The immiscible to miscible transition and its consequences for 3-phase displacements in porous media of arbitrary wettability: Comparison of 3D pore network results with basic theory

The theory of the immiscible to miscible transition and its consequences for 3-phase displacements in a system of arbitrary wettability distribution has been studied in a previous paper using a “process based” model through the simplest case possible; a capillary bundle (CB) model. As expected, the CB model was not able to capture many realistic phenomena such as phase trapping, film and layer-flow, topological effects etc. However, it was recognized that it should be able to indicate certain expected qualitative trends in 3-phase displacements in real 3D connected networks.

The current paper extends that earlier work by showing the consequences of changing the model of the porous medium to a 3D connected lattice pore network model (PNM) to capture phase trapping and rock connectivity. Initially a simple 3D regular cubic bond lattice is used for this initial work to make a clear comparison with the CB model. Two important characteristics of 3D network refer to (i) displacement phase accessibility, and (ii) defending phase trapping/escape. Three trapping/escape cases based on the probability of invading phase accessibility and defending phase escape have been defined in the 3D lattice network. Networks with different coordination numbers (z = 6 and 4) were studied in order to mimic different rock connectivity and study how and in which way the applied rules result in specific changes in 3-phase saturation paths, 3-phase pore occupancies and phase trapping. Again the focus is on the changes which are predicted to occur in all these quantities as the system moves from an immiscible (IFT σ_go ∼ “large”) to a miscible (σ_go → 0) case.

In this paper, a weakly oil-wet (WOW) wettability distribution is considered and various phase invasions (gas, water, and oil) in the corresponding 2-phase systems are investigated. Both saturation paths and pore occupancy sequences of different phase invasions under different miscibility conditions are presented for both the CB and PNM models. It is found that having a well-connected network and considering a relaxed rule on defending and displacing phases, the results are quite similar to that of the CB model. Also, applying restricted trapping/escaping rules and less connected network mean deviation from the CB model, but the underlying physics of the immiscible to miscible transitions and the central physical parameter of 3-phase displacements in all cases are the same as those observed for the CB model.