The Shear and Extensional Rheology of Polymer Dispersions
For the oil and gas extraction process, the understanding of well construction fluids (WCFs) were found to be fundamental towards the success and safety of the operation. This is due to the multiple functions the fluid provides such as maintaining the well bore formation pressure, lubricating the drill bit and transporting any drill cuttings to the surface. The well construction process methodology had been adapted towards carbon storage which promotes the research towards the understanding of well construction fluid rheology. WCFs generally present thixotropic or yield stress rheological behaviours. Additives are often used to promote the rheological properties. This causes the WCFs to have multiple phases present and much more complex interactions of polymers, emulsions, and particles. With the complex formulations, the process involves the WCFs to be pumped alongside a highly complex drill bit geometry. The shear rates required to pump from the surface of the well bore to the drill bit can range from 5 to 105 s-1. When WCFs are pumped to the drill bit, the fluids are contracted and expanded into the well bore. This causes the fluid to undergo both shear and extensional strains. With the process involving the complex formulations and flow regimes, it is ideal to understand the rheology in the shear and extensional regime over a range of shear rates. It is also essential to understand the interactions within the fluid and its impact on the rheology of the fluid. The topic of research for this PhD is to understand the rheological behaviours of WCFs. To understand the rheology of these complex fluids, the goal was to develop a simulation that can accurately calculate the rheological behaviour for WCFs. Simulations are an important and useful tool to obtain a ballpark range of any parameters under investigation. The methodology was defined by examining three types of fluids: Newtonian, viscoelastic and finally complex, multiphase fluids. This led to used silicone oil, xanthan gum aqueous solutions and WCFs respectively. Each fluid would then undergo three stages: experimental, rheological classification and simulation. For the experimental stage, the rheological data was obtained using the rotational and capillary rheometer. This is to ensure that a range of shear rates can be explored for rheological behaviours. A filament thinning rheometer was used to examine the fluid behaviour in the extensional regime. Using the experimental data, a numerical model is selected in the rheological classification stage. Numerical models are compared with the experimental data for the best fit with the fluid. The selected numerical model is used for simulation, the final stage, which uses the geometry selected in the experimental stage. The simulation stage compares qualitative and quantitative data with the experiments. This is the first step of understanding the applicability of numerical models to simulations and, if any, limitations arise from experiments or simulations. For the comparison of experiments and simulation data, contraction tests were used. These tests were chosen as it was a method to recreate similar conditions of WCFs contracting/expanding by the drill bit. Silicone oil, a viscosity standard fluid, had been used to test the methodology and to compare with literature. The experimental results had shown at a critical shear rate, there is a shear thinning trend. An investigation was made to identify the reasoning for the shear thinning trend and it was due to the high shear rates from the capillary rheometer. This had also been captured using the software Ansys PolyFlow with the Carreau-Yasuda model. For the contraction flow tests, the Carreau-Yasuda model was found to be a good fit for the OpenFOAM simulated pressure drop and streamlines with experimental data. Xanthan gum aqueous solutions were observed to act as a simpler viscoelastic fluid than the WCFs. There is a strong dependence on concentration with the viscoelasticity of the fluid. The Giesekus multi-mode model had been used to characterise the fluid. MatLabwas used to obtain a single set of parameters to capture the behaviour in both shear and extensional flow. The unified set of parameters were used on OpenFOAM using the contraction flow die geometry dimensions. The simulations had successfully obtained quantitative and qualitative data. However, discrepancy between the experimental and simulation pressure drops were found with a large vortex being visualised in the simulated streamlines at low piston speeds. Parallel superposition tests had shown the elasticity of xanthan gum decreasing when a controlled shear rate is applied to the fluid. From this, the relaxation time range was found and applied to the simulations. The simulation pressure drop was largely affected by the change in relaxation time and shown a closer value to the experimental data. The simulated streamlines, with the relaxation time limit, had shown a very small and absence of vortex at lower piston speeds. This matched well with the experimental streamline images using the capillary rheometer. For the spacer fluid used in the well construction process: DUO-VIS, the fluid is a grade of xanthan gum and was expected to produce similar rheological data. Yet, it was found DUO VIS presented a larger elastic modulus and much more entangled regime compared to the conventional xanthan gum. This dominance of elasticity caused difficulty in fitting the Giesekus model and thus, requires a more specific model for highly elastic polymer solutions such as the Phan-Thien-Tanner model. KCl Polymer was also analysed, a water based WCF, that presented a viscoelastic rheological behaviour. The Giesekus model was found to be a good fit with the experimental data and was used for simulations of the contraction geometry. Similar to xanthan gum, there was a large discrepancy between the simulated and experimental pressure drop data. The streamlines shown on OpenFOAM had also shown large vortex formations which were not visible in the experimental images. Using the parallel superposition tests, the elasticity is shown to decrease with increasing shear rates applied to KCl Polymer. Using the relaxation time limit, the simulations showed a much closer representation of streamlines to experimental and quantitively lower pressure drops. Despite this good match, it is noted that a multiphase model may be required for KCl Polymer when reaching high Reynolds numbers and Weissenberg numbers.
Contraction Flow, Polymer Solutions, Rheology, Simulation
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