Air sparging (AS) is an in-situ soil/groundwater remediation technology, which involves the injection of pressurized air/oxygen through an air sparging well below the region of contamination. The efficiency of the in-situ sparging system is mainly controlled by the extent of contact between injected air and contaminated soil and pore fluid. Hence, characterizing the mechanisms governing movement of air through saturated porous media is therefore critical to the design of an effective cleanup treatment. However, in spite of the success of air sparging as a remediation technique for the clean-up of contaminated soils, to date, the fundamental mechanisms or the physics of air flow through porous media are not well understood.
In this research, the micro-mechanics of air sparging are studied in order to understand the physical processes of air migration and air spatial occurring on the pore scale during air sparging. First, the void space in the porous medium is characterized by pore network consisting of connected pore bodies by bonds. Two approaches of generating 3D stochastic pore networks were proposed. The first methodology is to directly extract pore structure from a computer simulated packing of spheres. The second methodology is to generate an equivalent 3D pore network of porous media, in which the centers of voids are located in a regular lattice with constant pore center distance. Both algorithms were validated by comparing the predicted permeability of randomly packed spherical particles with published experimental data. The results showed that the predicted permeability values were in good agreement with those measured, confirming that the proposed algorithms can capture the main geometrical information and the topological information of random packing of spheres.
Secondly, based on the developed network model, a rule-based dynamic two-phase flow model was developed in order to study the dynamic flow properties of air water two-phase flow during air sparging. The rules for phase movement and redistribution are devised to honor the imbibition and drainage physics at pore scale. The system is forward integrated in time using the Euler scheme. For each time step, the distribution of the phases leads to a recalculation of the effective viscosities in the network, capillary pressures across the menisci, and thus the coefficients in the equations for the pressure field. When the pressure field is known, the flow field follows automatically and the integration step can be performed.
Finally, the developed dynamic model was used to study the rate-dependent drainage process during air sparging. Two types of numerical tests were performed: one is with one-step air injection pressure while the other one is with multistep air injection pressures. During the two types of numerical tests, the effect of the capillary number and geometrical properties of the network on the dynamic flow properties of air water two-phase flow including residual saturation, changing rate of water/air saturation, air and water spatial distribution, dynamic phase transitions, nonwetting fractional flow, air and water relative permeability and capillary pressure curve were systematically investigated. The results show that all this information for describing the air water two-phase flow properties is not an intrinsic property of the porous medium, but on the contrary, it is affected by the air water flow rate and historical distribution of air water phase. It is also shown that the developed model can capture the dynamic effects on all these data.
At the end of this research, two different ways of performing micro to macro level analysis to obtain the macro behaviors of air water two-phase flow by using micro-mechanics were also briefly discussed. In all, this research has laid substantial basis on understanding the mechanisms of the air water two-phase flow process during air sparging.
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