Lab Evaluation of Long-Distance Propagation of CO2 Foam for Deep Mobility Control

Abstract Long-distance foam propagation is crucial and necessary for deep mobility-control applications of foam in geological formations. The long-distance propagation of foam could be realized via two means: in-situ generation of foam at the displacement front and mobilization of foam generated upstream. Foam catastrophe theory predicts a minimum-pressure-gradient and associated minimum-velocity threshold needed for foam generation (∇pgenmin and Ut,genmin), mobilization (∇ppropmin and Ut,propmin) and stability (∇pstablemin and Ut,stablemin), respectively. However, the threshold conditions for CO2-foam propagation are rarely studied, posing uncertainty in the evaluation or optimization of long-distance CO2-foam propagation in the field. This study is aimed at determining the critical-threshold conditions for CO2-foam propagation and quantifying the impact of influential factors on the thresholds, e.g. injection quality (volumetric fraction of gas in total fluids injected), fg, permeability, K, and foaming agents. For the measurement of the critical thresholds for foam propagation, we deploy a varying-diameter coreflood approach. Compared with conventional uniform-diameter coreflood, varying-diameter coreflood represents more realistically the flow conditions of foam from near-well region radially outwards into formation where pressure gradient and velocity drop greatly. For the first time, we have determined the minimum pressure gradient and minimum velocity needed for CO2-foam generation, mobilization and stability maintenance. The results also show that increasing injection quality, fg increases the values of the critical thresholds for CO2-foam generation, stability and mobilization, making foam propagation more difficult, whereas increasing permeability, K reduces the values of the three thresholds, facilitating CO2-foam propagation. Thus, the favorable conditions for foam generation and propagation are lower fg and higher K. This is because, as the percolation theory of Rossen and Gauglitz (1990) predicts, a lower fg means wetter condition, leading to more liquid lenses occupying pore throats for blocking gas-filled pore clusters and lower capillary pressure. As a result, the values of ∇pgenmin and Ut,genmin required to trigger strong CO2-foam generation via lamella division are reduced. Also, at wetter conditions with lower fg, foam film is thicker, where the drag force between foam film and rock wall is reduced, leading to a reduced value of ∇ppropmin needed for mobilizing foam. In higher-permeability media, capillary pressure is lower, resulting in lower values of ∇pgenmin and ∇ppropmin, making foam generation and propagation easier. The physical correlation for the need of a minimum pressure gradient and velocity for foam-stability maintenance requires further research. We also find a new way of enhancing long-distance propagation of CO2 foam via reducing CO2-water interfacial tension. our measurements show that a surfactant achieving a lower gas-liquid interfacial tension can reduce the values of the thresholds for CO2-foam generation, stability and mobilization. This suggests that, especially in low-permeability media, when the pressure gradient or velocity required for deep CO2-foam propagation are high, one could select or develop surfactants that give low CO2-foam interfacial tension to enhance the propagation of CO2 foam into the formation.