Iron Precipitation During Acid Treatments Using HF-Based Acids

Abstract

Abstract Description of the Paper Iron precipitation during spending of four HF-based acids was examined using an experimental procedure similar to that of Taylor et al.1,2 Regular mud acid (HCl:HF weight ratios of 12/3 and 6/1.5) and three retarded HF acids based on AlCl3, boric acid, and a phosphonic acid were examined. Iron (III) was added to these acids at concentrations of 1,000 and 10,000 mg/L and the acid neutralization tests were conducted using sodium carbonate at ambient conditions. Results, Observations, Conclusions For full strength regular mud acid, the onset of iron precipitation occurred at pH 0.6, and complete precipitation of iron occurred at pH 1. These values are lower than those noted with 15 wt% HCl. Values of pH at the onset and complete precipitation of iron were found to be a function of initial HF acid concentration. For the retarded HF acid based on AlCl3, aluminum compounds (sodium hexafluoroaluminate) precipitated at all pH values examined, while iron compounds (sodium hexafluoroferrate) precipitated at pH values that depended on initial iron (III) and HF acid concentrations. For the retarded HF acid based on boric acid, iron compounds precipitated at pH values similar to those noted with 15 wt% HCl. For the retarded HF acid based on phosphonic acid, iron compounds precipitated over a narrow pH range (4.7-5.3) because of the chelating characteristics of the phosphonic acid. Applications Sandstone formations are commonly acidized using an HF-based acid system. Precipitation of iron compounds during acid treatments can cause formation damage by plugging the formation and stabilizing acid/oil sludges. Unlike acid treatments using HCl, previous studies on iron control with HF-based acids are sparse. These new findings reflect the complex chemical interactions that occur with various HF-based acid systems. They will be used to better control iron precipitation during acid stimulations of sandstone reservoirs. Technical Contributions 1. Iron precipitation in the presence of various HF-based acids was identified. 2. Precipitation of aluminum and iron compounds was noted with the retarded HF acid based on AlCl3. 3. Iron precipitation in the presence of boric acid was similar to that noted with 15 wt% HCl, and 4. Minimal iron precipitation was noted over a narrow pH range with the HF acid based on phosphonic acid.