Dissolved organic compounds in geothermal fluids used for energy production: a review

Dissolved organic matter (DOM) can be found in a variety of deep subsurface environments such as sedimentary basins, oil fields and mines. However, the origin, composition and fate of DOM within deep geothermal reservoirs used for energy production is relatively unknown. With well depths reaching a few kilometers, these sites give access to investigate deep subsurface environments. Natural DOM as well as artificial DOM (e.g., from chemical scaling inhibitors) might serve as nutrients for microorganisms or affect chemical properties of the fluids by complexation. Its composition might reveal hydraulic connections to organic-rich strata, giving insights to the fluid flow within the reservoir. This review presents an overview of a total of 143 fluid samples from 22 geothermal sites (mainly central Europe), from the literature and compiling data to address the importance of DOM in geothermal fluids and how it might affect geothermal operation. The environmental conditions of the sites included varied greatly. Temperatures range from 34 to \(200\,^{\circ }\hbox {C}\), depths from 850 to 5000 m, chloride content from 0.1 to \(160\,{\hbox {g}\,\hbox {L}^{-1}}\), and dissolved organic carbon (DOC) concentrations from 0.1 to \(30.1\,{\hbox {g}\,\hbox {L}^{-1}}\). The DOC concentrations were found to be generally lower in the fluids with temperatures below \(80\,{}^{\circ }\hbox {C}\). DOC concentrations were higher in fluids with temperatures above \(80\,{}^{\circ }\hbox {C}\) and showed a decrease towards \(200\,{}^{\circ }\hbox {C}\). Microbial degradation might be the main driver for low DOC concentrations in the lower temperature range (below \(80\,{}^{\circ }\hbox {C}\)), while thermal degradation likely accounts for the decline in DOC in the temperature region between \(80\,{}^{\circ }\hbox {C}\) and \(200\,{}^{\circ }\hbox {C}\). This review shows that DOM can be found in a variety of geothermal reservoirs and that it could be an additional essential tool to better understand fluid chemistry and reservoir conditions, and to optimize geothermal operation.