Steam injection in heavy oil reservoirs produces hydrothermal reactions which can lower producing potential by reducing permeability and causing well bore damage.Effluents from once-through steam generators often have pH values above 12 due to the use of high bicarbonate surface waters as feeds. Injection of these high pH and low ionic strength waters causes expansion and dispersion of water-sensitive clays and solubilization of silica and aluminum minerals. The hydroxide and carbonate ions in the effluents react with magnesium, calcium and other salts in formation waters to form precipitates.Laboratory experiments showed that these alkaline boiler effluents irreversibly reduced the permeability of cores (by up to 70%), dispersed clays and solubilized quartz sands.In the field, large quantities of amorphous precipitates containing magnesium, calcium and aluminum as well as clays, micas and quartz grains were produced from two wells with low oil/steam ratios. The quartz grains showed substantial changes in surface morphology similar to those produced by quartz solubilization experiments in the laboratory.To minimize reservoir damage, steam generator effluents with reduced pH and increased ionic strength are being injected at two field pilots.