Numerical and machine learning modelling of the impact of hydrolysis on oil recovery by polymer flooding
Creators: name: Khelifa, Mansour
Publisher: name: Imperial College London
Year: 2020

description: We investigate the impact of non-isothermal neutral hydrolysis on polymer flooding and determine which parameters have the greatest impact on the modelling of hydrolysis. Experimental investigations using polyacrylamide polymers in solution have shown that neutral hydrolysis can lead to a significant increase or decrease in the solution viscosity. This could change the performance of polymer flooding using these chemicals as the magnitude and nature of hy drolysis depends upon several factors including the water salinity, pH and temperature. There is also a paucity of data in the literature for many of the parameters controlling the rate of hydrolysis resulting in further uncertainty as to the importance of hydrolysis in reservoir ap plications. A fully implicit, two-phase, finite volume simulator was developed that models the influence of salinity and temperature on the degree of polymer hydrolysis with polymer viscosity and hence on the dynamics of a polymer flood. This model includes heat exchange with the formation surrounding the reservoir. A degree of hydrolysis and salt concentration dependent adsorption model was developed and implemented. The interactions between the polymer ions and the counter ions occurring naturally in the connate water or injected wa ter lead to a non-monotonic variation of the polymer viscosity with the degree of hydrolysis of the polymer. This was accounted for by the development of a novel correlation based on Huggins empirical equation. A machine learning algorithm was developed and trained by the data generated by the developed simulator to be able to identify the parameters that had the highest impact on the polymer flood performance. The sampling of data in the space of parameters was done using an experimental design strategy. A two-level factorial fractional experimental design was implemented to determine which parameters had the most influence on polymer flooding performance. Dimensionless analysis shows that the system of equations depends upon 23 independent dimensionless scaling groups. Eight of these groups were evalu ated in the experimental design. A systematic literature review provided the range of values of these dimensionless groups. The subset of groups was chosen following preliminary analysis of the dimensionless groups. The analysis shows that neutral hydrolysis can increase or decrease cumulative recovery by up to 4%. It can further modify slug injection by changing the degree of viscous cross-flow or fingering of chase water into the polymer. This is due to the hydrolysis zone lagging behind the polymer front. The main controls on recovery are the polymer, water and oil viscosity ratios together with the salt-polymer interaction coefficient. In the reaction model, the Damk¨ohler number and the Arrhenius numbers are most important in determining the degree of hydrolysis at the polymer shock front and behind the shock front. The impact of the degree of hydrolysis at the shock front and behind the front are highly dependent on the salt concentration. This confirms that it is important to control the solution salt concentration when planning injection of hydrolysed polyacrylamide. The relationship between adsorption and the degree of hydrolysis was reviewed using data from the literature and it was shown that the adsorption decreases with the degree of hydrolysis until 30% of hydrolysis. Above 30% hydrolysis, the amount of adsorbed polymer shows little variation with a change of the degree of hydrolysis. The study of the impact of adsorption on reservoir performances has shown that the impact of this mechanism can be minimized in reservoirs that have a high temperature. Indeed, under high temperatures, the reaction is extremely fast which allows a rapid increase of the degree of hydrolysis and therefore a decrease of adsorption. This observation makes it possible to inject polymers that have a very low degree of hydrolysis in reservoirs that have high temperatures without significantly increasing the risk associated with adsorption. The injection of polymers having a low degree of hydrolysis can potentially reduce the mechanical degradation of the polymer near the injection region and improve the injectivity of the polymer. The impact of hydrolysis on cross-flow and on oil recovery and on water-cut was assessed by simulating polymer flooding through heterogeneous reservoirs. First, a two-layered reservoir was studied and it was showed that hydrolysis can have an important impact on both cross-flow and on the recovery factor when the initial oil-polymer viscosity ratio is close to one, when the permeability values between the two layers are close to each other and when the initial degree of hydrolysis of the injected polymer is low (i.e. less than 0.2). More complex permeability distributions were also included in the investigations. We focused on inter-bedded reservoirs, upward or downward fining reservoirs, and on normally distributed permeability fields. We also investigated the impact of hydrolysis on SPE10 model2. It was shown that the hydrolysis has the highest impact on the cross-flow and on the recovery factor in the inter-bedded reservoir. This was explained by the fact that in these reservoirs, high pressure gradients exists due to the difference in the permeability values between the adjacent layers which leads to a large amount of the cross-flow and therefore to a better oil recovery. In the same time the cross-flow itself is affected by the viscosity field that is highly dependent on the degree of hydrolysis.