Iron sulfide, as one of the main products of sour corrosion in oil and gas production systems, has become a focal point for flow assurance research. The formation of iron sulfide can cause many production problems such as the malfunction of downhole devices which can lead to a significant decline in oil production. Once iron sulfide forms in the production system, it is difficult or impossible to remove chemically and costly to remove physically.
Accurate prediction models for iron sulfide formation at reservoir conditions are currently lacking in the industry and are necessary to help control scale and improve flow assurance. Solubility product (Ksp) of iron sulfide is the key parameter to make accurate scale predictions. However, research towards iron sulfide including precipitation, dissolution, inhibition, and removal are notoriously difficult not only due to the complexity of iron sulfide phases and their transitions but also due to the involvement of hydrogen sulfide in the gas phase.
Tomson Technologies has developed new technologies to simulate realistic field downhole conditions for scale research. A reliable flow-through apparatus has been customized to perform mineral solubility studies under xHPHT (up to 1720 bar and 250 °C). In order to simulate the strictly anoxic environment and prevent dissolved ferrous iron from oxidizing, dissolved oxygen in the test solutions has been reduced to far less than 1 ppb. This paper is the first to examine the solubility of iron sulfide under these realistic downhole conditions with temperature up to 250 °C, pressure up to 1720 bar in 1M and 3M ionic strength solutions, under a strictly anoxic environment (<< 1 ppb dissolved oxygen).
Under the HPHT and high salinity conditions studied, iron sulfide tends to form pyrrhotite (Fe1-xS) and troilite (FeSt) phases instead of mackinawite, the metastable phase (FeSm), which is most common at lower temperatures. Phase transition between pyrrhotite and troilite at elevated temperatures was observed during the solubility experiments. Solubility of iron sulfide decreases with increasing temperature and increases with increasing pressure which is consistent to previous reported results (Kharaka, et al., 1988). Experimental details and major findings from this research will be discussed.