Injection of sulphate rich seawater into reservoirs with formation brines rich in calcium, barium or strontium may result in the precipitation of sulphate scales. One technique for managing mineral scales in fields where scale inhibitor squeeze treatments may prove very difficult or ineffective is injection of low sulphate seawater. The CNR operated Tiffany field in the North Sea is one of the oilfields that has been swept with low sulphate seawater for the longest period (> 10 years) and from the start of water injection, and consequently yields useful information on brine mixing and brine-rock interactions during low sulphate seawater sweep. This paper presents the evolution of individual well brine chemistry data, backed up by reservoir simulation and reactive transport flow modelling, which demonstrates the effect that low sulphate seawater injection has had on the produced brine chemistry. The main impact is that scale inhibitor squeezes have only been required for carbonate scales.

The modelling has been extended to predict future individual well brine compositions to identify potential barium and sulphate concentrations to end of field life, and hence identify any remaining potential barium sulphate scaling risk. However, the predictive modelling has also been used to study the sensitivity to timing of a switch from desulphated to full sulphate seawater injection towards the end of field life, to address the question of when will low sulphate seawater no longer be required on the Tiffany field? This study has significant implications for other basins around the world where desulphation projects are about to commence (Brazil and Angola).

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