Previous studies have shown that acidization of sandstone with 12% HCl/3% HF acid systems can lead to secondary precipitation of many different compounds. Formation of these precipitate often have been found to be pH dependent. If the pH of spent HF formulations can be kept low, the likelihood of secondary precipitation may be reduced. This paper presents the results of a study showing the effect of acidizing a Berea Sandstone (76 × 5 cm), containing 9% carbonate, with a buffered HF acid system (12% H3PO4/3% HF). These results are then compared to results with 12% HCl/3%HF.

Treatment with one pore volume of the buffered acid system was found to increase the native permeability of the Berea sandstone core to a depth of 60 cm. No evidence of damaging precipitation or unconsolidation was found in any section of the core. The pH of the reacted acid was found to be low (2.0 – 2.5).

Fourteen pore volumes of HCl/HF increased the permeability only to depth of 25 cm and caused unconsolidation of the first 10 cm and 0.5 pore volume of HCl/HF caused formation of damaging precipitates.

The absence of secondary precipitation with 12% H3PO4/3% HF is most likely due to two factors: the buffering ability of the phosphoric acid kept the pH low and hence prevented precipitation of silicon or aluminium complexes, and the unique surface reactions between calcite and 12% H3PO4/3% HF prevent large amounts of calcium ions from being brought into solution from reaction with the carbonate content of the sandstone. These two factors were confirmed by reacting 12% H3PO4/3% HF with synthetic sandstone like mineral blends.

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