Numerical Study of a Flow Field Near the Bit for a Coiled-Tubing Partial Underbalanced Drilling Method

Abstract

A new drilling method called coiled-tubing partial underbalanced drilling (CT-PUBD) was proposed in this paper. The method is not only able to enhance rate of penetration (ROP) just like the conventional underbalanced drilling technology but can also maintain borehole stability in the upper formation. In the new method, the wellbore pressure system is divided into two parts by a packer: (1) normal pressure system in the upper formation used to balance formation pressure and maintain borehole stability and (2) an underbalanced pressure system in the annulus near the bit used to enhance ROP. Because the pressure system and the circulation system are different, the cuttings transportation process of the method is different from the conventional way. Therefore, it is essential to study how to carry cuttings away efficiently. The flow field and cuttings distribution in the annulus near the bit were analyzed by computational fluid dynamic (CFD) methods. Cuttings transportation trajectory, velocity distribution, and cuttings concentration distribution were obtained under different holes’ parameters of the backflow device (including holes number, diameter, distance, and angle) and different drilling fluid viscosities. The results show that these parameters all have influence on cuttings carrying efficiency, and the most influential parameters are viscosity, angle, and diameter. According to the result of an orthogonal test, a suitable combination of the holes’ parameters was obtained. In the combination, the value of holes number, diameter, distance, and angle is 4, 50 mm, 300 mm, and 120 deg, respectively. This paper provides a theoretical basis for an optimization design of the new method.