Underbalanced drilling facilitates the effective control of wellbore pressures - amongst several other important advantages - when compared to conventional drilling technology. However, this involves the flow of multiphase fluids, which introduces additional complexities due to highly transient flow patterns, unpredictable wellbore hydraulics and increased tendency for the settling of drilled cuttings in the wellbore. An accurate prediction of fluid dynamics and cutting transport efficiency is required to achieve an effective pressure-management hole-cleaning operation. In this research, a theoretical, numerical and experimental study was performed to analyse and investigate cutting transport dynamics and wellbore hydraulics. The analytical study involved the development of several mechanistic models, which are valid for both single phase and two-phase flows in the concentric and eccentric annuli, both with and without inner pipe rotation. The study derives and presents Reynolds number and effective viscosity equations valid for annuli flow of both Newtonian and non-Newtonian Power law, Bingham plastic and Yield power law fluids. The study also used the solution of the continuity equation of motion for axial steady-state flows to formulate new Laminar and turbulent friction geometry parameter and friction factor equations, which take into account the combined effect of the fluid rheology, fluid circulation rate, pipe eccentricity and inner pipe rotation speed for the evaluation of the flow dynamics and pressure losses in the annuli. In addition, the study developed new flow gas-liquid pattern-dependent multi-layered models for the different cuttings transport mechanisms, valid for both horizontal and inclined annuli flows. Numerical computational fluid dynamics simulations were performed to discretise and solve the governing equations for fluid flow, using a finite volume mathematical approach to obtain velocity, viscosity and pressure fields for different input conditions. Furthermore, an experimental study was carried out to evaluate the interplay between the two-phase gas-liquid flow patterns and the major drilling parameters, and to investigate the influence on the cuttings and fluid flow dynamics in a horizontal and inclined drilling wellbore. Results showed that the effect of the drillpipe rotation on cuttings transport in the annuli is highly dependent on the fluid rheological properties, the drillpipe eccentricity, the wellbore inclination and fluid flow pattern. The annuli pressure gradient was found to be dependent on the fluid flow pattern and the prevailing cutting transport mechanism. The minimum requirements to clean an eccentric annulus is higher than that required for the concentric annulus. Furthermore, the local mixture properties and gas-liquid flow pattern of the fluid is strongly influenced by the inclination angle of the wellbore, which consequently influences annuli pressure losses and cutting transport dynamics. Although drillpipe rotation can improve cuttings transport through the annuli, the influence of drillpipe rotation on the cutting's movement in the two-phase gas-liquid drilling fluid is much less than that of the single-phase drilling fluid. Overall, a good match was found when the mathematical models were compared to the experimental data. The output of this research is very useful for implementing an efficient cutting transport operation, hydraulic program optimisation and effective wellbore control, particularly for managed pressure drilling operations.
SALUBI, V. 2022. Cuttings transport and hydraulics optimisation for underbalanced drilling (UBD) operations in concentric and eccentric, directional and extended reach wells. Robert Gordon University, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-1893048