To improve the rate of penetration (ROP) in drilling deep and hard formations,this paper proposes a new drilling method called coiled tubing partial underbalanced drilling (CT-PUBD).As a preliminary investigation into the new drilling method,this paper presents predictions of hole cleaning efficiency,drilling speed,cuttings migration and pressure loss in the drilling process with CT-PUBD.Based on numerical simulation and full-scale experimental studies,we conclude that using CT-PUBD,an underbalanced drilling condition can be achieved near the bit while maintaining wellbore safety at the same time.This condition can be achieved using a cuttings discharge device,a rotary packer and a backflow controller.According to the numerical simulations performed in this study,CT-PUBD can achieve high efficiency of hole cleaning.Along the cuttings migration process,the fluid velocities can reach the maximum values in the backflow holes.A full-scale laboratory experimental system was used to test the hydraulic characteristics and obtain the drilling performance of the new technology.The result shows that CT-PUBD significantly improves the ROP compared to the conventional drilling method.