Enhanced Migration of Surface-Treated Nanoparticles in Sedimentary Rocks

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Abstract
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.

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