Analysis of Influence of Hydrate Decomposition on Underwater Wellhead Stability

Abstract

During deep-water drilling, it is easy to encounter hydrate interlayers. Due to engineering disturbances, hydrates may decompose and cause the formation strength to decrease, which will cause the formation settlement and the wellhead instability. In this paper, based on the ABAQUS finite element platform, a wellhead stability calculation model for deep-water drilling encounter hydrate formations is established, and the influence of hydrate decomposition and drilling time on wellhead stability is analyzed. The research results show that the circulation of high-temperature drilling fluid in the annulus will change the formation temperature distribution. The top formation will heat slowly due to its low temperature and the cooling effect of seawater. The middle formation is greatly affected by the drilling fluid, and the lower undrilled stratum remains unchanged because it has not been disturbed by the engineering. The increase in formation temperature will cause the decomposition of hydrates, leading to a decrease in the elastic properties and strength of the near-well formation. The formation with weakened mechanical properties will produce secondary compression and consolidation, and secondary stress concentration will be formed in the hydrate saturation transition zone. This is the essential reason for the effect of hydrate decomposition on the vertical stability of the formation and wellhead. As the drilling time increases, the hydrate decomposition range increases, the bearing capacity of the formation in a larger area decreases, and the risk of underwater wellhead instability increases. However, since the hydrate decomposition area is located below the underwater wellhead at the initial moment, it will have a more serious impact on the stability of the underwater wellhead. The research in the thesis can provide a theoretical basis for the maintenance of wellhead stability in deep-water drilling.