Research on Stratum Settlement and Wellhead Stability in Deep Water during Hydrate Production Testing

During hydrate production testing in deep-water non-diagenetically altered formations, subsea stratum settlement may occur after the decomposition of gas hydrate in a large area of weakly consolidated formation accompanied by an increase of production test time, resulting in wellhead and subsea manifold damage as well as the failure of the production testing project. To solve this problem, an analytical model of subsea stratum settlement and wellhead stability in non-diagenetically altered formations during hydrate production testing was established. Negative friction and drop-down load produced by hydrate decomposition were considered to be the main factors leading to wellhead instability, and the finite element strength reduction method was used to simulate the influence of hydrate decomposition on stratum settlement and wellhead stability. The results showed that the negative friction resistance around the pipe string after hydrate decomposition was mainly distributed in the area about 1/3 the length of conductor above the bottom, and the larger the hydrate decomposition radius, the higher the hydrate saturation, and the greater the negative friction resistance. The laboratory simulation test was carried out to explore the influence of hydrate decomposition on wellhead stability by means of the self-developed wellhead simulation device for gas hydrate production. The results showed that the relative error between the test result and simulation result of negative friction resistance was less than 10%, which verified the reliability of the calculation model and numerical simulation results. The research results can provide a theoretical reference for the time control and wellhead safety assessment of hydrate production test in deep water non-diagenetic formations.