Nanoparticles (diameter of approximately 5 to 50 nm) easily pass through typical pore throats in reservoirs, but physicochemical attraction between nanoparticles and pore walls may still lead to significant retention. We conducted an extensive series of nanoparticle-transport experiments in core plugs and in columns packed with crushed sedimentary rock, systematically varying flow rate, type of nanoparticle, injection-dispersion concentration, and porous-medium properties. Effluent-nanoparticle-concentration histories were measured with fine resolution in time, enabling the evaluation of nanoparticle adsorption in the columns during slug injection and post-flushes. We also applied this analysis to nanoparticle-transport experiments reported in the literature.Our analysis suggests that nanoparticles undergo both reversible and irreversible adsorption. Effluent-nanoparticle concentration reaches the injection concentration during slug injection, indicating the existence of an adsorption capacity. Experiments with a variety of nanoparticles and porous media yield a wide range of adsorption capacities (from 10 -5 to 10 1 mg/g for nanoparticles and rock, respectively) and also a wide range of proportions of reversible and irreversible adsorption. Reversible-and irreversible-adsorption sites are distinct and interact with nanoparticles independently. The adsorption capacities are typically much smaller than monolayer coverage. Their values depend not only on the type of nanoparticle and porous media, but also on the operating conditions, such as injection concentration and flow rate.

Superparamagnetic nanoclusters may be used in imaging in biomedicine and in mapping of petroleum reservoirs, by generating either ultrasonic or acoustic signals with oscillating magnetic motion. For a given magnetization per weight of iron oxide, nanoclusters with diameters from 20 to 100 nm experience a much larger magnetic force than that of the primary sub-10-nm primary particles. Aqueous dispersions of 0.1 wt % superparamagnetic iron oxide nanoclusters were stabilized with citric acid on the particle surface, with a high loading of 90% iron oxide. The dispersions were stable for months even with high salt concentrations up to 4 wt % at a pH of 6 and 8 based on the hydrodynamic diameter from dynamic light scattering. The citrate ligands provide electrostatic repulsion, as characterized by the ζ potential. The small size of the clusters, superparamagnetic properties, and high salt tolerance are highly beneficial in various applications including the mapping of petroleum reservoirs with magnetomotive techniques.

Engineered nanoparticles have properties potentially useful for certain oil recovery processes and formation evaluation. Nanoparticles are small enough to pass through pore throats in typical reservoirs, but they nevertheless can be retained by the rock. The ability to predict retention with distance traveled, and to predict the effect of different surface treatments on retention, is essential for developing field applications of such particles. We inject concentrated (up to ~20 wt%) aqueous suspensions of surface-treated silica nanoparticles (D = 5 nm and 20 nm) into sedimentary rocks of different lithologies and permeabilities (10–14 to 10–12 m2). The particles generally undergo little ultimate retention, nearly all being eluted by a lengthy postflush. Nevertheless the nanoparticles do not propagate as classical non-retained solutes or particles (e.g. conservative tracers). Effluent nanoparticle concentration histories show breakthroughs later than 1 PV injected, plateau concentrations less than the injected value, and long tails. Longer elution times occur in samples with greater specific surface area. This set of observations is consistent with weak, reversible attachment of particles to pore walls. Such attachment is predicted by DLVO theory for very small particles when van der Waals attraction is the dominant force. This is the situation in our experiments, as the nanoparticles carry virtually no surface charge due to their surface coating. Compared to viscosities measured on bulk suspensions, the apparent viscosities of suspensions flowing through sedimentary rocks are significantly smaller. Bulk phase viscosities show little or no dependence on shear rate, and all experiments involved single-phase flow in water-wet samples. The simplest explanation for these observations is that a moderately thick layer (several hundred nm) of water depleted of particles exists at the pore walls. The mechanism for depletion of nanoparticles is presumably analogous to the mechanism for depletion of colloidal particles near rough confining surfaces. Introduction A recent surge of interest on possible use of nanotechnology to help locate bypassed oil and improve oil recovery raises a crucial question: Whatever ingenious nano-sensors or highly effective nano-EOR agents are developed, is it possible to deliver them to where the oil exists deep in the reservoir? Nano-sized devices and agents will be solid aggregates, and the transport of colloidal dispersions (length scale between 100 nm and 10,000 nm) in reservoir rock is known to be very difficult. (We use the prefix "nano" to indicate length scale between 1 and 100 nm, and hereafter we refer to all such objects with the generic term "nanoparticles.") Clearly transportability is a pre-requisite for any nanoparticles for reservoir applications. Characterization of transport in reservoir rock has two major components: the nanoparticle retention and the mobility of the nanoparticle dispersion. The former quantifies the fraction of injected nanoparticles that survive and reach the target zone. The latter defines the operating conditions (e.g., injection pressure and/or flow rate) to bring the injected nanoparticles through the desired pathway and time to the target location.

Nanoparticles (D ~ 5 to 50 nm) easily pass through typical pore throats in reservoirs, but physicochemical attraction between nanoparticles and pore walls may still lead to significant adsorption. We conducted an extensive series of nanoparticle transport experiments in core plugs and in columns packed with crushed sedimentary rock, systematically varying flow rate, type of nanoparticle, dispersion concentration, number and sizes of dispersion slugs, and column grain size. Effluent nanoparticle concentration histories were measured with fine resolution in time, enabling evaluation of nanoparticle adsorption in the columns during flow of dispersion and of postflushes. We also apply this analysis to transport experiments reported in the literature.Our analysis indicates that nanoparticles undergo both reversible and irreversible adsorption. Effluent nanoparticle concentration reaches the injection concentration during slug injection, indicating the existence of an adsorption capacity. Experiments with a variety of nanoparticles and lithologies yield a wide range of adsorption capacities (from 10 -7 to 10 -2 g nanoparticle/g porous medium) and a wide range of proportions of reversible and irreversible adsorption. Reversible and irreversible adsorption sites are distinct and interact with nanoparticles independently of each other. The adsorption capacities are typically much less than monolayer coverage but are not an intrinsic property of the porous medium nor of the nanoparticle. Instead, they are influenced by operating conditions, i.e., increasing with larger injection concentration and smaller flow rate.