Effect of the Immiscible to Miscible Transition on 3-Phase Displacements in Systems of Arbitrary Wettability

Summary The understanding of 3-phase flow in porous media phenomena is crucial for various applications, especially in processes such as CCS (carbon capture and storage), H2 storage, fuel cell design and gas EOR. In this study, a process-based model introduced by van Dijke et al. is applied to study the 3-phase saturation paths and pore occupancy sequences in porous media flows. Systems with water-wet and oil-wet wettability distributions are studied using a capillary bundle (CB) model to simulate 3-phase displacement processes occurring over the immiscible to miscible transition. This transition occurs where gas and oil move toward miscibility(i.e. σgo → 0). Different phase invasions (gas, water, and oil) are modelled which elucidate the underlying physics which occurs in the porous media. The CB model has been applied as the first step in a planned research scheme with the purpose of achieving a fully understanding of 3-phase displacements in a system of arbitrary wettability under various immiscible/near-miscible conditions. The applied theory has been verified in detail in our previous work, and in this study we present results comparing modelled pore occupancy sequences with experimental results from x-ray image analysis. This is achieved with our simple CB model with minimum of parameter adjustment and good qualitative agreement is found despite the simplicity of the model. The CB model cannot reproduce several realistic phenomena, such as phase trapping, film flow and multiple-displacements. However, it is able to highlight the key physical parameter that controls 3-phase displacements and pore occupancies under different experimental conditions. Four wettability distribution cases are considered for a spreading oil; weakly water-wet (WWW), strongly water-wet (SWW), weakly oil-wet (WOW) and strongly oil-wet (SOW). The results of 3-phase saturation paths for gas, water and oil invasions are presented for immiscible to miscible transitions. The results show that for both water-wet and oil-wet systems, saturation paths of gas and oil injection at immiscible condition are determined by their wettability conditions while by moving toward miscibility all become the same. On the other hand, for water injection, no matter of wettability, saturation paths are not affected by immiscible to miscible transition. The agreement of such behaviour has been observed by X-ray experimental pore occupancy results of phase invasions in water-wet and oil-wet systems found in literature. The underlying physics of such phenomena is discussed in terms of wetting order effect on oil and gas invasion, using the van Dijke-Sorbie development of the Bartell-Osterhof chart.