Investigation of Gas Breakthrough of Two-Dimensional Capillary Barriers using Transparent Soil

Abstract

The prediction of gas distributions in the subsurface is critical for many applications, including the remediation of volatile contaminants in groundwater by in situ air sparging, or releases from either shale gas development or geological carbon dioxide sequestration. These distributions are strongly affected by porous media heterogeneities. It is important to know if our current theoretical knowledge can predict gas flow in realistic two-dimensional systems. This study’s objective was to investigate the breakthrough of gas through layered heterogeneities, with a particular focus on pool dynamics and gas entrapment. Two controlled gas injection experiments were performed for two heterogeneous transparent soil packs, which enabled detailed gas visualization at the pore scale. Digital images were collected at high temporal and spatial resolutions and converted to saturation images using a newly derived and validated relationship. A two-point logarithmic saturation – pixel intensity (S-I) relationship was evaluated, with the comparison to the macroscopic endpoints being the most useful for validation, and influential in satisfying the mass balance. The methodology to derive and validate this relationship may be reduced to a single test, which is superior to the labour-intensive methods used previously in studies that used light reflection techniques. Using this new S-I relationship, transient pool behaviour, which included saturation profiles and pool heights, was examined for three gas pools during numerous filling and emptying events: two gas pools were beneath undulating capillary barriers (CBs) and one gas pool was beneath a simpler one-peak CB. Gas migrated upwards and collected as highly gas-saturated pools, beneath a series of CBs. The pools broke through the CBs via narrow channels. As the pool released gas, the pool height reduced until flow through the CB ceased. The behaviour of the two pools beneath undulating CBs behaved differently than predicted. Firstly, the pools disconnected laterally due to pinch-points trapping highly gas-saturated pockets. Secondly, some pools completely emptied to near residual gas saturation with near-zero pool heights. This may be explained by the high local air flow velocity through a single pore throat. Complete emptying of gas pools must be considered in order to accurately predict gas storage in heterogeneous systems.