Silica Removal From Steamflood-Produced Water: South Texas Tar Sands Pilot

Abstract

Summary Steamflood-produced waters commonly contain suspended solids, oil, hardness-causing minerals, sulfide, and silica. Removal of these contaminants would make many of these waters suitable for recycling as steamer feedwater. Reuse of steamflood-produced waters increases steamer feedwater supplies and reduces water disposal requirements. This paper describes a field pilot study of silica removal from steamflood-produced water in the south Texas tar sands region. A hot-lime precipitation process was used to reduce dissolved silica (SiO2) concentrations from 400 to less than 50 mg/L SiO2 in Mary R. Saner Ranch produced water. Most water systems using hot-lime precipitation for silica removal require the addition of magnesium salt, as well as lime, to enhance silica removal. In this field study, however, addition of magnesium salt did not improve silica removal efficiency. Hydrated lime, Ca(OH)2, alone was sufficient to attain desired silica residual, 50 mg/L SiO2. The dissolved silica adsorbed onto the CaCO3 crystals formed by lime reacting with the alkalinity present in the produced water. Required lime dosage was approximately 900 mg/L Ca(OH)2. Residual silica concentration was found to be strongly related to both precipitator effluent pH and calcium ion concentration. Therefore, on-line pH and hardness monitoring may be used to estimate and to control residual silica concentration. A 50,000-B/D [7950-m3/d] produced-water treating plant has been designed from the results of this pilot study.