Evaporation-Induced Instability during Dry Gas Injection into Water-Saturated Porous Media

The dynamics of immiscible fluid-fluid displacement in porous media have been investigated widely. However, geological flow systems involving partially miscible fluids, such as unsaturated gas draining evaporable liquid, have not been investigated to an equivalent extent.

Here we experimentally investigated the role of slow evaporation on gas-water displacement dynamics, by continuously injecting dry gas into water-saturated porous media. We observed a surprising global collapse of the gas-swept domain—a process in which water invades back into the gas-saturated region in relatively short time, which is initiated several hours after gas breakthrough.

We show that this global pattern instability is caused by the evaporation of residual water clusters surrounded by the channels that deliver the gas flow. A mathematical model of this evaporation-induced instability is developed that predicts the onset of the pattern collapse and also explains the recurrence and intermittency of collapse at low gas-injection rates, and its absence at high rates.

Understanding such evaporation-flow coupling dynamics may be important for applications such as CO₂ sequestration, geothermal recovery, gas-enhanced oil recovery, and gas delivery to the cathode in low-temperature fuel cells.