Determining Trapped Gas in Foam From Computed-Tomography Images

Abstract

Summary Gas trapping by foam is a key mechanism of foam mobility and foam effectiveness in applications such as acid diversion in well stimulation, enhanced oil recovery (EOR), and aquifer remediation. Previous studies have attempted to quantify the extent of gas trapping by injecting a tracer gas within the foam and then fitting the effluent profile to a 1D capacitance model. In this model, at any given axial position along the core, all flowing gas and all trapped gas are each characterized by a single tracer concentration. Computed-tomography (CT) images of experiments using xenon (Xe) tracer show that this characterization is not accurate: Trapped gas near flowing gas comes rapidly to equilibrium with flowing gas long before tracer diffuses into trapped gas farther away. We introduce a method that uses the CT images directly to estimate flowing-gas fraction. In the CT images, tracer advances in many small channels and diffuses outward into surrounding regions of trapped gas a few millimeters in diameter. The difference between the higher tracer concentration at the center of these channels and the lower concentration at the edge can be related to the diffusion coefficient of the tracer and the flowing-gas fraction within the channel. For the CT images of Xe tracer in one experiment, this method gives flowing-gas fractions one or two orders of magnitude smaller than what is estimated using the 1D capacitance model. The model can be used to estimate flowing-gas fraction in different regions of a core in spite of different average gas velocities in the different regions.