Pore-network prediction of residual oil saturation based on oil layer drainage in mixed-wet systems

Since EOR methods mobilize oil trapped by capillary and viscosity forces during waterflooding, prediction of residual oil saturation (S_or) after waterflooding is very important before carrying out any EOR process. The mechanism through which a particular EOR method, such as gas displacement, actually works to reduce residual oil depends in turn on how that oil is trapped at the pore scale. In this respect, pore-scale network modelling can be used to estimate both the nature of the trapped residual oil and the relevant flow parameters in its subsequent mobilization, if the correct physics of oil drainage are properly included. During water flooding of mixed-wet systems, oil may drain down to relatively low residual saturations. A number of studies have indicated that such low saturations can only be reached when oil layers in pore corners are included in the pore-scale modelling, van Dijke and Sorbie (2006) obtained accurate thermodynamically derived criteria for oil layers existence in pores with non-uniform wettability caused by ageing, which is more restrictive than the previously used geometrical layer existence criteria. Recently, Ryazanov at el. (2009) have included these thermodynamic criteria in a two-phase pore network model, which takes as input geometrically and topologically representative networks, to calculate realistic S_or values for mixed-wet and oil-wet sandstones. In this work, the network model has been validated through modelling of experimental data for water-wet and mixed-wet systems. A very good match has been obtained between experimental and simulated relative permeabilities for strongly water-wet Berea sandstone and oil-wet sandstone. Comparison with a range of mixed-wet core waterflooding experiments (Jadhunandan and Morrow 1995) showed good agreement with experimental residual oil and oil recovery efficiency. This paper introduces the correct thermodynamically based physics of how residual oil is trapped in systems of arbitrary wettability. It is extremely important to understand this process, since the residual oil is the "target oil" for most EOR processes. Copyright 2010, Society of Petroleum Engineers.