Rigorous Carbonate and Sulphide Scale Predictions: What Really Matters?

Predicting the formation of pH-dependent scales requires full thermodynamic calculations of the compositions of all hydrocarbon and aqueous phases present. This is necessary in order to determine the distribution and speciation of CO₂ and H₂S between the gas, oil, and water phases in the system. Several commercially available software packages combine PVT calculations with scale predictions, but such packages are more targeted to the modeling of aqueous systems and often have limited hydrocarbon capabilities. Likewise, PVT modeling software focusing on the hydrocarbon phase does not fully model the aqueous phase or can only predict a limited number of scales/complexes. Moreover, within each type of software, there are a large number of different equations of state, activity models, and equilibrium parameters, which may ultimately have a significant impact on the final carbonate and sulfide scale predictions. The questions addressed in this work are as follows: 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 so important? In this paper, the previously published Heriot-Watt scale prediction workflow is applied using different software packages and EOS models to evaluate their impact on the final carbonate and sulfide scale prediction profiles. The results show that, despite the large number of modeling options available, two parameters play a key role in pH-dependent scale predictions: (i) the partition coefficients of CO₂ and H₂S between the gas, oil, and water phases and (ii) the relative mole (and volume) distributions 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.