compression, tension + internal pressure, and tension + internal and external pressure. They found that the stress of the middle thread is obviously less than that of the end thread, and the distribution was more uniform; the radial and axial displacement of the thread decreased with the increase of internal pressure load, which was different from the performance under other loading conditions. Xu et al. [10] performed a full-scale fatigue test on special threaded connections and realized the existence of two failure modes, stress concentration at the fillet and fretting fatigue, and found that they occurred at different locations and stress levels. In terms of the sealing performance of threaded connections, Murtagian et al. [11] carried out full-scale tests to study the effectiveness of metal-to-metal sealing structures and obtained critical values of contact pressure and length. Combined with the numerical simulation method, the leakage or transmission factor, a parameter that characterizes the tightness of the structure, was proposed, and a sealability criterion suitable for the threaded connection used in the petroleum industry was developed. Through two-dimensional finite element analysis and physical experiments, Wang et al. [12] pointed out that the effect of bending load on thread sealing performance was significantly greater than tensile load. The recommended values of contact length and pressure between the sealing surfaces were given by comparing different main sealing structures. Li et al. [13] improved the torque shoulder and sealing surface form of API threaded connections and proposed a cone-arc sealing structure, and its good sealing performance was verified through theoretical analysis and full-scale experiment. Mo et al. [14] proposed a double main seal threaded connection structure with a combination of conical surface and cylindrical surface and found that the dual main seal structure has better sealing performance through numerical simulation. The physical test also proved that the connection has good adhesion resistance.
Scholars have done a lot of research on the sealing structure, sealing performance parameters (mainly contact length and stress), and mechanical properties of special threaded connections and many regular understandings have been obtained. However, there are few systematic analyses on the sealing performance of tubular string in underground gas storage specifically. Most of the research methods rely on finite element simulation, and the full-scale experimental procedures carried out are mostly based on ISO 1369 and API RP 5C5. During load analysis, the operation characteristics of periodic strong injection and production in gas storage are ignored, and there are few studies on the performance of injection-production string under cyclic loading [15, 16]. Therefore, based on the statistics of string sealing failures of 6 domestic gas storages, the author comprehensively considers the load conditions during the injection-production operation of string, simulates the operating conditions of the gas storages, and carries out a full-scale airtight cycle indoor test of injection-production string.
2. Statistical Analysis on Sealing Failure of Gas Storage Tubular String
According to related technical reports, TPCQ, VAM-TOP, BEAR, 3SB, BGT1, and other special threaded connections are widely used in the construction of major oil and gas fields and gas storage in China. Due to the different geoenvironment in which the gas storages are located, the wells of the gas storages are run into different depths, so that the pipe string is affected by the corrosive medium in the formation to different degrees. At the same time, there are differences in gas injection-production capacity between gas storage wells, and the string is subjected to different degrees of temperature and pressure during injection-production operation. Therefore, the size of the string, the steel grade of string, and the type of thread used in each gas storage are different.
The sealing test results of 6 gas reservoirs in China (see Table 1) show that the sealing effect of the tie-back casing is relatively poor among the tested casings, with an average passing rate of 98.56%, which is more likely to become a weak part in the production of oil and gas wells. The size of the string also affects the sealing effect to a certain extent, and the performance of the large-size casing is relatively poor in this regard. In addition, the casing is not fully sealed regardless of the type of casing thread used. Affected by the underground environment, the sealing effect of the same type of casing with the same thread in each gas storage is different. The threaded connections used in different positions of the same well are also different. This is somewhat related to the geoenvironment where the wellbore is located and the medium of injected gas, which needs to be further explored in combination with the actual working conditions. Different steel grade string with different thread type, and its sealing performance is also a certain gap. The possibility of casing seal failure may reduce if the best combination of the two is found.
