Rheology and adsorption of hydrolysed polyacrylamide polymer in complex geometries
Creators: name: Ekanem, Eseosa M.
Publisher: name: Imperial College London
Year: 2020

description: The study of viscoelastic polymer solutions in complex porous media geometries has been a major interest in the energy field for the purpose of improving sweep efficiency in subsurface formations of porous rock during hydrocarbon recovery. This is because a viscoelastic polymer solution flowing in porous media, undergoes a repetitive expansion and contraction resulting in spatial and temporal flow instabilities reminiscent of turbulent-like behaviour at Reynolds number &lt; 1. Furthermore, these polymer solutions possess retention tendencies, within the porous medium, which changes the permeability of the porous media and exacerbates the rheological complexity. Hence, the work carried out in this thesis investigates these challenges: Elastic instabilities of viscoelastic polymers in porous media and their retention behaviour. The viscoelastic polymer considered is hydrolysed polyacrylamide polymer (HPAM). The elastic instabilities were investigated using a single and multiple microchannel, while polymer retention was investigated using atomic force microscopy and positron emission tomography (PET). Based on the results, we show that "elastic instabilities" occur in both microchannels and polymer retention is dominant in low permeability rocks. In the single contraction microchannel, there is a critical De number at which the transition in rheological flow behaviour, from shear thinning to shear thickening occurs, which is consistent with previous literature. However, it is strongly dependent on the ionic strength in the polymer solution, the type of cation in the anionic HPAM solution, and the nature of pore. Consequently, the "elastic instabilities" were as a result of two factors: the viscoelastic normal stress of the fluid in shear flow and the extensional stresses imposed on the fluid by the obstruction of the contraction. The multiple contractions proved that increased instabilities were present at high contraction ratios (CR) and in channel bodies whose contraction throats were relatively close than further apart. During core scale investigations for polymer retention, characterisation of the rock cores by direct pressure measurement, showed that retention was more dominant in Estaillades with permeability ~ 112 mD than Bentheimer of ~ 2500 mD. The permeability of Estaillades decreased by ~ 55 % to a permeability of ~ 55 mD due to polymer retention. The polymer zero-shear rate viscosity after polymer flooding in Estaillades decreased by ~ 75 % from 40 mPa-s to 10 mPa-s. Estaillades core characterised by Atomic force microscopy suggests that HPAM retention was due to a combination of mechanical entrapment, chain bridging and adsorption by electrostatic interaction. Positron emission tomography provided spatial and temporal maps of the evolution of the tracer in the core and results were indicative of high levels of polymer retention in Bentheimer, especially at the core entrance. The distribution of the tracer concentration was characterised by reduced mobility, long tailing and higher dispersion with the polymer solutions than with water/water. In the displacement of brine by polymer solution, more retention occurred compared to the polymer slug, where polymer displaces polymer. The results demonstrate that the flow of viscoelastic polymer solutions in porous media is indeed very complex, comprising an interplay between polymer rheology and polymer retention. Also, it is dependent on the porous media geometry and the nature of the polymer solution. For a better polymer flooding technique, we suggests that the effect of polymer retention can be reduced by the use of low molecular weight polymers and biopolymers. Furthermore, the "elastic instabilities", which are preceded by a flow focussing, should also be considered during polymer flooding design, as they have the capabilities of reducing residual oil saturation, although this is beyond the scope of this thesis.