Reservoir Engineering

Wettability, Viscous Fingering & Capillary Fingering

1. If viscous force remains to play a critical role in the fluid invasion process and if the displacement front propagation keeps depending highly on the viscous ratio, under high flow rates; and if the viscous force acting against the capillary force produces unstable fronts; and if the resulting pattern formation remains to be sensitive to the wettability, then, to what extent, the instability of displacement front keeps influencing (a) oil/gas recovery efficiency; and (b) CO2 storage capacity – towards characterizing a two-phase fluid flow?

2. To what extent, the competition remains further complicated by the pore surface’s wettability, apart from the competition between the capillary force and viscous force – associated with a fluid-fluid displacement (in the absence of gravity)?

In such cases, what exactly dictates the immiscible displacement pattern to remain to be either capillary fingering or a viscous fingering, or to a stable front?

3. Whether, phase diagrams would be able to efficiently distinguish displacement patterns as a function of Capillary Number and Viscosity Ratio – during a typical drainage process, where, a non-wetting fluid keeps displacing a wetting fluid?

4. Whether a precise scaling of capillary displacement with cos θ remains to be strictly limited to straight capillaries; and in turn, can’t we quantitatively account ‘cos θ in Capillary Number’ for the impact of wettability on the immiscible displacement patterns?

5. Feasible to bridge the gap between the local pore-scale filling events governed by young-Laplace law to the regional cooperative and competitive filling behavior, towards describing the impact of wettability on the displacement patterns?

6. In a capillary-dominated displacement, when the pore surface becomes more and more wet to the invading phase (i.e., when contact angle keeps decreasing), whether, the width of fingers would keep on increasing?

If so, then, how to capture the critical contact angle, when the width of fingers keep appearing to diverge?

7. Whether the increase of capillary number would always enhance viscous fingering, regardless of wettability, given the interplay between viscosity, capillarity and the wettability (@ high flow rates)?

8. Whether decreasing the invading fluid contact angle between 180 degrees and critical contact angle (i.e., from strong drainage to weak imbibition) would always stabilize the displacement patterns?

9. Whether the viscous pressure field would always reside behind the displacement front under favorable displacement conditions; and in turn, whether, the interface would remain to be stable against the capillary pressure fluctuations that roughen the interface?

10. Why does the mechanism of wettability together with the flow-rate controlling the invasion morphology relatively remain to be difficult to capture?

What exactly is the role of viscous force at the interface, (a) when M1?

Dr Suresh Kumar Govindarajan, Professor [HAG]

IIT Madras 17-March-2025