Abstract

This paper reports the main results of a study of the physics of in-depth particle deposition inside porous media at high velocities. This study was motivated by the need to determine the equations relevant for the deposition of "large" micronic-size particles to introduce them into a numerical simulator and thus to be able to predict more reliably well injectivity decline. Indeed, as distance from the well decreases, velocity increases and convection becomes dominant over brownian diffusion at smaller and smaller length scale. This changes drastically the deposition laws and consequently those of permeability damage. Indeed, at high velocities, convection is expected to reduce drastically particle deposition density near pore throats by two mechanisms we refer to as hydrodynamic shadowing and surface entrainment. Our analysis shows that hydrodynamic shadowing may be active up to relatively large distances from the wells for micronic-size particles. A direct consequence is that equations based on convective diffusion at pore length scale are irrelevant. Indeed, hydrodynamic shadowing hampers deposition of new arriving particles downstream from already deposited ones, leading to a very significant reduction in maximum deposition density. This prediction is in good agreement with our experimental results. The second mechanism, namely surface entrainment, is limited to much smaller distances from the wells but also contribute to reducing injectivity damage. In this surface entrainment regime, particles can move on non-negligible distances near the pore throats before being attached by attractive surface forces. As a result, deposition probability in pore throats is smaller than predicted from conventional analysis and decreases with velocity. These two mechanisms may explain why the formation of internal cakes, which is initiated by pore bridging, is often observed at low velocity but rarely in the high velocities range. Such a drastic effect of flow velocity suggests that, in field injections, internal cakes may be initiated only in-depth far inside the reservoir, grow up towards the well and then cause very severe and irreversible injectivity damages. A third point, particularly important for modeling permeability damage kinetic, is that the particle deposition flux is several times higher onto pore throats than the mean value. Such a result is expected only in a converging flow but not in a flow over a collector. This confirms that the prediction of transport phenomena requires the use of a dual porous medium model such as the Grain-and-Pore-Throat model which accounts for both flow-over and flow-through situations which are obviously two characteristics of flows through porous media.

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