| Title | Geothermal Carbonate Scaling: Forensic Studies Applying High-Resolution Geochemical Methods |
|---|---|
| Authors | Boch, R; Szanyi, J; Leis, A; Mindszenty, A; Deák, J; Kluge, T; Hippler, D; Demény, A; Dietzel, M |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | geothermal scaling, carbonate precipitates, environmental archive, crystal growth, Pannonian Basin, steel corrosion, heat exchanger clogging, microbial activity, geothermal energy |
| Abstract | Exploitation of thermal water frequently entails the precipitation of various minerals (scaling) due to elevated salinity and gas contents in deep aquifers. Progressive clogging of wells, pumps, pipes, valves and heat exchangers can cause major problems in geothermal heat and electric power production. In this context scale deposits constitute an environmental archive capturing installation-specific fluid-solid interaction over time, i.e. natural vs. man-made variations in fluid chemistry, temperature, pressure, outgassing (e.g. CO₂), flow rate and microbial activity. These factors control distinct crystal growth, variable porosity/density and lamination, accessory mineral and organic contents, chemical and isotopic compositions of the scales – and therefore the rates of inner-diameter reduction (scaling progress) and mechanical/chemical resistivity. In an approach of process understanding intuitively termed “Scaling Forensics” we apply state-of-the-art mineralogical and geochemical techniques combined with hydrogeochemical modeling and facility-specific data to reconstruct individual – favorable vs. unfavorable – environmental and technical conditions of scale formation. We collected carbonate scales and thermal waters from geothermal facilities distributed over the Hungarian Pannonian Basin covering a broad range of geogenic and man-made production conditions. First results support distinct nucleation and crystal growth mechanisms influencing carbonate scale appearance and consistency. Crystallization on surfaces (substrates) typically results in more compact and resistant fabrics, whereas spontaneous particulate crystallization within an aqueous solution (in suspension) promotes increased porosity and therefore scaling progress. Steel corrosion layers can provide a highly attractive substrate for nucleation and ongoing crystal growth due to high specific surface areas and abundant defect sites of typical corrosion products. Moreover, corrosion layers of reduced mechanical resistivity (low crystallinity, hardness) may lead to preferred shearing-off along these layers. A prominent example of interplay between steel corrosion and carbonate scaling progress is discussed regarding the formation of cemented scale fragments (shards) damaging and blocking heat exchangers. We also conducted a first series of laboratory experiments to investigate some of the findings. Purely inorganic can further be distinguished from microbially influenced carbonate precipitation. Microbial tissues might interact with basal corrosion layers and could be the explanation for prominent feather-like calcite crystals. The filamentous to dendritic structures probably foster the entrapment and accumulation of particles from suspension or provide nucleation sites for crystal growth. Next to an applied research focus we also discuss benefits from carbonate scales for fundamental geochemical method improvement. This involves the evaluation and calibration of novel and established isotope geothermometers using scales of diverse but well characterized precipitation conditions. |