Numerical Simulation and Experimental Study of Cuttings Transport in Narrow Annulus

With the development of petroleum industry, it needs an efficient drill method such as under balanced drilling (UBD) to enhance the rate of penetration (ROP). However, borehole instability is a problem that UBD must face. The coiled tubing partial underbalanced drilling (CT-PUBD) is proposed to try to solve this problem while keeping an underbalanced condition with high ROP. This paper analyzes the laws of cuttings transport in the narrow annulus focus on this new technique through the simulations and experiments. From the results of simulations, it obtains that the particle velocity declines with the increase of rotational speed and increases with the increase of flow rate. The particles become concentrated as the flow rate increases, and the high flow rate limits particles in a small area. The particle distribution undergoes a process of concentration, dispersion, and concentration as the rotational speed increases. The high rotational speed makes particles deviate from the high fluid velocity area, which causes low particle velocity. The relationships between particle velocity and rotational speed and between particle velocity and flow rate are fitted through the equations, respectively. The phenomenon of collision of particles, sinking and rising of particles, and variation of particle velocity are observed in the experiments. The error between the particle velocity in the experiment and numerical simulation is less than 8.5%. This paper is an exploratory study conducted for the cuttings transport in narrow annulus.