Effects of air injection on flow through porous media: Observations and analyses of laboratory-scale processes

[1] The effects of air injection on flow through porous media were explored in a series of 1-m and 2-m laboratory flow cells. Our motivation was to examine air barriers as an alternative to hydraulic barriers to inhibit saline intrusion in coastal areas. Steady flow conditions were created in homogeneous and heterogeneous unconsolidated sand systems. Dry air was injected at progressively higher flow rates through a well in the center of each flow cell. Discharge and NaCl - tracer breakthrough data were measured at the outflow reservoir of each cell. In addition, a dye tracer was used to visualize the flow patterns. In all cases, air injection was found to produce stable, low-conductivity barriers that reduced discharge by an order of magnitude or more. Effective hydraulic conductivity values determined from discharge and hydraulic head data showed exponential declines with increased air-injection rates in all cases. Numerical simulation was used to quantify hydraulic conductivity and effective porosity values in the saturated and aerated regions created by air injection, and to study advective flow behavior. Pore-filling cement formed in the air-injection region and was analyzed to determine its composition, mass, and volume. Approximately 60% of the cement consisted of soluble minerals, and 40% was less soluble carbonates. Evaporation and increase in solution pH due to stripping of CO2 by the injected air were responsible for creating the cement. The cement occupied < 10% of the pore space in the sand-cement aggregate. Both the air and mineral pore-fillings dissolved when air injection ceased, indicating that both barriers are temporary. These results serve as a preliminary proof of concept that air-injection barriers might effectively inhibit undesired subsurface flow, such as saline intrusion or contaminated groundwater.