Scaling Analysis and Seismic Resolvability of Transition Zone in an Air-Water System

Unsaturated flow in aquifers commonly includes a transition zone caused by capillary forces. Saturation varies within this zone, affecting many properties of interest, including effective permeability and acoustic velocities. Moreover, in systems with a rising or falling water table, the shape of the transition zone varies. This problem is of interest because it affects water deliverability and supply, and is analogous to segregated flow in oil and gas reservoirs. If transition zone shape is understood, it can be used to predict saturations and acoustic velocities, and our ability of using seismic methods to resolve water table locations in aquifers will be improved. The inspectional analysis isolates the effects in terms of a capillary and gravity number. We show that transition zone thickness scales on these using numerical simulation. Since Carmen Kozeny equation is used to convert the pore scale to the reservoir scale, the numerical simulations will run with different grain sizes and injection or production rates are run and. Water distribution and transition zone thickness are recorded at different time steps. Based on that, we investigated how the thickness of transition zone changes with different capillary number and gravity number. The Hertz-Mindlin method is applied here to calculate acoustic properties from the numerical simulation results, then raytracing simulation runs and the seismic resolvability of transition zone is analyzed. The scaling analysis results show that dimensionless thickness of transition zone has a linear relationship with capillary number when gravity number are the same. Therefore, the transition zone thickness can be predicted in large scale aquifers. From seismic analysis results, we found the transition zone thickness and the water table location are the two main factors that affect the refracted signals. With realistic models for permeability and capillary pressure, we investigate the seismic resolvability of the air-water transition for two cases: a meter-scale experiment and a 100-meter thick aquifer. The results show that if the transition zone thickness and refracted signal are known, the water table location can be predicted.