Abstract

A model is presented for predicting the scaling tendencies of barium, strontium and calcium sulphates resulting from the mixing of injected and formation waters, as well as the effects of temperature and pressure. The model is able to predict competitive simultaneous co-precipitation of BaSO4, CaSO4 and SrSO4 of which sulphate is the common ion, reflecting closely the precipitation of more than one sulphate mineral. The sulphate scaling tendency may be predicted for a single fluid or solutions of mixed waters at all ratios by calculating the supersaturations and amounts of precipitation of the sulphates at temperatures and pressures covering surface and reservoir conditions. For mixed scale occurrence of more than one sulphate, an iterative process is used.

The model is based on the concept of thermodynamic equilibria between the scaling ions in the solution and their solid precipitates, which requires the knowledge of the sulphates solubilities at various brine compositions, temperatures and pressures representative of oilfield operation conditions. In the model, the solubilities of BaSO4, CaSO4 and SrSO4 are regenerated from the published solubility data by Pitzer's ion interaction approach, a widely accepted method because of its sound theoretical basis and accurate representation of the properties of aqueous electrolyte solutions. The generated solubilities by this method agree with the published data within the error of experimental measurement.

The paper presents the scale prediction results and comments on its application in both predicting formation damage scaling tendency and also within reservoir modelling studies where scaling within the near production well zone is a concern.

Introduction

In North Sea offshore operations where sea water injection is a common development practice, barium sulphate, calcium sulphate and strontium sulphate scale deposition is a concern. Barium sulphate and related scale occurrence is considered a serious potential problem of causing formation damage within the near production well zone. Sulphate scales may occur due to changes of temperature and/or pressure while a water flows from one location to the other, but the major cause of sulphate scaling comes from the chemical incompatibility between the injected sea water which is sulphate rich and the formation water which originally contains high concentrations of barium, calcium and/or strontium ions. An accurate, convenient and fast model capable of predicting such scaling problems may be helpful in planning a waterflood scheme and also an effective scale prevention technique may be selected in following the application of a prediction model which predicts the scaling tendency, scale type and potential scale severity. A number of prediction models have been reported by Vetter et al(1–6). This model is considered an improvement on the previous models.

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