Iron sulphide scale management in high H₂S and CO₂ carbonate reservoirs

Combined sulphide/carbonate scale formation in wells producing from high CO₂ and H₂S reservoirs represents a serious threat to production efficiency and system integrity. Understanding both (i) the main source of iron forming the iron sulphide scale, and (ii) the phase partitioning and effect of the acid gases (CO₂ and H₂S), are important in devising and implementing the correct sulphide scale control program. In this paper, a PVT software package was used to take production data and model water condensation/evaporation, calculate gas compositional changes and CO₂/H₂S partitioning between the liquid phases. This enabled reservoir fluid compositions to be predicted using an in-house scale prediction software, with particular focus on the stable concentration of iron in the aqueous phase. A sensitivity study was then performed to assess the parameters which impact on iron solubility within the reservoir. With the reservoir fluid compositions established, changes along the production stream (over a given range of temperatures and pressures) were determined and used to predict scale formation at those conditions. A modelling workflow was developed and tested against field data for the prediction of sulphide/carbonate scale deposition in gas wells producing from carbonate reservoirs. The workflow was then applied to a number of Middle East gas wells which produce 2-4% CO₂ and 2-6% H₂S. By understanding changes in flow rate, gas partitioning and fluid composition along the production stream it was possible to map the potential scale deposition through the system and to compare these results with scale deposits observed in the field. It was calculated that the pH in the wellbore is low and mainly determined by the partial pressure of carbon dioxide. The pH in the reservoir is higher due to the presence of calcium carbonate. Thus, it was possible to determine that dissolved iron is highly unlikely to be present in the formation fluids leading to the conclusion that the source of iron for iron sulphide deposition to occur must be the result of sour corrosion. In addition, the resulting likely profiles of iron sulphide deposition were predicted.