Scale-fragment accumulations blocking geothermal energy extraction - Interacting steel sulfide corrosion and calcite crystallization

Various minerals precipitating from deep saline thermal
water (scaling) often constitute a major obstacle during
geothermal energy production. The formation of scalefragments
in pipes, which accumulate and block pumps,
filters and heat exchangers, is of particular concern in power
plants hindering an efficient energy extraction. Carbonate
scale-fragments from different sections of two geothermal
facilities in S-Germany were studied in a high-resolution
scaling forensic approach including microstructural
characterization, elemental mapping and stable isotope
profiles. The obtained (isotope)geochemical data were
evaluated in the context of favorable vs. unfavorable natural
environmental and operational (man-made) production
conditions. Our results support a close relationship of metal
sulfide mineral layers forming from H2S corrosion of the steel
pipes and CaCO3 nucleation and rapid crystal growth. A
conceptual model of scale-fragment evolution is developed
considering two key interfaces: i) the corrosion layer between
steel and calcite scale; ii) the scale surface vs. thermal fluid
flow. The corrosion products with brittle and mechanically
weak consistency constitute an attractive substrate for
carbonate crystallization. The rough carbonate scale surface
promotes the occurrence of (micro)turbulences and higher
flow resistance (frictional forces). These effects can lead to
partial exfoliation, fragment mobilization, accumulation and
severe blocking.