In oil and gas industry, scaling prevention is one of the most important problems. While with more aggressive drilling and exploitation, scale control for the unconventional scale under complex water chemistry becomes more challenging. There are more chances to encountering with high temperature, high pressure, high TDS and some unconventional scale conditions. The modeling of the sulfide scale is notoriously difficult due to the extremely low solubility and complex water chemistry. Thus, the thermodynamic data is rare for sulfide minerals. Metal-sulfide-bisulfide complexes bring a large uncertainty for scale prediction. Another challenge is scale prediction in brine with high TDS, especially with high calcium concentration. Thermodynamic data with common ions Ca2+ and SO42- is needed to improve thermodynamic models. The objective of this paper is to extend our knowledge for these exotic scale solubility predictions with both experimental studies and model validation. Some remaining questions in Pitzer theory framework have been thoroughly reviewed and discussed to improve the scale prediction for iron sulfide and high calcium condition. The newly derived models are able to predict the saturation index (SI) within ±0.3 unit for iron sulfide and ±0.15 units for common sulfate scales, respectively. These developed models have been incorporated into ScaleSoftPitzer for practical use in the oil and gas production.

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