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Abstract

Reasons for injectivity decline were investigated at a geothermal site located in SE Hungary. Due to low injectivities, production rates have to be reduced and the site faces negative commercial implications.In addition to historical operation data, fluid and rock samples were sampled on-site and further investigated in the laboratory. Analysis and experiments focus on physical, chemical and biological processes and their interaction.Results show fivemain processes being responsible for injection-triggered occlusion of flow pathways: Reservoir thickness, low permeability in the reservoir, precipitation of minerals, microbiological activity and fines migration. The reservoir geometry indicates unfavorable conditions like low sandstone thickness and low permeability. Fines migration is caused by washouts in loosely cemented rocks, from where fine sand or clay particles are transported and injected into lower aquifer layers. Precipitation of minerals is caused by cooling or oxygen exposure. Biofilm is a result of sulfate-reducing bacteria being present at injection depth. Biofilm and physicochemical conditions also cause corrosion in pipelines and wells.In order to fully understand the processes taking place in the injection well, borehole measurements will bedone in 2019. After evaluating the results, a specially tailored stimulation concept willbe applied in the injection well. A combined chemical-mechanical treatment will takeplace at different depth. Borehole measurements and hydraulic tests willbe done again after the stimulation to show the effect of the stimulation. A multiple monitoring and sampling program comesalong with activities onsite.