At the same time, it was discovered during the investigation that (1) each layer of the casing was only tested for air tightness when entering the well under tension, and the effect under tension + compression was not considered; (2) moreover, the multicycle reciprocating gas seal tests were not carried out on the airtight threaded connections, and the working characteristics of circulating injection-production of gas storage wells were not reflected; (3) so that there is no clear basis for the selection of gas-sealed thread in gas storage wells, and it is difficult to achieve the best sealing effect of injection-production string.
3. Sealability Indicators
3.1. Load Analysis of Tubular String during Injection-Production Stage
The annual gas injection and production process of the gas storage lasts for several months, during which gas flows bidirectionally between the tubing and the formation, and the temperature and pressure in the tubing change periodically. The stress and deformation of tubing are affected by this phenomenon, which affects the fit between airtight threads of the connecting tubular string. The load-carrying capacity and sealing performance of tubular string are reduced and increase the possibility of damage to the packer or tubular string [17]. During the operation of gas storage wells, the injecting-production string mainly bears forces in radial and axial directions. The axial force mainly comes from the gravity of the tubing itself and the friction between gas flow and tubing wall, while the radial force mainly comes from the internal pressure and external squeeze pressure caused by the change of temperature and pressure in the wellbore. Among them, the internal pressure generated by the direct action of gas pressure will lead to the change of external squeeze pressure and axial force of tubular string, and the pressure generated by the indirect action of temperature will lead to the change of radial force of tubular string. All of the above forces can be converted into axial force through the effect of temperature, ballooning effect, or piston effect [18]. According to [19], the effective axial force of injection-production string during injection-production cycles can be obtained as follows:where is the axial force on the tubing as a result of running or sealing, N; is the axial force of tubing due to the temperature effect, N; is the axial force of tubing due to expansion effect, N; is the axial force of tubing due to piston effect, N; is the axial force on tubing due to friction, N.
The original axial force of tubing is lost during the gas injection and production process, then the effective axial force at this time can be expressed as follows:
Ignoring the friction during gas injection and production, the effective axial force will be expressed as follows:
Considering the maximum pressure drop during gas injection and production , then the maximum friction be expressed as follows:where is the nominal inner diameter of the tubing, mm; and are the minimum and maximum operation pressure of gas storage, respectively, MPa.
The effective axial force can be expressed as follows:
Combined with the actual working conditions, the basic operation parameters (see Table 2) of injection-production well in gas storage are substituted into the theoretical calculation formula, and the load condition of the well during injection-production operation can be obtained, as shown in Table 3.
From the table above, we can see that without the influence of annulus pressure, the injection-production string has a maximum tensile load of 984.74 kN during gas injection, reaching 52.5% of the rated tensile strength; a maximum compressive load of 743.08 kN during gas production, which close to 40% of the rated tensile strength. Therefore, the effects of tensile and compressive loads on the sealing performance of gas-sealed threaded connections for gas storage cannot be ignored.
3.2. Evaluation Index
According to the above load analysis of tubular string, the internal compressive strength, tensile strength, and compression resistance of the threaded connections under the action of axial tensile load and compression load greatly affect the sealing properties and structural integrity of the whole tubular string. Therefore, when selecting special threaded connections, full consideration should be given to their tensile and compression efficiency under operating conditions. Within the 95%VME load envelope, the ratio of the critical compressive load of tubing threaded connection that leaks under the action of internal pressure + compression composite load to the compression yield load of tubing is defined as the compression efficiency. In the 95%VME load envelope, the ratio of the critical tensile load of tubing threaded connection that leaks under the action of internal pressure + tensile composite load to the tensile yield load of tubing is defined as the tensile efficiency [15], which can be expressed as follows:where is the compression efficiency of special thread connections, %; is the tension efficiency of special thread connections, %; is the maximum compression loading of tubing string during injection-production operations, kN; is the maximum tensile loading of tubing string during injection-production operations, kN; is the compression yield load of tubing, kN; is the tension yield load of tubing, kN.
Comparing the compression efficiency and tension efficiency calculated based on the load with their critical values, the special threaded connections that meet the engineering requirements can be preliminarily determined. However, its sealing performance needs to be further determined according to the ISO 13679 standard, and its connection performance can be divided into the following four levels (see Table 4) through the corresponding difficulty test.
