A Novel Approach to Predict the Co-Precipitation of BaSO4 and SrSO4

Co-precipitation of strontium sulfate with barium sulfate is often observed in oil fields. Some explanations have been given, but the mechanism is not fully understood. A numerical simulation approach gives a detailed explanation of its occurrence. This results in an improved tool for operators in dealing with prevention and treatment procedures.

The computer model simulates reactive flow through porous media since injection of an incompatible fluid can induce numerous chemical reactions in a reservoir. The model uses the Gibbs free energy minimization technique to calculate chemical reactions that include precipitation from dissolved ionic species and dissolution of formation rocks. The model applies finite difference technique to solve mass transfer of the dissolved ionic species by convective fluid flow and diffusion.

A case study of a North Sea seawater flood revealed the mechanism of strontium sulfate formation and its relationship to barium sulfate precipitation. Once seawater contacted barium (in the carbonate form) in the reservoir, barium sulfate dropped out immediately. This reaction also created excess carbonate ion (CO₃⁻⁻). Furthermore, although strontium carbonate is more stable than strontium sulfate, redistribution of the available CO3⁻ forced strontium carbonate to dissolve and strontium sulfate to form. The results also suggest that the formation of the sulfate scales is not only controlled by their equilibrium constants but also dependent upon the concentrations of all other ionic species in the immediate vicinity.

The simulation technique leads to a better prediction of the quantity and composition of sulfate scale formation. It also provides in-depth understanding of the interactions of the reservoir rocks with injected fluids. The application can also be extended to quantify the formation of other types of scales which occur during production operations.