Lithium Silicate Scale In Oilfield Steam Generation

Abstract

Abstract At Wolf Lake, steam for in situ recovery of bitumen has been generated from produced water for over two years. Conventional softening of the produced water with warm lime followed by weak acid cation exchange without silica removal was inadequate to prevent scaling of the boiler tubes. Analysis identified the scale as lithium silicate, a material not previously identified as a scale in steam generation. Treatment of the produced water to lower the silica concentration below 50 mg/L has been found necessary and at 25 mg/L adequate to prevent scaling under the operating conditions at Wolf Lake. The concentration of lithium in produced waters is expected to vary with the reservoir, and to be a previously unrecognized determinant in the allowable level for silica in boiler feed water. Introduction In situ recovery of bitumen is done primarily by steam assisted methods. This type of bitumen production requires substantial volumes of water for steam generation. The steam to bitumen ratio may be as high as 6:1 which means that to produce 1 m3 of bitumen, 6 m3 of steam as water equivalent are required. In many areas of heavy oil deposits, fresh water supplies are not readily available. Those fresh water supplies that are available are needed for a wide variety of uses, including domestic requirements, wilderness preservation, agriculture, recreation and industry. Moreover, increasing stress and more stringent quotas for fresh water usage are likely to be imposed on the heavy oil producer. Therefore, the oil companies producing bitumen, recognizing the competing demands, must recycle the water that is co-produced with the bitumen. This was recognized a long time ago and extensive research has been conducted on the treatment that could allow reuse of the so called produced waters. Basically, produced water to be recycled for steam generation must be de-oiled and components that will form scale in the boilers must be reduced to innocuous concentrations. Traditionally, for oil field steam generation, the scale forming elements/compounds that have to be treated have been calcium and magnesium (hardness), iron and silica. There is a general consensus what level of hardness and iron is acceptable in the boiler feedwater (BFW) to the oilfields oncethrough steam generators: total hardness expressed as CaCO3 should be less than 0.5 mg/L and iron less than 0.1 mg Fe/L. However, a much more complicated problem arises with silica. The prevailing opinion among water treatment specialists appears to be that if the feedwater is free of hardness and its alkalinity is high enough (ratio of alkalinity to SiO2 equal to 3:1 being the minimum) silica in the BFW is not a particular problem and higher levels can be tolerated. The two main manufacturers of the oilfield boilers, Struthers TIW Limited and C.E. Natco, suggest that satisfactory operation of their boilers can be maintained with silica content 100 mg/L and 150 mg/L respectively(1,2): trouble free operations were reported with silica exceeding 200 mg/L(3).