The Study of High-Building Rate Horizontal Downhole Casing and Drilling Fluid Technology for Deep Water Shallow Soft Strata of South China Sea Gas Hydrates

The development of horizontal wells is one of the important means to increase the production of natural gas hydrates in offshore areas and accelerate the commercial exploitation of natural gas hydrates. However, due to the characteristics of the natural gas hydrate reservoirs in the South China Sea, such as poor geological consolidation, small pressure gradient of formation fractures, and narrow safety windows, it is easy to cause problems such as leakage of target layers, formation collapse and key grooves, as well as secondary generation of hydrates during the construction of horizontal wells. The combination of downhole casing and drilling fluid technology is one of the important ways to solve the problems encountered during the process of horizontal well development. Based on the information from a deep water shallow high-building rate horizontal well in a certain area of the South China Sea, this study used the downhole casing method with riserless drilling technology for the first time in the deep water area. Taking into account the characteristics of shallow-buried formation, large inclination angle, and shallow burial depth of the inclined and horizontal sections at the wellbore during the drilling process, drilling fluid technology measures before casing are proposed, and the stress analysis and effect of two casing times in the horizontal well are compared. Combining with the stress characteristics of downhole casing in this well, the key technologies such as wellbore cleaning, lubricity, and wellbore stability in the highly inclined section and horizontal section are discussed respectively. By optimizing the drilling fluid technology during the downhole casing period, this study provides construction guidance and technical support for the development of shallow-soft mineral resources such as natural gas hydrates and deep-water shallow gas reservoirs in the South China Sea.