Analysis for Selection of Tubular Connections for HPHT Gas Well Completion

Currently, in high pressure and high temperature (HPHT) gas well, the selection of tubular connection is based primarily on ISO13679 standard. However, based on well failure investigation, connections which meet ISO13679 CAL IV test requirement, may not meet the needs of high temperature and high pressure as well as high corrosion environment gas wells. It is therefore necessary to revisit the fitness-for-service analysis and assessment methodology based on downhole working condition. For example, there were 44 development wells in petrochina southwest oil and gas field a block. Among them, 35 wells had the annular filled with anomaly pressure, leading to the tubular failure rate at 80%. During the production, serious leakage was found due to the tubing connection fracture and seal failure. When comparing the downhole working conditions with the ISO13679 standard,the main difference was found to be the coexistence of internal and external pressures under the varying axial load and temperature in different operation stages, and the existence of environmental corrosion. In this paper, Finite Element Analysis (FEA) model was established to study the premium connection limit tolerance. The analysis revealed the connection stress distribution, the seal contact pressure, as well as length change due to the different makeup torques. The minimum make-up torque was analyzed to study its impact on the seal contact pressure and length change of connection under the working conditions. In addition, different make-up torque conditions were analyzed to study the impact of the vibration load that may cause connection fatigue failures. In premium connections, the reverse shoulder is generally used. The analysis found that the greater shoulder reverse angle and shorter sealing length may reduce the ability to resist the torque due to the plastic deformation in the shoulder. It was found that as the compressive stress reached to greater than 80% of the material yield strength, the corrosion rate increased significantly, causing possible seal failure. However,the test method was not conformity with the coexistence of internal and external pressure in downhole in ISO13679 international standard. The FEA found that the connection seal ability to gas significantly decreases as the external pressure increases under axial load. The FEA also found that as the tubing string lateral vibration load becomes more than 9g acceleration, the fatigue endurance limit of connection reduces to less than 10 million times. The paper identifies that the coupling thread is prone to fatigue fracture and the seal prone to leakage failure. Therefore, it is recommended that make-up shoulder interference laps be controlled for premium connection with reverse angle shoulder. Under tension and high external pressure, test and analysis are required to verify the gas seal of premium connections, to meet resistance requirement to avoid the thread fatigue fracture and seal leakage under lateral vibration load in downhole. This paper presents the tubing connection fitness-for-service analysis and assessment methodology for high temperature and high pressure gas wells.