Abstract: Well Hua H50-7 is a horizontal well with an ultra-long horizontal section in Longdong area of Changqing Oilfield. The ultra-long horizontal section as well as the expected mudstone intervals and faults bring difficulties in resistance reduction, wellbore cleaning, prevention of the mudstone layer from collapsing and fault leakage prevention and plugging. The goal involves optimizing the water-based drilling fluid, the lubricity and inhibition of the water-based drilling fluid are improved to reduce friction and resistance and prevent the mudstone layer from collapsing. Different technical measures for leakage prevention and plugging were developed, depending on the severity of leakage to solve problems of fault leakage prevention and plugging. By applying the optimized water-based drilling fluid and adopting the developed leakage prevention and plugging technology, the drilling of Well Hua H50-7 had been smoothly completed. During the drilling process, the lubricity and inhibition of drilling fluid were stable. Compared with the adjacent wells using the original water-based drilling fluid, the resistance encountered in drilling string and casing running wassignificantly reduced. Lost circulations occurred were successfully plugged without the problem of borehole instability. The successful drilling of Well Hua H50-7 showed that by optimizing the formulation of water-based drilling fluid to improve its lubricity and inhibition, and adopting pertinent leakage prevention and plugging technologies, problems can be solved. Further, the problems involved in drilling the ultra-long horizontal sections of horizontal wells in Longdong Area, such as borehole instability, severe mud-making in the horizontal section, and difficulties in lost circulation control in the horizontal section could be solved effectively. At the same time, the successful completion of this well provided experience for drilling longer ultra-long horizontal section horizontal wells in the future.