ABSTRACT: This paper presents a new efficient rock breaking method of Coiled Tubing Partial Underbalanced Drilling (CT-PUBD). Unlike conventional underbalanced drilling, it can achieve a partial underbalanced condition near the bit rather than the whole borehole. The rate of penetration is influenced by hole cleaning. In this technique, most cuttings are translated in micro annulus which is between drill pipe and coiled tubing. Moreover, the micro annulus where the drill pipe rotates and coiled tubing is static is different from normal annulus which is between borehole and drill pipe. In this paper, High Speed Photogrammetry System is used to observe the cuttings transportation. It can be found that the cuttings move as spiral movement in the micro annulus and overturn continuously. With the increase of rotational speed, the cuttings tangential velocity increases. As the displacement increases, the cuttings axial velocity increases. The relationship between the cuttings velocity and the drilling fluid velocity is established by defining cuttings tangential velocity coefficient and cuttings axial velocity coefficient. The cuttings tangential velocity coefficient and cuttings axial velocity coefficient range from 0.83 to 0.88 and 0.29 to 0.46 respectively.
With the consumption of energy, more challenges are faced in petroleum industry such as oil drilling and reservoir reconstruction (He and Hayatdavoudi, 2018, He et al. 2018). There are more opportunities to drill into poor drillability formations. Drilling time is usually an important part of the total cost of drilling (Fear 1999). And a survey shows that drilling time occupies 1/3 of the oil well construction (Anderson, Maurer et al. 1990). Low rate of penetration (ROP) causes long drilling cycle and long drilling cycle may cause lager drilling risk. So enhancing ROP is always a hot spot in drilling industry. Overbalanced drilling maybe influence the pore pressure around the wellbore by filtration or stress (Black, Sandstrom et al. 1984, Pessier 1994), which may cause low ROP. Underbalanced drilling (UBD) is a method to enhance ROP (Kolle and Marvin 1999, Doan 2003). During drilling process, the pressure in the wellbore is lower than the pore pressure (Lockett, Queen et al. 2000).
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