Abstract
We have investigated the conditions of colloidal stability of silica nanoparticles smaller than 100 nm for their applications in enhanced oil recovery (EOR), especially pertaining to chemical flooding processes. Using zeta sizer and dynamic light scattering techniques, the stability of silica nanoparticle (SNP) dispersions has been investigated by variation of the pH, composition of salt solutions, addition of surfactants and polyelectrolytes. Such conditions can be encountered in oil reservoirs. It was found that changing pH from 5 to 10 had a negligible effect on the size of SNPs, whereas its zeta potential increased with increasing pH. Aggregation of SNPs is a partially reversible process for low degrees of aggregation in 500 mM NaCl, whereas observed strong aggregation in 1000 mM NaCl was irreversible. A critical aggregation concentration (CAC) was defined for the different salts investigated, above which the SNP dispersion became unstable at a fixed pH of 9.5. The CAC for NaCl was approximately 200 times higher than for CaCl2 and MgCl2. Our observations could not be explained completely by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Therefore, we have included non-DLVO interactions such as cation bridging, hydration forces, and steric effects. The additional presence of anionic alcohol alkoxy sulfate (AAS) surfactant slightly destabilized the SNP solution, but by the addition of polyacrylate (PA) was effectively stabilized. With increasing PA concentration, the CAC for both CaCl2 and MgCl2 increased. Upon addition of 100 ppm PA, the CAC increased by a factor of five compared to the situation in the absence of PA. Reducing the solution pH below 8.5, SNP can be stabilized in higher salinity in the presence of PA. The obtained results contribute to a better fundamental understanding of the SNP stability mechanism and a guide to optimize the SNP injection process with EOR chemicals.
Highlights
Owing to the increased demand for energy, it is extremely important to maximize oil recovery yields before abandoning existing wells to newly discovered ones [1,2,3]
The stability of silica nanoparticle (SNP) was examined in aqueous solutions under changing pH, salt concentrations, anionic alcohol alkoxy sulfate (AAS) surfactants and PA polyelectrolytes
It was found that the critical aggregation concentration (CAC) for NaCl was above 500 mM and DLVO well explained the size change of SNPs in different NaCl concentrations
Summary
Owing to the increased demand for energy, it is extremely important to maximize oil recovery yields before abandoning existing wells to newly discovered ones [1,2,3]. A better recovery can be achieved by chemical EOR (cEOR). It involves injecting a variety of chemicals such as alkali, surfactant, and polymer. Polymers attempt to increase the viscosity of injected solutions, and improve the mobility ratio and volumetric sweep efficiency of the reservoir [4,10], whereas alkalis increase pH, reduce adsorption of anionic surfactants, activate the reaction with an acidic constituent in crude oil, and facilitate the generation of in situ surfactants [11]. Various cEOR methods were found to be effective and successfully applied in the oil fields, their applications are limited in high salinity and high hardness conditions, and challenged with surfactant losses by adsorption to rock surfaces [12,13,14]
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