Mechanism and parameter study of jet hydraulic drilling for decreasing bottomhole pressure

Narrow window between fracture and pore pressure brings huge dificulty to drilling engineering. To solve the problem, a new jet hydraulic principle to decrease bottomhole pressure is proposed in this paper, i.e., using the suction power resulted from high speed jet to decrease bottomhole pressure (equivalent circulation density). And the basic idea of drilling tool design is introduced. The jet hydraulic bottomhole pressure decreasing tool includes main and secondary low path. Drilling luid lowing through the secondary path exerts sucking effect on annular at the high velocity and low pressure part of the nozzle. Calculation shows that the pressure decreasing effect gets better as the dimensionless nozzle-to-throat ratio, the low rate, and the nozzle diameter increase, but it gets worse as the drilling rate increases. The bottomhole pressure decreases by 1.5 MPa at the most in the condition that the low ratio between main andsecondary low path is 5% and the mud density is 1.25 g/cm3 .