TotalEnergies Gulf of Guinea field is located in water depth of 40 meters, where the waters of the Niger Delta meet those of the Atlantic Ocean. This field has been discovered in 1990 and started production in 1998. Since 2006, calcium carbonate scale is present in the production wells. Because formation water contains low levels of barium (around 10 mg/L) and low levels of sulphate (around 20 mg/L), the scaling risk of barium sulphate as a result of mixing of formation water and seawater has been considered as low. Recently barium sulphate scale deposits have been collected in some wells. This study will determine the cause of both calcium carbonate and barium sulphate scales and establish the required scale mitigation.

The analysis of the evolution with time of the production water chemistry, the determination of the mixing ratios between formation water and seawater in the production water, the scaling risk prediction modelling and field observations have allowed the mechanisms of formation of the different scales to be determined.

This study showed that the production waters contain different percentages of injected seawater varying from 0 to more than 90%. CaCO3 scale at bottomhole is the result of high-pressure drawdown. In order to avoid calcium carbonate scale at bottomhole of production wells, it is recommended to maintain a pressure drawdown below 50 bar. In case a pressure drawdown below 50 bar cannot be maintained a curative treatment with the help of acid washes is recommended. BaSO4 scale is formed as a result of reduction of pressure and temperature from the bottomhole to wellhead, although the scaling risk is low, but the limited predicted mass of scale can accumulate with time. In case barium sulphate scale is observed in the production wells, curative treatment using sulphate scale dissolvers should be applied. CaSO4 scaling risk is predicted for wells where the production water contains less than 10% formation water and more than 90% seawater. For wells showing formation waters containing more than 90% of seawater, scale inhibitor squeeze treatment should be carried out. This study has showed that a general scale mitigation cannot be applied for all the production wells of the field; the treatment needs to be adapted for individual wells.

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