Foam Trapping and Foam Mobility in a Model Fracture

Abstract

Gas injection often suffers from the poor sweep efficiency because of conformance problems, including gravity override, viscous fingering and channelling, as gas has a lighter density and a lower viscosity compared to in-situ fluids. Foam, by encapsulating the gas into separate bubbles in surfactant-contained liquid thin films (lamella), can effectively solve the conformance problems and hence improve the sweep.
Strong foam can reduce gas mobility by a factor of hundreds, by trapping gas and reducing its relative permeability in situ[1]. To efficiently improve the sweep, foam needs to propagate and maintain its strength at locations further away from the injection well. Foam trapping and propagation are highly dependent on porous media geometry, injection rate, foam quality, etc.
Microfluidic system, a medium integrating flow channels of manipulated structures on the order of tens to hundreds of microns, have been increasingly attractive to oil and gas, chemical and pharmaceutical industries[2]. Microfluidics are also becoming one of the most stimulating research field in foam EOR, because it provides the opportunities to visualize foam behaviour directly, such as foam generation, propagation and foam coarsening[3], etc.
We employ a model similar to microfluidics, directly applicable to flow in geological fractures. The 1-meter-long model represents a fracture channel with one roughened and one smooth wall. It has a width of 15 centimeters and a hydraulic aperture of 128 µm. The model is made of glass plates, therefore enabling direct investigation of foam behaviour through the channel using a high-speed camera. Since roughened glass is available with a range of roughness scales[4], one can relate foam behaviour to the roughness pattern in the channel.
We conduct a series of foam experiments in the model. Local equilibrium of foam (i.e. the rate of bubble generation equals to that of bubble destruction) is reached within our long model. We study the dynamics of gas trapping at different velocities and gas fractional flows.
We observe that velocity affects the fraction of gas which is trapped in the model at low foam qualities. Gas trapping decreases and foam mobility increases as superficial velocity increases. At high foam qualities, the relation between trapped gas and foam mobility is weaker. Gas trapping is insignificant and has little effect on foam mobility. When gas fractional flow increases at high foam qualities, flow alternates between slugs of gas and foam.