Wellhead stability in gas hydrate formation during deep-water drilling

During the deep-water drilling, the underwater wellhead weighing hundreds of tons mainly depends on the surrounding seabed soil to provide the bearing capacity before cementing. If the natural gas hydrate layer is drilled, the stability and bearing capacity of the formation will decrease significantly with the continuous decomposition of the hydrate, which may lead to the instability and sinking of the underwater wellhead and surface conduit, resulting in major safety risks such as borehole scrapping. The influences of drilling hydrate layers with different buried depths and distribution positions on wellhead stability are analyzed, physical tests are carried out, and the influence laws of different saturations and standing waiting time on bearing capacity are investigated. The results show that the buried depth of hydrate is shallow. After passing through the hydrate layer during injection, the bearing capacity of the formation is the lowest, and the risk of underwater wellhead instability is the highest. After hydrate decomposition, the maximum reduction of the ultimate bearing capacity of the formation can reach 35%. The standing waiting time after injection has a great impact on the recovery of bearing capacity of the formation after hydrate decomposition. The bearing capacity of the formation increases obviously in the initial period of 1 ~ 12 h, then slows down gradually and increases approximately in logarithmic form. A fitting model between the standing waiting time and the reduction coefficient of bearing capacity is established. Based on the results, for a well in the hydrate formation in the South China Sea, the design of surface conduit running depth and the optimization of wellhead are performed. During the drilling of hydrate formation, the mud running depth of surface conduit can be appropriately increased, the standing waiting time can be increased, and the suction piles can be used to improve the bearing capacity of wellhead and prevent the wellhead from sinking. This study may provide a theoretical basis for the drilling design and wellhead safety assessment in deep-water hydrate formation.