Dynamical feature of particle dunes in Newtonian and shear-thinning flows: Relevance to hole-cleaning in pipe and annulus

Cuttings transport under laminar and turbulent liquid flow in horizontal and deviated pipes were investigated. Several experiments were conducted both for single-phase liquid flow and liquid-particle flow in order to study the impact of rotation of a drill string, as well as fluid rheology and particle size on dynamic feature of particle dunes. Long time series of pressure gradients and flow patterns of the liquid-particle flow were obtained for off-line analysis. Fast Fourier Transform (FFT) was the methodology used to study the time series of pressure gradient signals. The time series were recorded as long continuous experimental sequences, while the flow conditions were changed in time stamped steps. The off-line analysis could then be carried out on each sequence which was easily extracted from the original based on the time intervals. The analysis indicated an existence of dominant frequencies corresponding to liquid and particle dynamics, and to the rotational speed of the drill string. It was found that the distribution and width of the frequency spectrum can be used as an indicator of the particle concentration at given flow conditions. Particle size has a large impact on the frequency spectrum, mostly when the drill string is rotating. The results of this study could help in real time prediction of downhole conditions in petroleum geology, exploration, and drilling operations.