Summary Injectivity decline during produced-water reinjection (PWRI) originates not only from filter-cake buildup but also from in-depth deposition of oil droplets or solid particles. Physical modeling of particle-deposition mechanisms in porous media is thus of key interest for optimizing PWRI operations. The present work brings new insights on oil-droplet and solid-particle-deposition mechanisms in porous media. The experimental conditions were selected such that the ratio between pores and particle sizes is sufficiently large to ensure in-depth propagation. The parameters are the nature of the particles injected and a Peclet number calculated on the size of the collector grains (Pg) that encompasses in a nondimensional form the impact of both the flow rate and the particle size. The results are analyzed within the framework of the "colloidal approach." For oil droplets and solid particles, the collection efficiency (?) shows a transition from a behavior in which ? varies as a power law of Pg, with exponent values -? [(diffusion-limited deposition (DLD)] to -1 [reaction-limited deposition (RLD)] that are typical of the convection/diffusion regime, to a behavior characterized by an increase of ? vs. Pg, typical of the hydrodynamic deposition regime. In the case of oil droplets (slightly charged), the transition occurs at a critical Pg value, PgC ˜ PgCgeom/10, corresponding to a diffusion-layer thickness around the collector grain of the same order of magnitude as the droplet diameter. In the case of electrosterically stabilized solid particles, the transition takes place at PgC << PgCgeom for small particles and at PgC > PgCgeom for larger particles. Introduction As the environmental regulations for water discharge are becoming increasingly stringent, PWRI is now recognized as an important issue with respect to environmental protection and profitability. In many mature zones, PWRI is becoming an economically attractive option. It enables reduction in processing costs of the produced water before disposal and allows handling the ever-increasing volumes of produced water without increasing environmental risks associated with water discharges. PWRI also can be used for waterflooding or pressure maintenance in the initial stage of production. Therefore, most of the produced water has to be reinjected either in a suitable formation for disposal or in the producing formation for improved-oil-recovery purposes. However, great uncertainties still remain about the consequences of PWRI and the actual injectivity behavior. If injectivity decline is severe, sustainable injection rate for extended periods will become impossible, jeopardizing the entire reinjection operation. From field observations, it was acknowledged that:With produced water, plugging is important and injectivity is lower than expected. The solids and the oil do not behave separately.With matrix PWRI, a continuous loss of injectivity is obtained, even in high-permeability formation (soft rock) (Detienne and Po 2005).Successful PWRI is likely to require fracturing (Detienne and Po 2005; Raaen 2005; van den Hoek and Bjoerndal 2005; Sweeney 2005). Thus, the injectivity performance is viewed increasingly as being dictated by a dynamic coupling between fracture growth and plugging of fracture faces. This general scheme raises additional challenges related to the issues of fracture containment, sweep efficiency, and conformance, especially in soft-rock formations. Simulation of PWRI under fracturing conditions is therefore necessary to optimize the injectivity behavior and to establish a PWRI strategy (Detienne et al. 2005; Ochi et al. 1999; van den Hoek et al. 1996). However, until now, development of models with reliable prediction ability suffers from the lack of a clear insight into the actual physics of the damage process. Understanding the damaging mechanisms when injecting water containing solid particles and oil droplets and evaluating the impact of such damage on injector performance are, thus, important research needs. PWRI-induced damage results from in-depth particle penetration into the surrounding formation and from particle accumulation on formation/fracture face to form a heterogeneous and highly compressible filter cake. The filter-cake permeability is among the important parameters that have a great impact on injectivity. Hence, a representative estimation of this parameter is required to forecast the injection behavior (van den Hoek et al. 1996). More generally, the impact of particles, including mixtures of oil and solids, on injectivity has received increasing attention recently (van den Hoek et al. 1996; Al-Abduwani et al. 2003; Al-Riyamy and Sharma 2002; Bedrikovetsky et al. 2001; da Silva et al. 2004; Hofsaess and Kleinitz 2003; Sæby et al. 2005). Regarding oil-in-water-emulsion flow and deposition in porous media, despite significant experimental and theoretical work (Soo and Radke 1984, 1986; Soo et al. 1986), impact on injectivity is still not understood clearly, especially under severe injection conditions (high flow rate into low-permeability formation). The objective of this study was to bring new insights into the impact of flow rate on in-depth deposition mechanisms of the colloidal particles present in produced water. The present work was focused on two kinds of particles:uncharged oil droplets in dilute and stable oil-in-water emulsions andelectrosterically stabilized latex microspheres. For both kinds of particles, a broad range of velocities was investigated. In the next section, some useful results regarding the theoretical background of the colloidal approach are presented. In the section after that, materials and experimental procedures are described. Experimental results are presented and discussed in the last section.

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Injectivity decline during PWRI originates not only from filter-cake build-up, but also from in-depth deposition of oil droplets or solid particles. Physical modelling of particle deposition mechanisms in porous media is thus of key interest for optimizing PWRI operations. The present work brings new insights on oil droplets and solid particle deposition mechanisms in porous media. The experimental conditions were selected such as the ratio between pores and particle sizes is sufficiently large to ensure in-depth propagation. The parameters are: the nature of the particles injected and a Peclet number calculated on the size of the collector grains (Peg), that encompasses in a non-dimensional form the impact of both flow rate and particle size. The results are analysed in the frame of the "colloidal approach". For oil droplets and solid particles, the deposition efficiency (h) shows a transition from a behaviour in which h varies as a power-law of Peg, with exponent values -2/3 (Diffusion Limited Deposition, DLD) to -1 (Reaction Limited Deposition, RLD), characteristic of convection-diffusion regime, to a behaviour characterized by an increase of h versus Peg, characteristic of hydrodynamic deposition regime. In the case of oil droplets (not or slightly charged), the transition occurs at a critical Peg value,, corresponding to a diffusion layer thickness around the collector grain equal to the droplets diameter. In the case of electro-sterically stabilized solid particles, the transition takes place at for small particles and at for larger particles. Introduction As the environmental regulations for water discharge are becoming more and more severe, the re-injection of produced water (PWRI) is now recognized as an important issue with respect to environmental protection and profitability. In many mature zones, PWRI is becoming economically attractive. It enables reducing processing costs of the produced water prior to disposal and allows handling the ever larger volumes of produced water without increasing environmental risks associated with water discharges. PWRI can also be used for waterflooding or pressure maintenance in the initial stage of production. Therefore, most of the produced water has to be re-injected either in a suitable formation for disposal or in the producing formation for IOR purposes. However, great uncertainties still remain about the consequences of PWRI and the actual injectivity behaviour. If injectivity decline is severe, sustainable injection rate for prolonged period will become impossible, jeopardizing the whole re-injection operation. It was recognized, from field observations that:with produced water, plugging is important and injectivity is lower than expected. The solids and oil do not behave separately;with matrix PWRI, a continuous loss of injectivity is obtained, even in high permeability formation (soft rock) [1];successful PWRI is likely to require fracturing [1–4]. Thus, injectivity performance is increasingly viewed as being dictated by a dynamic coupling between fracture growth and plugging of fracture faces. This general scheme raises additional challenges related to the issues of fracture containment, sweep efficiency and conformance, especially in soft rock formations. Simulation of produced water re-injection under fracturing conditions is therefore necessary to optimize the injectivity behavior and to establish a production water re-injection strategy [5–7]. However, up to now, development of models with reliable prediction ability suffers from the lack of a clear insight in the actual physics of the damage process. Understanding the damaging mechanisms when injecting water containing solid particles and oil droplets, and evaluating the impact of such damage on injector performance are thus important research needs.