Research Of Downhole Flow Field In The Near Bit Area Of Partial Underbalanced Drilling

During the process of underbalanced drilling,the pressure in the wellbore is lower than the formation pressure,safety accident would easily happens during the process of underbalanced drilling.In order to solve the safety problem of underbalanced drilling,a new method which combines coiled-tubing and underbalanced drilling was invented.This new method puts coiled-tubing into drilling pipes and uses special packer to separate the annulus into two parts-the upper part and the lower part.The annulus formed by drilling pipe and wellbore is called outer annulus and the annulus formed by coiled-tubing is slim annulus.High density drilling mud can be injected from the wellhead to the upper annulus to balance the formation pressure.Low density drilling mud is injected from the coiled-tubing to the lower part to form an underbalanced condition to improve ROP.The research of feasibility of this new method and the parameters of backflow device and slim annulus has important meaning.This paper is about the feasibility analysis of this new method and the flow regulation of the near bit area.Numerical simulation is used to demonstrate the feasibility of partial underbalanced drilling and the flow regulation in the lower annulus and small annulus.Orthogonal test is used to find the best combination of these parameters.The results of this paper show that this new method can make the liquid pressure of the new method lower than normal dirlling so that it can realize the purpose of underbalanced drilling.The parameters which influence the accumulation are hole numbers,hole distance,hole angle and hole diameter.According to the results of orthogonal test,the best combination can be decided and the maximum ROP of this combination is 13m/h.Rotation velocity influences the carrying capacity of cuttings in the slim annulus.For the slim annulus of which the diameter of drilling pipe is 108.6mm,the rotation help carry cuttings when the value is under 120 rpm but is bad for carrying cuttings as the value exceeds 120 rpm.So the critical rotation velocity is 120 rpm.For the slim annulus of which the diameter of drilling pipe is 118.6mm,the critical rotation velocity is 100 rpm.The results of this paper can provide the theoretical support for the f the partial underbalanced drilling method.