Mathematical Modeling of Secondary Precipitation From Sandstone Acidizing

Abstract

Summary Field and laboratory experience clearly demonstrates that, besides primary dissolution, precipitation from both secondary and tertiary reactions with aluminosilicates adversely affects matrix acid treatment success. In the secondary reaction, fluosilicic acid from the primary dissolution will further react with aluminosilicates to form hydrated silica on the matrix surface. In the tertiary reaction, aluminum fluoride also extracts aluminum from aluminosilicates to form silica gel by lowering the fluoride/aluminum (F/Al) ratio in the solution. In the presence of a high concentration of metal ions, metal fluorides and fluosilicates are also likely to precipitate. An optimum acidizing design package can be derived for a specific formation mineralogy and damage mechanism if this complex reactive system can be modeled properly. A geochemical model for acidizing has been developed for this purpose. The numerical simulator is a reactive flow model that solves the reactive system with both reaction kinetics and chemical equilibrium calculations. All possible reactions among all species in the system are solved simultaneously. As acid spends, the computer simulation shows the progress of aluminosilicate dissolution and the formation of different precipitates as the change of dissolved species along the reaction path occurs. The evolution of the damage skin can be evaluated as simulation continues. On the basis of simulation, different pumping schedules with different types of acids can be quickly evaluated to optimize treatment volume and minimize precipitation of hydrated silica and other precipitates. To validate the model, a series of laboratory coreflood tests has been fully analyzed. The results indicate that the formation of silica gel is significant, on the basis of hydrofluoric acid (HF) effluent analysis. The same set of kinetic data derived from the laboratory coreflood data adequately predicts and explains a sandstone acidizing field case.