Rigorous carbonate and sulphide scale predictions: What really matters?

Predicting the formation of pH- dependent scales such as carbonates and sulphides requires a full calculation of all hydrocarbon and aqueous phases present to determine the distribution and speciation of CO2 and H2S in the system. Several commercially available software packages combine PVT calculations with scale predictions, but such packages are more targeted to aqueous systems and have limited hydrocarbon capabilities. Likewise, PVT modelling software focusing on the hydrocarbon phase does not always fully model the aqueous phase or can only predict a limited number of scales/complexes. Moreover, within each software we can select a large number of different Equations of State (EOS), activity models, equilibrium parameters etc., which may ultimately impact the final carbonate and sulphide scale prediction profile. The questions we try to answer in this work are: How important is the software selection and which parameters really affect the final scale prediction profiles? In what scenarios do these values matter and when are they not important? In previous publications we laid out a clear rigorous procedure (workflow) for the prediction of carbonate and sulphide scales which can be applied using any commercial PVT and scale prediction software. Here we apply this general workflow using different software and EOS models to evaluate their impact on the final carbonate and sulphide scale prediction profiles for some specific carbonate/sulphide field scaling scenarios. The results show that despite the large number of modelling options available, there are two parameters that play a key role in pH-dependant scale predictions: partition coefficients of CO2 and H2S between gas, oil and water and the relative mole (and volume) distribution between each phase at selected temperature and pressure. The final scale prediction results can be accurate only when these values are accurate, irrespective of how they are obtained. This work shows the impact of choosing different software and equations on carbonate and sulphide scale predictions, not just as a "black box" software comparison exercise but with a clear connection between the aqueous and hydrocarbon phase thermodynamics, the scaling system and the final results..