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Displacement and dispersion effects in vertical eccentric annulus flows Zhang, Ruizi


The oil and gas industry, while offering numerous benefits, grapples with environmental and sustainability challenges such as uncontrolled gas emissions, greenhouse gas contributions, ecological damage, and risks to groundwater. This thesis focuses on a pivotal industry operation: primary cementing. It is a process aimed at mechanical stability and hydraulic isolation, achieved by placing cement slurries in the annular space between a steel casing and wellbore. Failing to meet these objectives could potentially have consequences in terms of wellbore integrity. Complications arise from the eccentricity of the annular regions and the complexities of the involved fluids. The main emphasis of this thesis lies in understanding dispersion behavior and performing classifications for displacement flows in vertical eccentric annuli. This type of flow underlies the primary cementing process which is fundamental to preventing leakage. We firstly make a comprehensive comparison between the 2D gap-averaged model (2DGA) and a 3D model. The cases show both the successes and failures of the Hele-Shaw approach. Acknowledging the main limitation of 2DGA model, we then re-derive it starting from the reduced shear flow equations by applying a different assumption regarding the distribution of fluids across the annular gap. We find significant improvement of the dispersive 2DGA (D2DGA) model in capturing the gap-scale dispersion. A series of experiments has been conducted using our lab-built setup. We start from the displacement flows of Newtonian fluids in a vertical annulus, studied from both experimental and computational aspects. The models have been validated and are found to be effective and robust in predicting different flow behaviours. We develop a set of criteria for flow classifications, applicable for both concentric and eccentric annuli. The eccentricity and buoyancy number are found to be the dominant parameters affecting the front behaviour as well as the displacement efficiency. Finally we extend our study to shear-thinning fluids, employing a combination of experimental and computational methods. The classification criteria also prove effective in discerning various flow types, aligning with the results from the Newtonian study to a degree. Additional information due to extreme viscosity ratios is provided, along with study of some intriguing shear-thinning effects.

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