Methane hydrates as a clean and abundant energy source, have attracted the attention of the world. However, the instability of hydrate reservoirs makes it difficult to drill production wells. After drilling, the creep properties of hydrate reservoir would cause wellbore shrinkage, which will affect the drilling safety. This study is to analysis law of wellbore shrinkage after a hydrate well drilled. The results demonstrate that wellbore shrinkage mainly results from plastic deformation and creep deformation. The largest and smallest values are separately found in the directions of the maximum and minimum horizontal in‐situ stress. With the increase of initial in‐situ stress ratio, wellbore shrinkage linearly rise. Under the condition of low horizontal in‐situ stress ratio, wellbore shrinkage is mainly determined by creep deformation. Under the condition of high horizontal in‐situ stress ratio, wellbore shrinkage commonly depends on plastic deformation and creep deformation. With deepening methane hydrate reservoirs, wellbore shrinkage shows a nonlinear increase. For formations with higher hydrate saturation, larger wellbore shrinkage would occur. The research results can provide a theoretical basis for the selection of drilling method while drilling in hydrate formations.

Natural gas hydrate has been identified as a new alternative energy resource which has characteristics of weak cementation, low reservoir strength and shallow overburden depth. Thus, the stability of subsea equipment and formation can be affected during the drilling process. To quantitative assessment on the vertical displacement of the strata induced by hydrate decomposition and clearly learn the influence laws of various factors on wellhead stability, this study established a fully coupled thermo-hydro-mechanical-chemical (THMC) model by ABAQUS. The important factor that affects the wellhead stability is the decomposition range of hydrate. Based on this, the orthogonal experimental design method was utilized to analyze the influence laws of some factors on wellhead stability, including the thickness of hydrate formation, initial hydrate saturation, overburden depth of hydrate sediment, and mudline temperature. The results revealed that, the decomposition of hydrate weakens the mechanical properties of the hydrate formation, thus leading to the compression of the hydrate formation, further causing the wellhead subsidence. The factors were listed in descending order as follows according to their significance of influences on wellhead stability: the thickness of hydrate formation, initial hydrate saturation, overburden depth of hydrate sediment, and mudline temperature. In particular, the effect of mudline temperature on wellhead stability is negligible. Hence, these findings not only confirm the influence of hydrate decomposition on wellhead stability, but also provide important implications for the drilling of hydrate-bearing strata.