Study On Joint Strength And Sealing Theory For Casing And Tubing Premium Connections

Casing and tubing premium connections have the advantages of high joint strength and good sealing performance, which have become one of the key technologies to ensure the safety of wellbore under harsh circumstances like high temperature, pressure and corrosive media in recent years. However, even the acknowledged advanced premium connections have also failed from time to time, including the cracking or fracturing and sealing failure. Premium connection is studied mainly based on finite element analysis and experimental method presently, which have difficulties in revealing its failure mechanisms and common laws. Meanwhile, there are no relevant recognized standards for premium connections, and the major casing and tubing companies possess their own sealing structures, core design theory and analysis software.This paper attempts to study systematically the joint strength and sealing theory of premium connections by employing modeling, including make-up torque model, mechanical model under axial working loads, joint strength model, the sealing surface contact stress model, the gas leakage rate model, a qualitative and quantitative sealability evaluation method, the parameters design methods for sealing structure, the leakage mechanism caused by material corrosion and contact stress relaxation. The studied contents are as follows:(1) The research status for casing and tubing connections has been investigated from the aspects of theory, finite element analysis and test. The main problems for premium connections have been analyzed and the research train and main contents in this paper has been put forward.(2) The make-up torque composition for premium connections has been analyzed and a theoretical model established based on the elasticity of thick-wall cylinder and rotary shoulder torque formula. The influences of different interferences and thread friction coefficient on the total torque and the ratio of torque acted on shoulder to total torque have been investigated by an example analysis for ? 88.9 mm tubing premium connections, which provide a theoretical foundation for thread parameters design.(3) The axial load and contact stress on the thread teeth distribution models under pre-tightening force have been established based on the elasticity theory and the 3D geometry description of thread teeth. And the model is validated by FEM method for a same premium connection. Example analysis show that the pre-tightening force mainly applies to the last 4-6 engaged threads, particularly near half of that to the last one. In order to prevent the thread galling, over torque acted on the shoulder should be prevented and the effect of shoulder pre-tightening force on the joint strength and sealing performance be considered.(4) A mechanical model of premium connection under the axial working loads has been established based on the elasticity theory. The distribution laws of the axial working loads has been obtained by an example calculation for a ? 88.9 mm tubing premium connection, which shows the main failure positions and forms and also provide a theoretical foundation for establishing the joint strength formula.(5) The deficiency of the joint strength formula for API thread connection has been analyzed. The new tensile and compressive joint strength formulas for tubing and casing premium connection have been proposed when considering the pre-tightening force from the shoulder, the thread strength and the preventions of the shoulder separation or yield. By an example analysis for ? 88.9 mm tubing premium connections with non-external upset, external upset and special clearance coupling designs, the failure laws of premium connection have been obtained and the measurements for enhancing the joint strength and efficiency proposed, including internal or external upset design, avoiding using special clearance coupling for thick wall tubing and controlling the reasonable torque acted on the shoulder.(6) The interference fit contact stress models for coordinate (cone to cone) and non-coordinate (ball to cone, ball to column and ball to ball) metal to metal seals have been established by thick wall cylinder interference fit theory and the Hertz elastic contact mechanics respectively. The influencing laws of the cone taper, tubing wall thickness, radius of the sphere and the elastic modulus of the deposited alloy on the contact stresses obtained by the sensitivity analysis. The methods for enhancing its sealability, including a reasonable cone taper, adoptions of the internal ball to ball or ball to column contact and a low elastic modulus deposited alloy, has been proposed.(7) The contact stress models of the main sealing and shoulder under downhole working loads have been established with the string mechanics theory. The influencing laws of the axial working loads, internal gas pressure, external annular pressure and wellbore temperature increase on the sealing length and average contact stress obtained by sensitivity analysis for cone to cone and ball to column seal structures. A reasonable torque acted on the shoulder and the prevention of the high sustained casing pressure suggested for sealability enhancement.(8) The sealability evaluation methods for premium connection has been established based on the sealing energy mechanism and sealing yield criterion respectively, also the gas sealing enveloped diagram drawn. The sealability of the main sealing and shoulder under downhole loads evaluated and the parameters design method for metal to metal seal proposed.(9) A gas leakage rate model of metal-to-metal seal in premium connection has been established based on the fractal elastic-plastic contact model between rough surfaces and the non-compressed viscous fluid laminar flow theory. The model was validated by the gas sealing enveloped diagram from Murtagian test. The influencing laws of the average sealing pressure, length and roughness on the gas leakage rate obtained by example calculation. The proposed model is beneficial to reveal the leakage nature, evaluate the sealability qualitatively and the parameters design.(10) The corrosion tests under the simulated conditions of the downhole and wellhead temperature, pressure, formation water and CO2 corrosion environment for 13CrS-110 and carbon steel has been conducted. The stress corrosion test for 13CrS-110 tubing premium connection has been enforced, which reveals the seal failure mechanism and laws caused by material corrosion. The view that crevice corrosion and leakage of premium connection caused by contact stress relaxation of sealing surface have been proposed, the performance of stress relaxation for the commonly used material steels in oil and gas wells evaluated. Furtherly, different sealing design theory for tubing and casing premium connection with high grade and low grade steel have been recommended.In conclusion, through the research work of this paper, the joint strength and sealing theory for casing and tubing premium connections have been established systematically for the first time, which not only provides the theory basis and methods for analyzing the mechanical behavior, evaluating the joint performance and designing the new products of the casing and tubing premium connections, but also to be a great significance for reducing the failures of premium connections and improving the wellbore integrity for high temperature and high pressure gas wells. Therefore, the research work in this paper not only has important theoretical significance and academic value, but also important engineering significance and broad application prospects.