TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDescription 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 AlCl 3 , 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 AlCl 3 , 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 AlCl 3 . 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.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper discusses a new technique that was used to evaluate matrix acid treatment conducted on a cased water disposal well in a carbonate reservoir. The technique relies on calculation amount of corrosion products and minerals dissolved by the acid. It was used to calculate the amount of iron dissolved by the acid and the live acid present in the flowback following the pickle treatment. Chloride ion concentration was used to determine the degree of mixing between the spent acid and formation brine. The degree of mixing was then used to calculate the volume of the produced spent acid and the amount of minerals (calcite and dolomite) that were dissolved by the acid. The chemical efficiency of the acid, defined as the actual amount of calcite dissolved by the acid/the theoretical amount of calcite that the should dissolve, was finally determined.
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