Computer Simulation of Bottom Hole Cleaning in Oil-Well Drilling operations using the Coupled DEM-CFD Method

Oil-well cleaning operation is the ability of a drilling fluid to transport drilled cuttings from the bottom hole to the surface through the annular space between the drill string and the drilled hole (annulus). Better well cleaning leads to more efficient drilling operation which lowers the cost of development. In order to predict and prevent the cuttings accumulation at the bottom hole, it is essential to study the critical parameters affecting the cuttings transport. Computational model is one of the helpful methods to predict the hole-cleaning process as experiments are somewhat difficult and expensive to be carried out due to the harsh drilling conditions. Furthermore, having a good understanding about the physical properties of the particles (cuttings) and in-depth analysis of transport phenomenon can help the researchers to identify strategies to improve the cleaning efficiency. In spite of the extensive investigations carried out in this field, few numerical studies have been conducted considering the effect of particles dynamics and interactions in the fluid domain. Most of the modelling studies are limited to the methods which do not fully consider the discrete nature of cuttings in fluids. Nevertheless, very few investigations implemented fully coupled particle-fluid interactions, while there is a lack of focus and careful investigations of effect of cutting size with appropriate mesh configuration and refinement for particle-fluid interaction near the wall regions of annulus. This is significantly important for the transportation, sedimentation and suspension of cuttings. Moreover, a systematic study of the effect of mud rheology on cuttings transport in a fully coupled CFD-DEM is still lacking in the literature. This study is focused on modelling of the hole-cleaning process using fully coupled computational fluid dynamics and discrete element method (CFD-DEM) approach with carful mesh configuration, for particle-fluid interaction near the annulus wall regions. The aim is to identify the effective strategy for the removal of generated cuttings in oil-well drilling operation from bottomhole to the surface in order to avoid cuttings concentration in the wellbore. This study employed a coupled computational fluid dynamics/discrete element method (CFD-DEM) to predict and optimise the hole-cleaning efficiency of drilling fluid (mud) in different drilling conditions. Simulations have been carried out to investigate the dynamic behaviour of cuttings where the rheology of fluid phase is expressed by the Herschel-Bulkley non-Newtonian model, in an Eulerian framework (CFD) and the cuttings are modelled using the Lagrangian approach (Discrete Element Method, DEM). The CFD-DEM coupled approach, considers the particle-particle, particle-wall, particle-fluid and fluid-particle interactions. In this work, the effects of cuttings size, drill rotation, inclination angles, mud rheology and annular velocity on the cleaning efficiency are investigated. The simulations input parameters have been chosen based on the data reported in the literature. It has been found that the role of mud viscosity and annular velocity in improving the cleaning efficiency are dominant while they can be increased to their maximum/limiting values. Increasing the well deviation from vertical position leads to higher cuttings concentration particularly at the inclination angles close to horizontal. Interestingly at low annular velocity the cuttings concentration at the inclined 45° well is found to be higher than the horizontal annulus due to the sliding motion of cuttings on the lower section of the annulus. Overall, the drill pipe rotation has little effect on decreasing cuttings concentration but the effect is more pronounced at low annular velocity, nevertheless it does not change the behaviour of cuttings at 45° as compared to the horizontal well.
The overall results from fully coupled CFD-DEM in this study can be used to improve the cleaning efficiency in vertical and deviated annuli in oil and gas drilling.