Sulphate scale precipitation arising from seawater injection: a prediction study

A thermodynamic prediction model of sulphate scaling has been applied to a wide range of offshore field brines to investigate sulphate scale precipitation due to the mixing of seawater and formation water and the factors influencing this precipitation. The severity of sulphate scaling arising from seawater injection is shown with examples. The interdependence between different sulphate minerals in the simultaneous competitive coprecipitation of BaSO₄, CaSO₄ and SrSO₄ from brines is examined; the precipitation of one sulphate may be affected by the coprecipitation of other sulphate(s) in certain instances and such interactions should be reflected in a prediction model. From scale prediction carried out for brines with diverse ion concentrations, it is found that BaSO₄ precipitation tends to be most severe at low mixing ratios between injected seawater and formation water; SrSO₄ has the highest scaling potential in the middle mixing range between the two waters and CaSO₄ scaling becomes the most problematic only when the mixed brines have a high seawater content. The effects of changes in brine temperature and pressure on sulphate scale formation are also analysed. The implications of these findings are discussed in relation to oilfield scaling problems. © 1994.