Co-precipitation of calcium and magnesium carbonates is possible in real oilfield systems. Due to co-precipitation, the amount of salts formed may differ significantly from the amount calculated using saturation indexes for the various carbonates individually. Magnesium, along with calcium, is in the overwhelming majority of cases in the reservoir and in produced water, and its presence may have an effect on the calcite formation. The aim of the work was to study scale formation processes on the basis of experiments with synthetic and real samples of the produced water of the Piltun-Astokhskoye oilfield to confirm the possibility of formation of mixed calcium and magnesium carbonates. The composition of scale from oilfield equipment and sediments formed in laboratory experiments was determined by X-ray spectral analysis. To study the scale formation synthetic brines of Piltun-Astokhskoye field produced water were prepared with the following composition (ppm): Na+ – 8140, K+ – 170,
Ca2+ – 470, Mg2+ – 140, Cl- – 13000, HCO3- – 1500, SO42- – 730. The precipitation of mixed Ca and Mg carbonates was also studied on samples of real produced water from the PA-A platform and mixtures of produced and sea water. According to X-ray spectral analysis, the composition of scale from oilfield equipment is more complex than the composition of sediments formed from synthetic brines in laboratory experiments. On the diffractograms of sediments formed from synthetic brines in laboratory experiments, in addition to calcium carbonate, there are signals of mixed calcium and magnesium carbonates (magnesian calcite), with different stoichiometric ratios of these metals, with a significant predominance of the former. The formation of dolomite was not observed in the laboratory experiments. It was shown earlier that the amount of magnesium in the scale from oilfield equipment can reach 6.6%wt. in terms of MgCO3. The magnesian calcite found in the scale has several stoichiometric Ca / Mg ratios, so it is difficult to predict its precipitation. Since in real scales magnesium can be present both in the form of magnesian calcite and in the form of dolomite, the magnesium deposition forecast should be made for the dolomite, for which the thermodynamic characteristics are known, and it is possible to calculate the saturation index.
The results obtained indicate that the formation of magnesium carbonate should be taken into consideration when predicting scale precipitation in oilfield systems.
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