Deposition of CaCO/sub 3/ from natural geothermal fluid was studied by using porous substrates made from several granular materials. These experiments aimed to explore connections between the process of steam flash from superheated CO/sub 2/-charged water and the consequent changes in the carbonate chemical equilibria. The substrates were characterized pre- and post-experiment. Flow rates and compositions of input and output fluids were determined. The resultant CaCO/sub 3/ deposits were examined from several points of view. (MHR); LBL-10673