Scale-fragment formation impairing geothermal energy production: interacting H2S corrosion and CaCO3 crystal growth

Abstract

Background: Mineral precipitates (scaling) from deep saline thermal waters often constitute a major problem during geothermal energy production. The occurrence of scale-fragments accumulating and clogging pipes, filters, and heat exchangers is of particular concern regarding an efficient energy extraction. Methods: Carbonate scale-fragments from different sections of two geothermal power plants were collected and studied in a high-resolution scaling forensic approach comprising of microstructural characterization, elemental mapping, and stable carbon and oxygen isotope transects. The solid-phase analyses were evaluated in the context of natural environmental and technical (man-made) production conditions. Results and discussion: Our results indicate an interaction of metal sulfide mineral layers mainly from H2S corrosion of the steel pipes and CaCO3 nucleation and crystal growth. A conceptual model of scale-fragment development addresses the relevance of two key interfaces: 1) the corrosion layer between the steel substrate and calcite scale and 2) the scale surface versus thermal fluid flow. The corrosion products constitute an attractive crystallization substrate of brittle and mechanically weak consistency. A rough carbonate scale surface tends to induce (micro) turbulences and increased flow resistance (frictional forces). These factors promote partial exfoliation, scale-fragment mobilization, and rapid clogging. This investigation highlights the potential of detailed petrographic and geochemical analyses of mineral precipitates for evaluating favorable versus unfavorable processes in geotechnical environmental settings.