Time-dependent mechanical characteristics of natural gas hydrate production test casing under exploitation disturbance

Production tests of natural gas hydrate in deepwater have been successfully conducted twice in China in 2017 and 2019. However, exploitation disturbance caused huge threat to the security of production well, as hydrate production was characterized by complicated phase variation in coupled seepage process, which made conventional analysis approach of oil and gas production well futile. This paper aimed at proposing a mechanical analysis method for casing in hydrate production test well based on the self-developed phase transition solver, which was employed to address exploitation disturbance induced by the varying hydrate saturation, reservoir strain state and physical parameters. By developing a multi-field coupling model and the corresponding algorithm, dynamic changes of casing boundary condition were achieved, and temporal-spatial distribution of casing stress was revealed. A case study was presented to illustrate the application of the proposed methodology. Results indicated that casing stress presented a distribution of “8” and a trend of decreasing first and then increasing with the progress of production test, reaching 112.95 MPa at the 60th day. Meanwhile, this trend was affected by production pressure and in-situ stress inhomogeneity. Moreover, as in-situ stress inhomogeneity, initial hydrate saturation and initial pore pressure changed from 1.05 to 1.3, 0.3 to 0.6 and 19 MPa–15 MPa, casing maximum stress increased by 2.3 times, 13.9% and 11.2%, respectively. The proposed approach could be used to provide theoretical reference for safety design of hydrate production test well.