An accurate scale prediction is crucial for the design of field production systems and treatment strategy. Existing models do not predict accurately ZnS and PbS scaling risks.
In this paper, we share the findings of using ScaleSoftPitzer (SSP2014) model, developed by Rice University, for this purpose. SSP has been extensively evaluated against both the relevant density and solubility data related to the carbonates, sulfates, and halide scales. However, it has not been closely evaluated against the sulphidic scales, due to lack of high quality experimental observations.
The evaluation of solubility as a function of multiple parameters is presented. Simulations results are compared to literature and field data. The method used to generate comparison is based on the total metal concentration at equilibrium. Temperature, pH, and composition are found to be strongly interplaying effects since aqueous phase metal complexes control the solubility of sulphide minerals. Chloride complexes are believed to be mainly responsible for metals aqueous speciation transport in oil and gas systems, however, hydrosulfide/sulfide complexes may play a role when pH increases or when brine salinity is low. The equilibrium measurements and extrapolated calculations are in reasonable agreement with field observations assuming sulphide minerals are in equilibrium with the produced brine at reservoir conditions. Research showed that colloidal dispersion phenomena and other specific interactions might play a significant role in metal sulphide systems; calcium seems to impact speciation. Two approaches are proposed to improve the model: (i) a short term solution using existing literature data to implement the principal complexes (ii) a more rigorous approach involving additional laboratory study acquisition of metal complexes.