Observations of gas flow in porous media using a light transmission technique

A novel technique for quantitative nondestructive study of two-dimensional disposition of gas phase in unsaturated porous media is presented. Carbon dioxide was pumped through a backlit 1 cm thick chamber packed with translucent sand, which was variably saturated with water containing the pH indicator dye methyl red. As the carbon dioxide dissolved in the pore water, lowering the pH and changing the dye color, a CCD camera captured images of the resultant changes in transmitted light. These digital image files were then processed using a series of calibrated steps to relate light intensity to dye attenuation, dye attenuation to solution pH, and solution pH to aqueous and gaseous carbon dioxide concentration. The final product was a series of false-color images showing the development of the gaseous carbon dioxide plume. Limitations were found that will require further development, including more attention to calibration of dye concentration versus observed transmission and investigation of dye solubility across the range of pH values employed. With refinement, this technique may prove to be a useful tool in studying the complexities of gas phase transport in variably saturated porous media.