Foam generation and propagation in homogeneous and heterogeneous porous media
Foam generation plays a crucial role in foam processes for improved oil recovery, acid diversion and environmental remediation. This study extends previous work to layered media, the effect of surfactant concentration, and injection strategy, including alternating-slug (SAG) injection and pulsed injection rate and pressure. New experiments in sand packs and a sandstone core reach the following conclusions: • As in earlier studies, three foam states were observed: high-mobility (coarse) foam at low pressure gradient, low-mobility strong foam at high pressure gradient, and an intermediate, unstable regime including nearly complete gas trapping in between. • Foam generation occurred at lower pressure gradient in higher-permeability media. At lower surfactant concentration, creating foam required higher gas velocity. • In flow from lower- to higher-permeability layers, foam generation occurred at lower pressure gradient than expected in either medium by itself. However, a minimum threshold pressure gradient for foam generation was still observed, in contradiction to theory (Rossen, 1999). Modeling shows that non-uniform packing of sand near the edge of the pack might explain the discrepancy. • Briefly raising the injection pressure in steady co-injection helped trigger foam generation, i.e. persistent low gas mobility, in layered media, but had no lasting effect in homogeneous packs. • During gas injection in SAG processes, a low-mobility front traveled the length of homogeneous packs and then exited. In heterogeneous packs, SAG injection gave persistent low mobility near the transition in permeability. • With co-injection of gas and liquid, the low-mobility zone near the transition in permeability spread downstream if the pressure gradient was sufficient. In gas injection in a SAG process, we never observed the front moving downstream of the vicinity of the transition. We offer a modified population-balance model for foam that for the first time combines foam generation by snap-off at a layer boundary and by pressure gradient. This model for the first time reproduces two features of the experimental results: SAG foam injection that produces a stronger foam the higher the injection rate, and foam generation at a layer boundary that propagates downstream of the boundary if pressure gradient is large enough.