According to the API RP 5C5 standard, the sealing performance of the test piece is considered to have reached the corresponding level if it passes a certain strict sealing test. If it can pass a higher level test, the threaded connection meets the assessment level [22].
4. Improvement and Verification Evaluation of Test Scheme
4.1. Limitations of Existing Standard Test
According to the standard of ISO 13679 and API RP 5C5 adopted widely, the sealing assessment and optimization of special threaded connections can be preliminarily realized. For gas storage wells, the fluid flowing between the tubular string often contains acid gas such as CO2 or H2S, which corrodes the string and reduces its service life. However, the existing standards did not take the presence of aggressive liquid that may affect the performance of connectors and did not fully comply with the working environment of threaded connections in gas storage wells. In addition, the B-series gas seal test under this standard only performs cyclic loading of CCW (counterclockwise), CW (clockwise), and CCW for 1.5 cycles, which is challenging to meet the demand of gas storage for injection and production of 30 cycles. Although the tensile load applied during the test reached 80% (up to 95%), the maximum compressive load was only loaded to 75%, and the gas seal test was carried out without increasing the loading level, which is unable to fully reflect various pressure changes during the operation of gas storage well [20–22].
4.2. Improvement of Test Scheme
Given the limitations and incompleteness of existing standards, based on a comprehensive investigation of the use of tubing and casing in 6 domestic gas storages and referring to the gas seal test in the ISO 13679 and API RP 5C5, a multicycle sealing test procedure for threaded connections of injection-production wells in gas storage was proposed. During this period, the special threaded connection with no leakage was considered to be effective in sealing.
4.2.1. Sample and Test Load
Two Φ114.3 × 7.37 mm P110 13 Cr tubings (as shown in Table 5) of domestic gas storage were selected to conduct a multicycle gas sealing test under injection and production conditions.
Combined with the calculation results under cyclic loading of string in Section 3.1, the maximum tensile load of string during gas injection operation was 52.46% of the rated tensile strength. Considering the safety factor (>1.5), the tensile load was loaded to 85% of the rated tensile strength (1595.56 kN) during the test. The maximum compressive load during the gas production was 39.59% of the rated tensile strength. Considering the safety factor (>1.1), the compressive load was loaded to 60% of the rated tensile strength (1126.16 kN) during the test. The internal pressure of the test was 50 MPa.
4.2.2. Test Procedure and Phenomenon
The experiment was divided into three stages. In the first stage, the airtight test under high tensile load (95% of rated tensile strength) was carried out (see Table 6 for the loading scheme). In the second stage, the airtight test under cyclic loading was conducted for 30 cycles (see Tables 7 and 8 for the loading scheme). The applied tensile load and compressive load were 85% and 60% of the rated tensile strength, respectively, and the internal pressure was taken as 50 MPa. Finally, the entire test procedure with the B-series test in API RP 5C5 standard under 95% VME was completed.
Sample 1H was cycled 30 times in the following order: sealing test (see Table 6 for the loading scheme) + cyclic loading for 9 cycles (see Table 7 for the loading scheme) + cyclic loading for 1 cycle (see Table 8 for the loading scheme) + cyclic loading for 9 cycles (see Table 7 for the loading scheme) + cyclic loading for 1 cycle (see Table 8 for the loading scheme) + cyclic loading for 9 cycles (see Table 7 for the loading scheme) + cyclic loading for 1 cycle (see Table 8 for the loading scheme). In the 26th cycle of sample 1H, when it was loaded to the load Step 7 (load point 4) in Table 7, the tubular string suffered compression instability, but no leakage occurred during the airtight cycle test. The test result curve is shown in Figure 1, and the instability morphology of the sample is shown in Figure 2.
