Figure 1—The gap between immiscible viscous fingering and miscible viscous fingering.
Suzuki et al. (2016) noted in their study with sodium sulfate and polyethylene glycol that the phase separation via spinodal decomposition outside the stage of metastability is associated with multiple droplet formation (due to the thermodynamic instability as opposed to hydrodynamic instability). Following their study, Fu et al. (2017) reported in their study of gas-in-oil injection for enhanced oil recovery at high-pressure conditions, the transition of the fluid system from immiscible to partially-miscible results in higher occurrences of droplet formation than finger formations.
This leads us to a question regarding one of the rarely studied but commonly encountered case: What about emulsions ? Emulsions are defined as fluid systems that consist of the oleic phase and aqueous phase and can achieve mixing with the addition of surface-active agents. Even without the addition of chemicals, emulsification is a commonly encountered part of crude oil production (Lee et al. 2019; Bruns and Babadagli 2020; Wang et al. 2020)— especially in the case of bitumen as bitumen contains a higher level of indigenous surface-active components such as asphaltene, naphthenic acid, humic acid, etc. relative to the light crude.
In our recent study (Lee et al. 2020), we have provided an extensive set of visual images captured from emulsion visualization experiments, where a heavy oil-saturated Hele-Shaw model was injected with chemicals of various components and rheological properties—which allows us to categorize and capture the universal fundamentals of the chemically induced viscous fingering development. The compact view of the original experimental images of selected samples is available in Figure 2 which displays the viscous finger development with chemical addition including AS (alcohol propoxy sulfate), CTAB, SiO2, and XG (Xanthan gum). The classification methods for the finger morphologies are elaborated in the cited study.