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.
The modelling of WAG processes at the pore scale in the "near-miscible" regime is still not fully understood, where "nearmiscible" implies at low gas/oil interfacial tension (IFT, go ) near the transition from immiscible to miscible conditions.Micromodel experiments under near-miscible conditions have been performed previously at Heriot-Watt U. (HWU) and results from these show clear differences from the immiscible flooding cycles. In particular, there is significant oil production through "thick" films after the breakthrough of the gas finger and, in repeated gas floods, the gas finger tends to re-establish rather than to redistribute the phases as in the immiscible floods. Here, we address some of the major issues in modelling nearmiscible WAG and a new mechanism is proposed based on the interfacial physics of the process. At near-miscible conditions, mass transfer between phases occurs and the oil and gas hydrocarbon phases approach each other in properties, which leads to both the swelling and extraction of oil. The importance of both viscous and gravity forces may increase and it is also thought that water blocking (shielding) leads to bypassing of oil, indicating that gas-water and oilwater capillary forces remain important. The flow of oil through "thick" films and layers becomes more important, possibly as a result of a "wetting transition" or gas-oil contact angle change.To explain these processes, a consistent model is proposed for the IFT and contact angles (which must also change) as the three-phase system goes from immiscible to near-miscible conditions. A linear model is assumed for the variation of the gaswater and gas-oil IFTs, gw and ow , as functions of the gas-oil IFT, go , consistent with measurements. Along with some further linear assumptions on the solid-fluid IFTs, expressions are presented for the varying (cosines of) gas-water and oilwater contact angles, cos gw and cos ow . Surprisingly, cos go is predicted to be constant down to fully miscible conditions ( go = 0). Indeed, accurate measurements of the two-phase cos go , confirm this trend, but only down to a finite value of go , below which the values of cos go rapidly increases to 1 ( 0 go ). This behaviour has been incorporated in our model.The consequences of the various IFT and contact angle models are then worked through, using our previously developed theory of film and layer formation for three-phase configurations in angular pores. We demonstrate how the formation of these thick conducting layers is affected as the system goes from immiscible to miscible conditions. By incorporating the more realistic behaviour with cos go approaching 1 as the system goes miscible, much thicker and more conductive oil layers are predicted, very like those observed in the HWU micromodel experiments. This may not be the only explanation for the changes in oil recovery and WAG flooding behaviour in near-miscible systems, but we believe that it is an important and novel component of the mechanism. Additionally, it is s...
TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractGas injection and water-alternating-gas (WAG) displacements are becoming increasingly important as EOR processes in a wide range of reservoirs. Although the conservation equations can be written down, the resulting multiphase macroscopic flow equations contain quantities which are not well understood, viz. the three-phase relative permeabilities and capillary pressures (denoted 3PRPs and 3PCPs in this paper). There are a number of published empirical correlations for three-phase relative permeability (3PRP) based on the corresponding two-phase relative permeability functions e.g. Stone Models I and II 1,2 , the Baker saturation-weighting model 3 , etc. These models are purely empirical and have only been partially validated for data in strongly water wet rocks and most reservoirs are not strongly water wet. In addition, these models embody very little understanding of the pore scale physics of three-phase flow. Recently, a number of advances have been made on the understanding of the pore scale physics of three-phase flow in water wet porous media 4 and also in systems of non-uniform wettability 5-8 , where the water-wet case is a particular limiting case. How this pore scale physics works through to the effective macroscopic flow parameters is still a matter of active research.
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