Barium Sulphate (BaSO4) scale deposition is a serious problem encountered in oilfield operations. The precipitation of BaSO4 scale has been studied mainly for fields under water flooding. On the other hand, polymer flooding is a mature Enhanced Oil Recovery (EOR) method that has been applied successfully in many fields. This study investigates the effect of polymer flooding and salinity variations on oil Recovery Factor (RF) and on brine mixing and BaSO4 precipitation in porous media and the scale risk in producers.

Reservoir simulation has been used to carry out the study. We have performed simulations using a reactive transport simulator and heterogeneous 2D areal and vertical models and a field scale 3D model. Data from literature have been utilized to define parameters that control polymer viscosity, polymer adsorption and barium (Ba2+) and sulphate (SO42-) concentrations. We have also studied the effect of injecting a low salinity water as the make-up brine for the polymer slug to see its impact on oil recovery and BaSO4 precipitation.

The study shows that the injection of a viscous polymer slug reduces the mixing between injected and formation brines and so reduces the amount of BaSO4 deposition in the reservoir compared to a normal water flood. This reduction is not large and its effect on reservoir permeability is marginal. However, importantly the viscous polymer delays the breakthrough of injected water and hence the precipitation of BaSO4 at the wellbore. Including the effect of polymer adsorption makes the polymer front move slower than the SO42- front, and this accelerates BaSO4 precipitation at the wellbore and increases the total precipitation compared to the case without adsorption. A low salinity polymer slug, which contains low SO42- concentration, improves polymer viscosity, which enhances oil recovery, and reduces and delays the amount of BaSO4 deposition in the formation and in the producers.

The behaviour of brine mixing is different under polymer flooding compared to normal water flooding. This work shows for the first time that this impacts the amount of BaSO4 scale that precipitates in the reservoir, and thus the timing and amount of potential scale deposition in the wellbore.

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