Displacement stability revisited – A new criterion for the onset of viscous fingering

Any process in which a high mobility fluid is displacing a lower mobility fluid is prone to instability. Pertinent present-day examples are CO2 injection for carbon capture and sequestration (CCS), gas injection for underground storage or for pressure maintenance. Over the years, several criteria have been developed to predict the onset of viscous fingering. However, there is no consensus on what criterion is truly valid and more recently several studies have been reported to empirically decide on what criterion is the most reliable. In this work, we present a new approach derived from first principles. The results are shown to unify all earlier derived criteria whether for porous rock, or for Hele-Shaw cells, with and without gravity, immiscible and miscible, through proving that these are special cases, of limited validity, of a more general formulation. The new criterion is more restrictive in terms of viscosity ratio than the Hagoort shock-front mobility ratio, but less restrictive than the straightforward viscosity ratio. 2-Phase flow simulations have been conducted to demonstrate the validity over a wide parameter range. In addition, we have studied the impact of fingering in SCAL laboratory tests employing the unsteady-state (“Welge”) technique. The results were analysed through interpretation-by-simulation using the automatic history matching tool AutoSCORES.