Abstract

The present work is a part of a thorough and systematic laboratory study of oil-in-water emulsion flow in porous media that we have recently undertaken to investigate the mechanisms of oil droplet retention and its consecutive impact on permeability. One of our main objectives was to see how the in-depth propagation of produced water residual dilute emulsion could impair the permeability during Produced water Reinjection (PWRI). During this casework, we used granular packs of sharp-edged silicon carbide grains and stable and dilute dodecane-in-water emulsions. The flow experiments have been performed under well-controlled conditions and we studied the effect of most of the relevant parameters including flow rate, salinity, droplet size and permeability of the porous medium.

A careful monitoring of the salinity and the jamming ratio allowed us to consider and work separately on the two main mechanisms of droplet capture, i.e. surface capture and straining capture. In a previous paper, we reported on the effect of salinity and flow rate on emulsion flow through porous media where the pore size to droplet size ratio (jamming ratio) was very high, ensuring so, that only droplet capture on pore surface is operative. This paper reports on the impact of salinity and jamming ratio on both mechanisms with the main focus on the induced permeability impairment.

We demonstrated that surface capture could induce significant in-depth permeability losses even at high jamming ratio. The maximum permeability loss reached is very sensitive to salinity and flow rate (shear thinning effect). This maximum is always lower than a limiting value dictated by the surface coverage jamming limit of RSA theory. This limiting value increases while decreasing the jamming ratio according to a simple formula extracted from the Poiseuille's law with a mean hydrodynamic thickness of the deposited layer close to the droplet diameter (monolayer deposition). Regarding the straining capture, we determined a critical jamming ratio value of 7 for this mechanism to occur. Preliminary results using only two jamming ratio values and one flow rate are presented. Compared to surface capture, the results show that straining capture induces more severe plugging with a lower rate of propagation. The lower the jamming ratio, the more severe the plugging and the lower the propagation rate. However, more investigations are still required, notably, using various jamming ratios and flow rates to better characterize this important mechanism.

Introduction
PWRI-induced injectvity decline

Due to the large and increasing stream of oilfield produced waters and to the tightening of environmental regulations, water handling is becoming a major issue for oil industry. Produced water reinjection (PWRI) for oil production support through pressure maintenance and EOR methods is among the best options to convert waste to value and becomes more and more the main destination of oilfield produced waters (Furtado, C.J.A. et al., 2005; Souza, A.L.S. et al., 2005; Abou-Sayed, A.S. et al., 2007). However, even though PWRI is environmentally correct and economically attractive, its implementation is still facing challenges related to injectivity, geomechanics, corrosion, souring, contingency… Produced waters are complex systems that are reactive and biologically-enriched (schmoo, biomass, EOR additives, oil-coated solids, stable emulsions enhanced by microbiological and EOR surfactants, microbial corrosion products…). Even after advanced surface treatment, fluid to be injected still contains solids and residual dilute oil-in-water emulsions that are hard to remove and have a high potential for plugging (Zhang, N.S. et al., 1993; Al-Abduwani, F.A.H. et al., 2001). How this plugging impacts well injectivity depends on other important parameters such as well completion, injection scheme - matrix or fracture injection - and reservoir characteristics (soft or hard formation).

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