Challenges and prospect of the drilling and completion technologies used for the natural gas hydrate reservoirs in sea areas

There are abundant natural gas hydrate resources all over the world, and they are globally taken as the strategic replacement energy of oil and natural gas in the future. Natural gas hydrates are mainly distributed in the shallow layers of deepwater sea floor and the permafrost areas on the land. The natural gas hydrate resources in sea areas are richer, but at present there is less practical drilling and completion engineering in the natural gas hydrate reservoirs of sea areas and there are many technical difficulties and challenges. In order to study and apply the drilling and completion technologies to natural gas hydrate reservoirs of sea areas, this paper introduced the drilling and production test activities in the natural gas hydrate reservoirs of sea areas in America, Japan, India and China. Then, the development status of the drilling and completion technologies for natural gas hydrate reservoirs in sea areas was analyzed, and it was pointed out the basic technical difficulties in the drilling and completion engineering of natural gas hydrate reservoir mainly include narrow drilling fluid density window, hydrate phase change, wellbore instability, wellhead instability and sand production and control. Finally, in view of the technical difficulties in drilling and completion engineering, the future development of the key technologies for natural gas hydrate reservoirs of sea areas (including managed pressure drilling, casing drilling, horizontal well drilling, sand control and drilling fluid) was prospected and analyzed so as to provide the reference for the design of safe and efficient drilling and completion in natural gas hydrate reservoirs of sea areas in China.