Application of Polyelectrolyte Complex Nanoparticles in Increasing the Lifetime of Poly(Vinyl Sulfonate) Scale Inhibitor in Berea Sandstone Rock

Abstract

Water flooding is used extensively in oil fields to maintain reservoir pressure and displace oil. However, seawater containing high concentrations of sulfate ion may form scale precipitate when mixed with incompatible formation water containing barium and strontium ions. Formation of scales such as barium sulfate can pose costly operational problems by plugging the injection and production wells. Polymers such as poly(vinyl sulfonate) (PVS) are well-known scale inhibitors which can effectively prevent the formation of barium sulfate. Squeeze treatment is a common method which can be used to inject the PVS in the reservoir. In this process, PVS solution is injected into production wells and the inhibitor is adsorbed on reservoir rocks and released during subsequent production of reservoir fluids. Once inhibitor concentration decreases to its minimum effective concentration (MEC), the process needs to be repeated. However, the low adsorption of PVS onto the rock results in a very short squeeze lifetime rendering the treatment uneconomical. In this research, the application of polyelectrolyte complexes (PECs) to increase the squeeze treatment lifetime of PVS was examined. The objective of the project was to develop PEC nanoparticles (NPs) which would improve the PVS adsorption on the rock through charge alteration. The PECs entrapped the PVS in their structure and released the polymer gradually when pH or ionic strength of the surrounding brine increased. PVS adsorption followed by a slow release of the polymer can maintain the scale inhibitor concentration above MEC for longer, and therefore extend the squeeze treatment lifetime. Positively charged nanoparticles consisting of poly(ethyleneimine) and poly(vinyl sulfonate) (PEI-PVS) were prepared and optimized to maximize PVS entrapment in the PEC structure. The stability of the nanoparticles at different temperatures and over time was confirmed. Their stability in the presence of mono and divalent cations was also studied. Static and dynamic adsorption tests were performed which confirmed the nanoparticles rapid and strong adsorption on the Berea sandstone rock. Sand pack studies were also performed to study the adsorption and release of PVS from the PEC structure. The effect of ionic strength shock on the release of PVS from the nanoparticles was also studied. The results indicated that an increase in the ionic strength can decompose the PEC structure and release the PVS in the solution. Core flooding in combination with a dynamic tube blocking test was used to study the nanoparticles’ scale inhibition performance and squeeze treatment lifetime. Nanoparticles injection into Berea core did not alter the core permeability. PVS entrapment in nanoparticles increased the squeeze treatment lifetime by 22% compared to an equivalent injection of unentrapped PVS (P < 0.05). The results also showed that ionic shocks can be used to further improve the release of PVS, prolonging the squeeze treatment lifetime of the nanoparticles by 40% compared to unentrapped PVS.