Given the instability failure of sample 1H during the test, after making appropriate adjustments to the test procedure, sample 2H was recirculated for 30 cycles in the following order: sealing test (see Table 6 for the loading scheme) + cyclic loading for 15 cycles (see Table 7 for the loading scheme) + follow the loading steps 1–9 to load (see Table 9 for the loading scheme) + cyclic loading for 15 cycles (see Table 7 for the loading scheme). After the completion of sample 2H for 30 cycles, no leakage occurred in the tubular string. The curve of the test result can be seen in Figure 3.
B-series test at 95%VME was carried out for 2H according to Table 9. When it was loaded to load Step 23 (load point 7) in Table 9, the tubular string suffered compression instability, and no leakage occurred during the sealing cycle. The test result curve is shown in Figure 4, and the instability morphology of the sample is shown in Figure 5.
4.3. Analysis of Test Results
The Φ114.3 × 7.37 mm P110 13Cr B type of tubing did not leak after 30 cycles of the cyclic sealing test under alternating tension or compression load. The test path is shown in Figure 6.
Then the limit load envelope sealing test at 95% VME (API RP 5C5 standard B-series test) was carried out, and the test path is shown in Figure 7. In the third CCW (counterclockwise) cycle, the string failed when an airtight test was performed under the final compression load. It indicates that after 30 cycles of cyclic sealing test, the compression bearing capacity of tubing decreased, and it can no longer reach the 95% VME load envelope, as shown in Figure 8. Therefore, in the later operation process, the load on the tubular string should not exceed 85% of the VME.
A 25.4 × 50.8 mm strip longitudinal tensile specimen was taken from the tubing body after the cyclic sealing test and performed a tensile test in accordance with ASTM A370-14 under the ambient temperature in detail, as shown in Table 10. Compared with the performance of string material before the test, it was found that the yield strength of tubing material (average value 721 MPa) did not met the standard requirements, and the strength decreased by an average of 14% and 17.38% at the maximum. It further showed that the bearing capacity of string decreases after a multicycle of injection-production operation. Compared with a decrease in the actual yield strength of the material, the bearing capacity of the string was calculated according to the standard yield strength reduction of 15% (85% VME), as shown in Figure 8. From this, we can see that the injection-production operating pressure (in the working area) was still within the 85%VME stress ellipse, but the maximum compressive load and operating pressure point in the test area exceeded the 85% VME stress ellipse. Therefore, the tubular string still has sufficient bearing capacity under normal injection-production operations, but increasing operating pressure during later workover or stimulation operations requires recalculation based on the string’s residual strength.
If the yield strength was reduced by 15%, the actual tensile strength, internal pressure strength, and collapse strength of the Φ114.3 mm × 7.37 mm P110 13Cr B type tubing are shown in Table 11. Based on this, the safety factor of internal pressure resistance is 1.15, and the maximum wellhead pressure does not exceed 33.91 MPa.
5. Conclusions
Based on the experimental results and discussions above, some conclusions can be drawn as follows:(1)The sealing performance of existing gas-sealed threaded connections is different, and leakage phenomena of different degrees will occur in the working environment of gas storage with multiple injection-production cycles. The sealing effect achieved by combining different steel grade tubing strings and threaded connections is also different, and the efficient sealing combination of the two needs to be further explored.(2)Existing standards have good adaptability to the classification of ordinary oil and gas well tubing and casing airtight threaded connections. Still, they can no longer meet the 30-cycle injection-production requirements of gas storage, and the lower compression load cannot reflect the various changes in injection and production pressure. It is necessary to increase the cycle loading times and loading values of the original thread sealing test method or to adjust the test process to adapt to various working conditions.(3)After 30 cycles of cyclic loading of the tubing string, its compression capability is reduced, and it can no longer reach the 95% VME load envelope. Therefore, the tubular string should not be subjected to more than 85% of the VME in the later operation.
Data Availability
The testing data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This study was funded by the National Natural Science Foundation of China (Grant no. 42072166) and Natural Science Foundation of Heilongjiang (Grant no. LH2020D004).