Gas-Tightness Evaluation of Threaded Connections for Deep Oil and Gas Wells at High Temperature

This study systematically investigates the gas-tightness evolution of three threaded casing connections (511 straight-thread, TPG2, and BGT2) under extreme downhole temperature–pressure conditions through multi-cycle experiments. A novel cyclic testing protocol was developed to simulate three critical scenarios: 50 °C/21 MPa (low-temperature high-pressure), 350 °C/21 MPa (high temperature and high pressure), and 450 °C/7 MPa (high temperature and low pressure). Quantitative leakage analysis using the ideal gas law revealed significant performance divergence: TPG2 demonstrated superior stability with leakage rates of 0.61% (350 °C/21 MPa) and 0.39% (450 °C/7 MPa), attributed to its barb-type threads and multi-stage sealing design. In contrast, conventional 511 connections showed 2.54% leakage under high-temperature, high-pressure conditions, while domestic BGT2 exhibited intermediate performance (2.46% at 350 °C/21 MPa). The results establish temperature–pressure synergy as the dominant degradation factor, with combined 350 °C/21 MPa conditions causing 300–400% higher leakage than individual extremes. These findings provide critical empirical evidence for optimizing premium connection designs in complex reservoirs, particularly for thermal recovery and ultra-deep applications where sealing integrity determines operational safety and efficiency.