Kinetics of Silica Dissolution and Hydroxyl Ion Consumption In Alkaline Flooding

In this paper a mathematical model simulating the kinetics of silica dissolution and hydroxyl ion consumption in a typical alkaline flooding environment is developed. The model is based on the premise that dissolution occurs via hydrolysis of active sites through the formation of an intermediate complex, which is in equilibrium with the silicic acid in solution. We consider equilibrium ion exchange at the silica surface and ionization reactions of the silicic species in solution. Both static (batch) and dynamic (core flood) processes are simulated to examine the sensitivity of caustic consumption and silica dissolution to process parameters, and to determine rates of propagation of pH values. The model presented provides a quantitative description of the quartz-alkali interaction in terms of pH, salinity, ion exchange properties, temperature and contact time, which are of significant importance in the design of soluble silicate flooding processes. The latter are shown to minimize or eliminate silica dissolution and caustic consumption, in agreement with recent suggestions. An analysis of the batch processes shows that OH- consumption results from a combination of both ion exchange and silica dissolution. Higher values in the salinity level and in the ion exchange constant lead to a net increase in OH- consumption despite the decrease in the dissolved silica. The opposite effect arises in core flood processes, where the ion exchange is beneficial in arresting the OH- consumption and the decline of pH, in an upstream region. This (plateau) region is characterized by a constant concentration of Na+, the silica-alkali interaction follows a batch-like behavior and pH profiles become stabilized. A region of decline to the initial values precedes this ion exchange plateau. For all practical purposes, the relevant quantities are the pH level at the front of this plateau, determined from the batch analysis, and its velocity of propagation. Thus, the rate of the pH decline increases with the Damkohler number (C), and equilibrium pH values may be reached well before breakthrough for high enough C. The equilibrium values are independent of salinity or the ion exchange constant, although the conditions for establishing the equilibrium before breakthrough and the frontal velocities are functions of these parameters. For instance, the front velocity is found to decrease as the ion exchange constant increases.