1 INTERACTION BETWEEN DISSOLVED SILICA AND CALCITE

In this work, we investigated the interaction between dissolved silica and calcite during adsorption and coprecipitation in connection with the problem of retardation of recrystallization process in oil chalk res-ervoirs. To assessment the interaction a lot of attention was given to choose optimal experimental conditions: silica to calcite ratio (SiO2/CaCO3), concentration of calcite-forming reagents, calcite precipitation rate, etc. All experiments were performed in a glass jacked reaction vessel (T=25 °C) at different CO2 partial pressures (pCO2=10-3.5 atm and 1 atm). In the adsorption studies weighed amount of calcite powder (1-10 g) and 50 ml of 0.1 M NaCl solution were placed into the reaction vessel. When the equilibrium at given CO2 partial pressure between calcite and the solution was reached (controlled by pH and Ca-electrodes) the aliquot of 0.1 M SiO2 solution was added into the vessel. The calcite suspension concentration ranges from 20 to 200 g/L. The aqueous phase was periodically sampled, filtered with 0.2 μm (Sartorius) and analyzed for silica. Simultaneously we ran blank experiments with the same silica concentration but without calcite in the system. To determine con-centration of silica in the taken samples the colorimetrical method was used. We could not see any differ-ence between silica concentration in the samples and the blank samples even when the amount of placed calcite was high. We also analyzed silica concentration in solid phase. To do that, in the end of each ex-periment calcite was rinsed several times with calcite-saturated solution and dried at 60°C for 10-12 hours. A part of the solid sample was dissolved in 5 M HCl and analyzed for Si by colorimetrical method. Re-sults show small amounts of silica in calcite phase (less than 1.5 %), which increase with increasing SiO2 concentration in the solution (fig. 1).