Completely Cationic Drilling Fluid System Relieved Ultra-Deep Wells Drilling Difficulties

Abstract

Abstract When it comes to ultra-deep wells drilling, there are multiple difficulties to be considered: over-pull and blockage during tripping because of swelling of shale and clay, instability, and costly maintenance of traditional water-based drilling fluid in high temperature. In order to solve these problems, a completely cationic drilling fluid system with excellent inhibition and stability in ultra-deep wells has been researched. The completely cationic drilling fluid system, characterized by a zero zeta potential differential between the fluid system and the formation to which it is applied, consists of cationic inhibitors, cationic coating agents, cationic fluid loss additives, cationic plugging agents, etc. Laboratory experiments have shown that the zeta potential of the optimal fluid system lies between −10 mV and −20 mV, which is slightly higher than that of the formation. Furthermore, the system exhibited excellent rheology properties, filtration control properties, and anti-contamination abilities of salt, calcium, and clay at 180 °C. The completely cationic drilling fluid was successfully applied to more than 30 wells in an Haxinre oil reservoir located in the central of Tarim basin with depths over 6650 m and temperatures >160 °C. The results show that the completely cationic fluid has particularly good inhibition, rheology, and filtration control properties at high temperatures, while it resists salting, clay, and gypsum contamination effectively. Compared with traditional fluid systems, wells using completely cationic fluids have ~10% fewer drilling problems, ~15% greater penetration rates, and ~10% reduced drilling periods. The best records include 59 days to finish drilling a 6700-m-deep well and 69 days for a 7300-m-deep well, which were about 30–50 days earlier than planned. It is clear that the completely cationic drilling fluid system is an economic alternative for ultra-deep reservoir drilling.