Transport and retention of surface-modified nanoparticles in sedimentary rocks
With a number of advantages newly recognized, use of surface-coated nanoparticles is being proposed for various upstream oil applications, such as for Enhanced Oil Recovery or as nanosensors. The first requirement for many of these applications is the ability to transport the nanoparticles a desired distance from the injection well. It means the particles should exhibit little retention in sedimentary rocks and minimal formation damage. Also, a certain amount of particles should adsorb at target locations such as water/oil interfaces for response measurement, if they are used as nanosensors. Three kinds of nanoparticle dispersions are tested for coreflood experiment in sedimentary rock cores: silica nanoparticles, commercial iron-oxide nanoparticles, and in-house synthesized paramagnetic nanoparticles. The quantitative retention measurements from corefloods offer insight into the mechanisms for nanoparticle transport in various sedimentary rocks (Boise sandstone, layered-Berea sandstone and Texas Cream limestone), and also with and without oil in the core. The coreflood experiments helped to develop a procedure to identify efficiently a surface coating to a given nanoparticle, that will allow both long-term dispersion stability and long-distance transportability in a given reservoir rock. To achieve this objective, seventy-six coreflood experiments were conducted to investigate transport of nanoparticles at rock grain surface and at water/oil interface. The parameters analyzed in this dissertation are: dialysis of the nanoparticle dispersion; cross-linking of polymer on coating; hydrophobicity/hydrophilicity of surface coating; oil type; nanocluster size; flow velocity; pH; ionic strength; rock lithology; and injected nanoparticle concentration. Our results show that surface coating, ionic strength, and specific surface/interfacial area were dominant factors for nanoparticle retention at rock grain surface and water/oil interface. Nanoparticle retention concentration (adsorption density) at rock grain surface decreases with decrease in nanocluster size and increase in flow velocity. Some retained nanoparticles can be recovered by increasing flow velocity or decreasing ionic strength. It indicates that the nanoparticle retention at the rock grain surface is unlike the generally irreversible adsorption of surfactant or polymer molecules. Ionic strength affects both reversible and irreversible adsorption of nanoparticles at rock grain surface; in these corefloods the irreversible retention is mainly due to the instability of nanoparticle dispersion and subsequent aggregation under high salinity conditions. The nanoparticle synthesis method, whether dialyzed or not, and cross-linking of coating polymer, all have significant impact on dispersion stability, especially for aqueous dispersion with high ionic strength. Nanoparticle adsorption at water/oil interface can be increased by increasing hydrophobicity of surface coating, or to a certain extent by increasing ionic strength of dispersion.