Particle Settling in shear thinning non-Newtonian drilling fluids : Effect of oscillatory motion

Abstract

When drilling fluids circulate through the well and during the solids control operations, they are exposed to vibrations and oscillations of different frequencies and amplitudes. The secondary flow structures in oscillatory motion influence the liquid shear rate and rheological characterization of drilling fluids. Hence, oscillations understandably influence the cuttings carrying capacity of the drilling fluids and the solids separation efficiency as well. The majority of pipe flow investigations available in the scientific literature are related to the steady flow. However, more attention should be paid to unsteady flows, since there are many industrial and biological applications in the fields of applied fluid mechanics. As an example, even local geometrical variations and gas kicks in oil-well drilling operations might cause flow instabilities and fluctuations along the pipe trajectory. The problem of shear rate change in time-periodic flows of viscoelastic fluids is relevant in particular for the oil industry, as acknowledged by the research, and also for physiological flows such as blood flow in veins and arteries and the flow of mucus driven by cilia oscillations; hence the interest of the proposed research. This thesis presents a collection of six experimental studies and one numerical study which aims to investigate the effect of zero-mean oscillatory flow on particle settling in shear thinning polymeric non-Newtonian fluids of interest in drilling and maintenance of petroleum wells. In most of the cases, a mixture of water-based polymeric solutions of Polyanionic Cellulose (PAC) and Carboxymethyl Cellulose (CMC) has been employed as the test fluids and in some cases, an individual type of polymer solution has been employed with the viscosifier, Xanthan gum (Xg). The rheological properties of the slightly viscoelastic test fluids have always been characterized in all the sub-studies based on the specific experimental conditions and it is shown that the test fluidsexhibit a shear thinning viscosity in the linear range of the experimental viscosity data depending on fluid composition. After conducting a successful prestudy which includes; -rheological characterization of model drilling fluids, -an acoustic approach of providing the oscillatory motion to theliquid medium, -horizontal oscillation of a vertical liquid column and -a numerical investigation, a core study was planned, designed and executed successfully. For that, an oscillatory flow in a vertical pipe (1.2 m high, 50 mm inner diameter), driven by a zero-mean oscillatory pressure gradient has been used to mimic the practical scenario takes place within a vertical oil well and to achieve the main aim of the research. Driving frequencies were set constant at values ranging from 0 (stationary) to 0.75 Hz. The oscillation amplitude ratio (𝐴�=𝑎�/𝐷�) was set constant at values ranging from 0.3 – 0.5, where 𝑎� is the displacement amplitude of the piston, and 𝐷� is the pipe inner diameter. Flow visualization was deployed to compare the velocity distributions in Newtonian (deionized water) and non-Newtonian test fluids using particle image velocimetry (PIV) and high-speed imaging techniques. Care was taken to avoid any relevant entrance effects. Particle settling in oscillatory systems is a practically important example under dynamic settling since sinusoidal oscillatory fluid motion exhibits a condition of continuously changing acceleration and thus the flow patterns and drag phenomena could be significantly different from those at steady state. Achieving the goal of the research, an experimental investigation was carried out to study the effects of oscillatory motion on the settling of spherical particles in Newtonian and non-Newtonian fluids as the last part of the research study. Three different sphere diameters (1, 2, and 3 mm) were employed in the study and the particles were released at three different locations within the pipe diameter to study the effects of the shear region on particle settling in non-Newtonian fluids at oscillatory conditions. The velocity profiles were used to investigate the possible flow nonlinearities caused by shear thinning behaviour of the non-Newtonian fluids and to determine the shear rate profiles which arguably have a major influence on particle settling. Oscillatory flows of non-Newtonian fluids in wall-bounded large domains, such as the vertical pipe considered in this research, have received continued attention in the literature because they show strikingly different features than their Newtonian counterparts, including resonant behaviour, flow enhancement, and bifurcation to complex flow structures (vortex rings and low Reynolds number turbulence). The study reveals that the axial velocity amplitude along the pipe centreline increases with increasing frequency and with increasing oscillation amplitude irrespective of the fluid type. The thickness of the shear region close to the wall decreases with increasing frequency. The change of shear rate is maximum near the wall region of the pipe, and that is achieved at the maximum position of the sinusoidal velocity profile, where the axial velocity possesses its highest magnitude. The settling velocity was smaller if particles were released close to the pipe wall, independently on the rheology of the fluid. The main result of the investigation is the observation of a significant reduction of the settling velocity in the presence of an oscillatory flow when a fluid characterized by shear thinning viscosity is used. It was found that the liquid oscillations decreased the average settling velocity in Newtonian fluid up to 7% and a reduction of 23% in non-Newtonian fluids. Moreover, when the fluid oscillates, the combination of the shear-layer associated with the particle wake and with the pipe wall does not result in any reduction of the settling velocity. In other words, the effect of the near-wall shear layer, which reduces the viscosity of shear thinning fluids, dominates over the other effects that would not keep the particle longer in suspension.