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Thermal recovery projects, in which steam-water mixtures are injected into oil-bearing rocks at high temperatures, provide a laboratory to study rock-water interactions. The temperature range of 80°-220°C is appropriate for diagenetic settings and, because injected steam that condenses to freshwater mixes with more saline water during production, the effect of mixing waters with different salinities can also be examined. In this study of the Tucker Lake pilot site in the Cold Lake heavy oil deposits of Alberta, water and gas samples were obtained at regular intervals over a 7-month period. The chemical composition of these samples was used with solution speciation models to compare the stability of waters with respect to the calculated stability of minerals known to be formed during steam injection and thermal recovery as determined from thermodynamic data.

The activity ratios of dissolved species, such as Na+, K+ and Mg2+ to H+, tend to follow phase boundaries that represent silicate hydrolysis reactions between kaolinite, chlorite, illite, K-feldspar, smectite, and analcime. The Na concentration changes during production from approximately 500 to 4500 mg/L, and the observation that the aNa/aH ratio for the waters follows the smectite-analcime boundary during production suggests that the silicate hydrolysis reaction is buffering the pH to maintain the constant aNa/aH ratio. In contrast, the aCa2+/(aH+)2 ratio follows the dissolution phase boundary for calcite, implying that the reaction rate for calcite dissolution is more rapid than that for silicate hydrolysis. Other published studies of the isotopic composition of produced gas from the Tucker Lake pilot confirm that CO2 is produced by reaction of calcite, supporting the role of silicate hydrolysis in calcite dissolution. The buffering of fluid activity ratios by silicate hydrolysis reactions suggests that silicate hydrolysis plays an important role in dissolution of calcite and other carbonates in this thermal pilot, and potentially during natural diagenesis. Silicate hydrolysis and reactions involving silicates in general should be carefully considered in formulating diagenetic models aimed at predicting reservoir quality, particularly if prediction of dissolution porosity is considered an important factor.

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