Rippled scales in a geothermal facility in the Bavarian Molasse Basin: a key to understand the calcite scaling process

Mineral precipitates, scales, are wide-spread in geothermal facilities and hamper technical and economic efficiency. Scales reported from earlier studies in the Bavarian Molasse Basin consisted mostly of calcite, appeared rather homogeneous, grew flat and perpendicular to the surface, and showed little or no internal macrostructure apart from small gradients in crystal size and chemical composition. In this study, we investigated extraordinary calcite scales collected from the inlet chamber of a heat exchanger. These scales showed current ripples and a sequence of coarse and dense layers. By an extensive investigation of the ripples, the hydrochemistry, and by the application of existing empirical equations, the formation of the ripple scales could be reconstructed: Due to degassing of CO2 in the geothermal pump calcite scales precipitated on the pipes of the ground-level facilities. During maintenance works, these scales were partially dissolved, and remobilized as individual mobile grains. These particles subsequently settled again, and solidified as ripple scales in the horizontal pipes at surface level. For this ripple forming process, flow had to be small, because otherwise the ripples would have been altered or washed away. After deposition of the ripple structure, a dense new calcite layer precipitated on top of the ripples, and stabilized it due to transitory supersaturation. This sequence of ripples and a dense calcite cover layer is repeated two times. Finally, during start-up of regular plant operation, the whole composite scale was remobilized and got washed into the inlet chamber of a heat exchanger. The investigation presented here indicates that this type of calcite scale only forms in geothermal facilities in the Bavarian Molasse Basin if certain flow and hydrochemical conditions are present during the maintenance procedure.