Sandstone HCl/HF acid matrix stimulation is a complex process involving numerous minerals and reactions. Previous attempts to model these reactions employed a "lumping" procedure to classify sandstone minerals as either fast reacting or slow reacting depending on their reaction rates with hydrofluoric acid.

Bryant (1991)1  showed that the two-mineral dissolution model can not accurately account for the effect of flow velocity on HF consumption rates. Consequently, this model cannot reliably extrapolate laboratory data to field conditions which involve radial flow geometry. Bryant presented an improved model to include the secondary dissolution/precipitation reaction of fluosilicic acid (H2SiF6) with clays and amorphous silica. The improved model was previously tested only against limited published data from three laboratory acid core flood tests. To further validate the model, additional laboratory acid core flood tests were conducted using different sandstone cores and with different acid injection rates and concentrations. Test results show that the improved model not only predicts the HF effluent histories reasonably well but also predicts the dissolved Si and Al concentration profiles in the effluent acid. This suggests that the improved model provides a more reliable basis for extrapolating laboratory data and for simulating matrix acid treatments. The improved model also helps to understand the dissolution/precipitation reactions occurring during sandstone acidization processes.

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