A Field Case Study Validating the Use of a Kinetic Scaling Risk Model to Predict the Metastability of Barium Sulfate

Abstract A case study is presented of a field in which multiple wells are being produced (some for over 12 years) without downhole scale control and without scale formation despite the waters being supersaturated in barium sulfate. When seawater breakthrough occurs and sulfate concentration increases above a threshold, scaling is observed. The field data and observations are consistent with, and hence provide validation of, a new kinetic scaling risk model that incorporates temperature and water composition to predict the metastability of barium sulfate. Modelling of a range of the production wells was carried out at both wellhead and bottomhole conditions, and pre- to post-seawater breakthrough sulfate concentrations, using the developed kinetic model. The predictions of when the waters were expected to remain metastable were compared with field observations. The model has been developed based on empirical laboratory testing with an extensive range of conditions and so similar scaling tests using brine compositions specific to this field were conducted using a jet impingement method, to provide further validation of the correlation between experimental findings and field observations. The results of thermodynamic modelling indicated that, at both pre- and post-seawater breakthrough concentrations of sulfate, the produced waters are supersaturated in barium sulfate. However, the kinetic model can take into account the effect of temperature and brine composition (in particular the Sr/Ba ion ratio) to indicate what level of supersaturation is required for scale deposition to be observed in practice – the critical saturation ratio value below which the system remains metastable. It was found that the wells without scaling being observed correlated well with the kinetic predictions and those wells where scale had formed following seawater breakthrough and increasing sulfate concentrations were shown to be above the critical saturation ratio values. The results of the laboratory tests were consistent with the field observations and the model developed. Use of the kinetic model supported development of an improved scale management strategy where risk levels for wells were defined based on predictions using the sulfate ion concentration to determine whether they would remain metastable and could be safely produced without scale inhibitor and at what point they would become higher risk and require inhibitor treatment. The findings from this case study highlight the importance of understanding not just the thermodynamics of inorganic scale prediction but also the kinetics. Use of a kinetic model to define the conditions under which fields can be safely produced without application of scale inhibitors whilst being supersaturated in a particular mineral results in improved guidelines as to when mitigation activities such as scale squeeze or downhole chemical injection are required, and so allow for optimization of operational costs/reduced environmental impact.