Ultra-HTHP reservoirs are Contained in the deep strata of Yingqiong Basin in the South China Sea. The formation pressure coefficient is about 2.30, and the reservoir temperature is about 230℃. So the reservoir has the characteristics of low pressure bearing capacity and high collapse pressure, it poses great challenges to drilling operations.

Aiming at the difficulties of Ultra-HTHP wells, we carried out a series of studies. Firstly, We have done 2170 sets of experiments on rock properties to study drillability and viscoplasticity of rocks, the test conditions is 200℃ with confining pressures of 20 Mpa, 35 Mpa and 50 Mpa. Secondly, focusing on the problems of large surface casing and high risk of anti-collision in HTHP cluster wells, We improved drilling efficiency by pre-inclination drilling and anti-collision drilling. Finally, to protect reservoirs and reduce pollution, we invented a dual-effect drilling and completion fluid system with megatherm resistance.

After decades of research and public relations, we had completed a series of successful exploration and practice. Based on the law of viscoplastic change of rock under HTHP and rock breaking mechanism, we innovatively expounded a composite impactor acceleration tool with rotating and axial high frequency impact force characteristics. According to the principle of jet pump, a new type of speed-increasing jet-suction bit with bottom jet and reverse jet-suction dual-fluid channel had been creatively invented, it can increase the ROP of Ultra-HTHP wellbore by 162%. Ultra-HTHP cluster well surface drilling, pre-inclination, anti-collision one-trip high-efficiency drilling technology realizes the cluster well large-scale borehole drilling, pre-inclination, anti-collision three-in-one operation mode, it reduced the risk of anti-collision while improving the operation time, the single well construction period saved 2.5 days, and the average drilling cost saved 600 thousand RMB per day.Compared with conventional solid-phase drilling and completion fluids, the dual-effect drilling and completion fluids have high temperature, high density and low solid content decrease by 20%, mud cake thickness decreases by 60%, and permeability recovery value increases from 70% to more than 90%. Compared with the single well cost of cesium formate system in the world, single well saves 48 million RMB.

Ultra-HTHP wells had been successfully drilled 16 wells in the South China Sea, and the success rate is 100%. The average drilling cycle reduced from 175 days to 52 days, it is of great reference value for Ultra-HTHP drilling. It can be recommended to make Ultra-HTHP offshore areas such as the North Sea of Britain and the Gulf of Mexico of the United States.

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