Critical Thresholds for CO2 Foam Generation in Homogeneous Porous Media

Abstract

Long-distance propagation of foam is one key to deep gas mobility control for enhanced oil recovery and CO2 sequestration. It depends on two processes—convection of bubbles and foam generation at the displacement front. Prior studies with N2 foam show the existence of a critical threshold for foam generation in terms of a minimum pressure gradient r pmingen or minimum total interstitial velocity vmint,gen, beyond which strong-foam generation is triggered. The same mechanism controls foam propagation. There are few data for r pmingen or vmint,gen for CO2 foam.

We extend previous studies to quantify r pmingen and vmint,gen for CO2 foam generation and, for the first time, relate r pmingen and vmint,gen to factors including injected quality (gas volume fraction in the fluids injected) fg, surfactant concentration Cs, and permeability K. In each experiment, steady pressure gradient ∇p is measured at fixed injection rate and quality, with total interstitial velocity vt increasing and then decreasing in a series of steps. The trigger for strong-foam generation features an abrupt jump in ∇p upon an increase in vt.

In most cases, the data for ∇p as a function of vt identify three regimes, which are coarse foam with low ∇p, an abrupt jump in ∇p, and strong foam with high ∇p. The abrupt jump in ∇p upon foam generation confirms the existence of r pmingen and vmint,gen for CO2 foam. We further show how r pmingen and vmint,gen scale with fg, Cs, and K. Conditions that stabilize lamellae reduce the values of the thresholds: Both r pmingen and vmint,gen increase with fg and decrease with increasing Cs or K. Specifically, r pmingen scales with fg as (fg)2 and vmint,gen scales as (fg)4, and both r pmingen and vmint,gen scale with Cs as (Cs)−0.4. The effect of K on the thresholds for foam generation is greater than the effects of fg and Cs. Our data in artificial consolidated cores show that r pmingen scales with K as K−2 for CO2 foam, in comparison with K−1 for N2 foam in unconsolidated sand/bead packs. More data are needed to verify these correlations.

It is encouraging that r pmingen in the cores with K = 270 md or greater is less than 0.17 bar/m (~0.75 psi/ft), two to three orders of magnitude less than for N2 foam. Such low r pmingen can be easily attainable throughout a formation. This suggests that limited ∇p deep in formations is much less of a restriction for long-distance propagation of CO2 foam than for N2 foam. Foam propagation could still be challenging in low-K reservoirs (r pmingen ~10 bar/m for K = 27 md). Nevertheless, formation heterogeneity and alternating slug injection of gas and liquid help foam generation and can reduce the values of r pmingen.