The immiscible to miscible transition and its consequences for 3-phase displacements in porous media of arbitrary wettability: Basic theory

In this paper, a “process-based” 3-phase model is applied to explore how multiphase fluid flow behaviour in two non-uniform wettability systems, named weakly oil-wet (WOW) and mixed-wet large (MWL), change when gas and oil move towards miscibility (σgo → 0). In the course of the theoretical development, we indicate how parts of the theory have been validated and confirmed satisfactorily by summarising experimental data from the literature. To demonstrate how the complexities of the three-phase physics interact with the system wettability, we use a capillary bundle (CB) model for the porous medium. This is sufficient to show the main processes which occur in the transition from immiscible to miscible behaviour. Even although the CB model is the simplest possible case for a porous medium, the results demonstrate a high level of unpredictable complexity which cover key parameters of the physics of 3-phase flow within porous media. In addition, for each case, the results point to the specific physical parameter that controls the phase separation lines. In some cases, gas injection or oil injection in a WOW system, the phase displacement changes quite radically as the system goes towards miscibility since there is a transition in wetting order from O/W/G to O/G/W. The differences and similarities between saturation paths in WOW and MWL are described in detail. For a given pore size distribution (PSD), the 3-phase displacements are controlled by the parameters of the rock wettability structure (see text) and the 3 interfacial tensions. Moving towards miscibility, 3-phase fluid displacement for gas injection and oil invasion are controlled by IFTs, while for water flooding, wettability structure plays the key role at all miscibility conditions. Although the simple CB model cannot describe all phenomena, such as phase trapping, the phase displacement paths, still the pore occupancies are qualitatively correct. Results of various phase injection in WOW are broadly consistent with pore occupancy sequences observations from pore scale X-Ray image analysis in literature despite simplicity of the theory which is validated in terms of its underling physics.