LABORATORY INVESTIGATION OF GAS BUBBLE FLOW THROUGH LAYERED HETEROGENEITIES USING TRANSPARENT SOIL

The prediction of gas distributions in the subsurface is critical for many applications, including releases from shale gas development or geological carbon dioxide sequestration and the remediation of volatile contaminants in groundwater by in situ air sparging. Gas flow pathways are controlled by multiphase flow processes, but can differ depending on whether gas flow is discontinuous (bubble flow) or continuous (channel flow). These pathways are also strongly affected by porous media heterogeneities at both the local and macroscopic scale. The purpose of this study was to investigate the breakthrough of gas through layered heterogeneities, with a particular focus on bubble flow and the development of disconnections within gas pools during releases. A two-dimensional air injection experiment was conducted in a 147 cm x 117 cm x 4.5 cm tank packed with fused quartz and initially saturated with a mineral oil mixture as the wetting fluid. The pore fluid was selected such that its index of refraction matched that of the fused quartz to produce a transparent soil system, which enabled detailed visualization of gas bubbles at the pore scale. Horizontal layers of finer material were used to create multiple capillary barriers, which resulted in multiple breakthrough events. Digital images were collected at a high temporal resolution (every 5 seconds) to quantify dynamic gas saturations at a spatial resolution of 3 mm x 3 mm. Both local and macroscopic heterogeneities were shown to affect gas distributions. Advancing bubble flow was initially halted beneath capillary barriers but remained disconnected. As bubble flow continued, coalescence occurred beneath the barrier until a connected pool was formed with sufficient driving force to advance (breakthrough), which resulted in bubble flow above the capillary barrier. Pore-scale disconnections were visible as the pool emptied, both vertically (as oil replaced gas at the pool base) and laterally (between pools of different height created by undulating capillary barriers). The nature of these pool disconnections affected the frequency of breakthrough and the magnitude of the trapped gas values, and is expected to affect predicted volumes of stored and released gas in field-scale applications.