InterPore2024

CT Gas Tracer Study of Gas Trapping and Diffusion in Foam in Porous Media

16 May 2024, 09:05

15m

Oral Presentation (MS06-A) Physics of multiphase flow in diverse porous media MS06-A

Speaker

Dr JIAKUN GONG (Hohai University)

Description

Foam improves sweep efficiency in gas-injection processes to sequester CO2 in, and to produce hydrocarbons from, porous geological formations (Bellow, 2023, Rossen et al., 2022). Gas trapping plays a key role in foam’s ability to reduce gas mobility in porous media. We describe a study of gas trapping and diffusion in a sandstone core using nitrogen (N2) foam and krypton (Kr) as a gas-phase tracer.
Surfactant solution and nitrogen were injected, with gas fraction 0.6, superficial velocity 2 ft/day (3.5 x 10-6 m/s), into a 17-cm-long, 4-cm-wide Berea core oriented horizontally in a CT scanner. At steady-state, foam apparent viscosity was 0.4 Pa s (400 cp), and gas saturation was uniform across the core cross-section. Then injection continued with 0.6 gas fraction, but with half the N2 replaced by Kr gas. Krypton can be distinguished in the CT scanner from N2 at the pressure of these experiments (4 MPa). Kr can therefore serve as a tracer in high-pressure foam-flow experiments in cores in place of the more-expensive xenon used by Nguyen et al. (2009). With image filters, it was possible to determine Kr fraction in the gas from the image with a resolution of about 2 mm.
CT images show that the advance of Kr was almost entirely in a thin zone at the top of the horizontal core, with trapped, immobile foam below. (See graphical abstract, where Kr is shown in red.) This is likely the result of segregation of gas and surfactant solution in the core endplate. Similar, though less-severe, segregation originating in the endplate was observed in the foam CT experiments in Kil et al. (2011). Slowly Kr diffused down from the flowing foam at the top of the core, in a similar fashion to diffusion of gas through trapped foam in the coreflood experiments in Kil et al. Our resolution was not sufficient to resolve individual flowing-gas paths in our experiments as in Kil et al., however. A model fit to the CT data indicates that flowing fraction of gas in the core was roughly 0.06, and the Kr diffusion coefficient through trapped gas was 3 to 4 x 10-8 m2/s.
In addition to measuring the flowing gas fraction and diffusion rate of gas through trapped foam, these results highlight the usefulness of Kr as a possible gas-phase tracer in high-pressure foam experiments in porous media. They also highlight the need to account for possible nonuniform injection from the core endplate in multiphase displacements in core samples.