Modelling carbonate and sulphide scales in sour systems
Abstract
Carbonate and sulphide scales are pH-dependent inorganic deposits that often form in
oil and gas wells posing a significant threat to production efficiency, system integrity,
fluid quality and production safety. Despite the frequent occurrence of these scales, their
prediction and mitigation still present numerous challenges, particularly in the case of
sulphides. This work is aimed at laying out and addressing these challenges to
ultimately improve field scale management programs.
Carbonate and sulphide scales are directly coupled together and differ from other
inorganic scales because they are intimately linked to the in-situ concentration of carbon
dioxide (CO2) and hydrogen sulphide (H2S), which influence the local pH and
availability of reactive species. .
Pressure-volume-temperature (PVT) and scale prediction calculations must be
combined to accurately model three phase component distributions and to determine the
scaling risk profile from reservoir to separator conditions. Although some commercial
software integrates these two sets of calculations, these packages are usually more
targeted to one or the other applications; i.e. either oil/gas PVT or aqueous phase
chemistry.
This work defines a rigorous step-by-step procedure (workflow) which allows us to
obtain carbonate and sulphide scale prediction profiles from commonly available field
data using any choice of PVT and scale prediction software. The capability of
decoupling PVT and scale prediction software offered by this workflow (as opposed to
using an integrated software package) enabled a thorough study of the impact of field
measurements and software choice on the final pH-dependent scale prediction results. The newly developed workflow was also applied to two real field case scenarios to
show how it can be used to answer key operational questions.