Experimental study of rock cutting-carrying for high-pressure CO2 jet drilling based on non-spherical rock cuttings

Abstract:

In unconventional reservoir exploitation， underbalanced gas drilling technology overcomes the shortcomings of conventional drilling such as low drilling speed， long well construction period and low efficiency， and one of the key constraints to the gas drilling technology is the high efficiency of rock cuttings transport at the bottom of the well when drilling in deep formations. Since the gas drilling rock cuttings transport research mostly adopts spherical instead of real non-spherical rock cuttings， ignoring the influence of non-spherical contour on rock chip transport， resulting in the inaccurate characteristics of bottomhole jetting rock cuttings transport. In this paper， the non-spherical rock cuttings are produced by cutting sandstone with a PDC drill bit， and a PDC drill bit and high-pressure CO₂ jetting test platform are set up to investigate the effects of the sphericity φ of the rock cuttings， the jetting pressure， the rotational speed of the drill bit， and the volume of the rock cuttings on the rock transport. The results show that： the axial velocity of the non-spherical rock cuttings fluctuates greatly when they rise， and their start-up speed is faster than that of the spherical rock cuttings， but the velocity decays faster； with the increase of the value of φ， the axial velocity of the rock cuttings decreases and then increases in the wellbore， and the axial velocity of the rock cuttings decreases and causes the poor transport efficiency when the value of φ is in the range of 0.6~0.65； the axial velocity of the rock cuttings increases significantly with the increase of the CO₂ injection pressure， and the increase of axial velocity of rock cuttings at the distance of 35 mm from the bottom of the well can reach 221%； the increase of drill rotational speed increases the axial velocity of rock cuttings significantly close to the bottom of the well， and the average velocity is stable at 11.88 m/s when the rotational speed of the drill bit is in the range of 70~80 r/min， which shows a better effect of rock carrying； the axial velocity decreases and then increases with the increase of rock cuttings’ volume. The above study systematically elucidates the rock-carrying characteristics of PDC bit jet drilling， and provides a theoretical basis for the application of CO₂ and other gas jet drilling technologies.

Key words:unconventional reservoir;underbalanced gas drilling;jetting rock cutting-carrying;rock cuttings’ sphericity;PDC drill bit