Practice of Ultra-Deepwater Shallow Well Construction in Nature Gas Hydrate and Shallow Gas Formation

Abstract Due to the large water depth and geological structure, a large amount of nature gas hydrate (NGH) and shallow gas are buried in the shallow layer in the deepwater area of the South China Sea. Conventional deepwater oil and gas well design faces the risk of wellhead subsidence and collapse, which led to a greater risk of well construction in the surface layer. By venting shallow gas, optimizing the wellbore structure design, and using a unique surface cementing slurry system, a set of methods for comprehensively dealing with surface NGH and shallow gas has been established. According to the construction operation process, a set of risk disposal plans for dealing with NGH and shallow gas is established. Before drilling, the method of drilling pilot holes is used to release and spray NGH and shallow gas. Considering the influence of NGH decomposition and shallow gas leakage on the soil, the undrained shear strength of the surface soil is converted, and the minimum depth of the surface conductor to meet the bearing capacity of the wellhead is calculated to be 90.8 m. During the 0.6604 m holes cementing, the cementing quality is not good due to the existence of NGH and shallow gas. Assuming that there is no cement slurry sealing above the NGH and shallow gas layers, the penetration depth 547 m of the 0.508 m and 0.34 m combined surface casing is confirmed safe. The liquid lightening agent and early strength cement slurry system is used for cementing operation, and the suspected NGH layer and shallow gas layer are sealed to ensure the cementing quality. During the actual drilling operation, gas overflow occurred in the pilot hole, and NGH were formed in the subsea wellhead and the lower diversion cover of the blowout preventer (BOP), which confirmed the existence of NGH and shallow gas. Through a series of optimized design and prevention measures, the construction of the surface layer of Well S was successfully completed. A set of shallow operation methods suitable for ultra-deepwater wells are proposed for the shallow geological disasters in the South China Sea. The method effectively reduces the risk of shallow geological disasters, improves the safety of shallow well construction, and provides rich experience for the construction of such wells.