Analysis of Chemical and Biological Processes in Geothermal Systems – a Case Study

Abstract

The physical, chemical and biological processes taking place in geothermal systems due to changes of pressure and temperature can reduce the system’s efficiency. In case of injection into sandstone reservoirs, these processes can cause serious technological problems and can lead to the non-functioning of the system. The familiarization with, and, handling of, chemical and biological processes characteristic for a given geothermal system may be necessary for the sustainable operation of the injection phase. Our research was carried out in a porous sandstone reservoir in Hungary. We used two different sandstone samples from drillings near the reinjection location, and carried out water-rock relationship analyses. For the analysis of the bonding of grains and the eventual suspension, we used sediment volume calculation. The core samples were ground and the prepared samples were analyzed by means of XRF and XRD spectroscopy. We examined original and oxygen-treated water samples. During the analysis of the salt effect, potassium chloride solutions with different concentration were added to the water samples. Each treatment was carried out three times; the photos of the sediments were evaluated afterwards. We determined that besides the interaction of the mineral matter the presence of biological components is of high importance during water injection. There is a large number of species of bacteria in the extracted thermal water whose metabolism does not raise a problem during production as they are present in the form of spores. However, as the water becomes colder at later stages of the geothermal system, conditions become ideal for mesophilic microorganisms. Around 90% of mesophilic bacteria present in the system studied were able to form biofilms on different spots of the injection system, especially on the filters, reducing the efficiency of the whole operation. We collected samples from the filter material of the injection wells and carried out metagenomic analyses of the bacteria. This way, we were able to collect the DNA of the bacteria from the analyzed sample, and define the most efficient water treatment processes accordingly.