Calcium sulfate has been one of the major scales which cause many significant and serious operating problems in producing oil/gas wells and in water injectors. Impermeable hard scale deposits of calcium sulfate can severely impair the formation permeability or lead to down-hole equipment failure. Typically, preventative treatments, such as use of scale inhibitors, are the best economical methods for calcium sulfate mitigation. However, application of a cost-effective treatment is needed, in case of emergency, when calcium sulfate precipitation occurs.

A comprehensive lab investigation was conducted in order to assess the effectiveness of several remedial methods for calcium sulfate removal. This work shows the amount of calcium sulfate dissolved after its exposure to different reactive fluids at 25 and 50°C. In addition, it discusses the effect of different factors on the efficiency of each remedial treatment. These factors include: pH, temperature, reactive fluid concentration and presence of magnesium/iron ions.

Based on obtained results, several new findings were identified. The presence of gypsum (CaSO4.2H2O) has negative impact on the performance of mud acid treatments. After initial dissolution of gypsum in live mud acid, dissolved calcium will precipitate as both calcium fluoride and calcium sulfate in spent HCl/HF solutions. Similarly, gypsum has higher solubility limit in live HCl acid compared to its spent solutions. This resulted in re-precipitation of calcium sulfate in spent HCl. This solubility trend of gypsum in acidic solutions could result in severe formation damage. Live acids can initially dissolve any precipitated calcium sulfate solids in wellbore area and then it will re-precipitate in the formation rocks as the acid spends.

Gypsum has higher solubility limit in EDTA solutions, compared to acidic solutions. No re-precipitation of calcium sulfate occurred in these solutions due to the fact that calcium ions exist as complex ions and not free to interact with other ions. The dissolving power of EDTA was found to be a function of the solution pH value. Higher dissolving power of EDTA for gypsum was observed at high pH values. The presence of both magnesium and iron (III) ions had negative effect on gypsum dissolution in EDTA fluid. Compared to magnesium, iron (III) ion resulted in significant decrease in gypsum solubility in EDTA. Dissolved magnesium ions in EDTA solutions could re-precipitate as magnesium sulfate when gypsum is dissolved. This re-precipitation is more at low pH EDTA solutions.

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