The Dynamic and Static Behaviour of Foam in a Model Porous Media

Foam has many useful features as an injection fluid for EOR. One of the most important aspects, is to reduce the gas mobility in a porous media. The manners in which the mobility of gas is reduced, is both by the increase of the gas apparent viscosity and by the trapping of the gas within the porous media, which reduces the relative permeability of gas. However, the dynamics and stability of foam in a porous network are not fully understood due to the complexity of foam behaviour in a porous media. A visual understanding of the dynamic processes is therefore of great value. In this project a better understanding of the aging mechanism of foam namely coarsening, and the gas entrapment (together with its effects) in an ideal micromodel with uniform permeability is pursued.

Previously, all methods to identify the quantity of trapped gas in a 3D porous media required injection of a tracer gas. Similarly, direct measurements of the dynamics of coarsening in a porous media are scarce in contrast to the coursing studies of foam in bulk. The shortcomings can be addressed by using a 2D porous media to directly measure the trapped gas fraction of foam flow at steady state conditions and also give an insight into how the quantity of trapped gas depends on the Darcy velocity, the foam quality and the position in the micromodel. Furthermore, the coarsening time scale for microfoam in a confined porous media was investigated with the deviation from Von Neumanns law.

Foam flood experiments were conducted in a 2D idealized micromodel with an average pore size of 60 ┬Ám and total dimensions of 60mm x 0.8mm x 5um. Firstly the mechanism of foam generation and flow in a porous media was observed. Secondly, the different stages of coarsening was investigated, by analysis of the increased average bubble size and the quantification of the characteristic times for the evolution of the numbers of bubbles. Thirdly, coarsening experiments were conducted in a core, to test the hypothesis that a additional pressure would be required to reactivate flow after coarsening. Fourtly, foam quality scans were made at steady state conditions to measure the pressure drop and the apparent viscosities for different solution concentrations of Alpha Olefin Sulfonate (AOS 14-16).

The characteristic time for a foam to coarsen is found to be approximately 4 minutes, while bubbles with a size around 40 um disappear in under 1 minute. Coarsening in a porous media is limited to the pore size, the bubbles and lamellae gain a stable, minimum energy position in the pore throats. Coarsening observed during foam flow caused a conversion of plug foam flow into preferential flow paths. Results show trapped gas fractions ranging from 64% to 11% for velocities from 0.05 m/s to 0.4 m/s and the fraction of trapped gas as function of increasing foam quality ranged from 21% down to 6%. Apparent viscosities found were low but in line with the apparent viscosities obtained in core flood experiments. Fg* seperating two foam regimes of 0.65, 0.44 and 0.4 were observed for three different AOS concentrations of 0.1wt%, 0.05wt% and 0.01wt% respectively in the micromodel.

Furthermore, with the required alterations made, the microfluidic device provides great information on foam flow in a 2D porous media and holds great potential for further research. 

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