Mineral deposition has reduced the injection rates in an injection well at the Salton Sea geothermal field. Scanning electron microscope images combined with emiquantitative energy dispersive analyses show that the scale deposits found in cuttings from Elmore IW3 RD-2 consist of layers of barite and fluorite and minor anhydrite, amorphous silica and copper arsenic sulfides. Geochemical modeling using TOUGHREACT has been initiated to further assess the behavior of the injection fluids and their effects on well performance. A onedimensional model is used to simulate injection into sandstone containing a fracture zone with 95% porosity. Initial models predict that barite is the mineral responsible for porosity declines when silica and bicarbonate are removed from theinjection fluid. This prediction is consistent with the observed mineral relationships