Indicators of cutting removal efficiency when drilling directional wells

Borehole cleaning is still a major challenge for most directional wells despite a significant progress achieved in drilling fluids, tools and field practice, as well as in the development of academic and industry research. Insufficient cleaning of the borehole causes undesirable phenomena in the form of various complications. Therefore, the purpose of the conducted research was to present the main correlations that determine the nature and cleaning degree of the annular eccentric space of the well and to analyze these correlations presented in the form of indicators in order to improve them. The object of the research is the mechanics of cuttings removal from an inclined or horizontal well. In the frameworks of this research mechanics is mathematically displayed in the form of indicators reflecting one or another physical aspect of the cuttings removal process. The research methodology consists in the analysis of the composition of indicators, evaluation of the significance of the indicator constituents, identification of the quantitative contribution of the indicator to the cleaning degree of the annular space of the directional well from destruction products while sinking the borehole. The study resulted in the introduction of the following indicators: equivalent circulation density, slip velocity and cuttings removal index. The physical essence of indicators in their mathematical form is shown with the focus on the slip velocity. The definition of the latter, which is widely accepted among the specialists indicates the rate of washing of cutting particles by the drilling fluid during its transportation to the day-light surface. This indicator is one of the main ones that determine the hydrodynamics of the cuttings transport process. To simplify the calculation of the indicator, it is proposed to use the dimensionless Lyashchenko criterion.