The Rotliegend sandstone reservoir of Voelkersen (Northwest Germany) is a low permeability (kabs 2–4 mD), high pressure high temperature gas field (650 bar, 160°C). Its formation water is of high salinity (250 g/l) and characterised by its very high calcium content (≥ 40 g/l). The reservoir section of a partly depleted acceleration well was drilled using saturated water based sodium/potassium formate brine. For corrosion protection the pH value of the brine was adjusted to pH 10.5 using a bicarbonate/carbonate buffer. After gas production of 70 million m3(Vn) the production collapsed as a result of an unexpected build-up of calcium carbonate scale in the well bore area and the tubing. Two mechanisms have been identified as the cause of this scale formation. First the interaction of the caustic filtrate of the formate brine with the calcium rich formation water leading to a direct calcium carbonate precipitation. Second the enrichment of the formation water with bicarbonate by interaction with the caustic mud filtrate. This has resulted in calcium carbonate precipitation caused by the rise of the pH value of the water. The latter is resulting from decarbonisation caused by the pressure relief in the well bore area as a result of production. After subsequent mechanical and chemical scale removal, the gas production was even higher than at the initial state of production. However, having additionally produced 90 million m3(Vn) the production dropped again. This breakdown is attributable to an increase of bicarbonate in the formation water by a slow process of decomposition of formate left in the formation. This has resulted in subsequent scale formation following the decarbonisation process described above. To minimise the scaling potential of the formate brine while maintaining sufficient corrosion protection, careful adjustment of the pH value of the formate brine while drilling the reservoir section was implemented for successive wells. For already damaged wells several potential treatments have been identified and introduced, such as scale inhibition treatment integrated in hydraulic fracturing.

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