This study investigated the three possible main causes of injectivity reduction in geothermal wells (suspended solids, scaling, biological activity). That has been done by obtaining water samples at three operating geothermal doublet locations in the Netherlands. This paper contains new geothermal data and the interpretation of them. At the moment, injectivity reduction is the biggest and least understood issue in geothermal projects in the Netherlands. Investigating this problem is valuable for current and future geothermal injectors and essential for the upscaling of geothermal energy in the Netherlands.The three locations differ from each other in several aspects (reservoir, surface facilities, chemicals in use, injectivity reduction), which yields a good comparison. Water samples were taken at different moments in time, on different points in the geothermal systems and for different purposes: TSS, chemical composition and ATP concentrations. The data is obtained in the laboratory and used for scale analysis (using the Stream Analyzer software from OLI Studio®) and microbial kill tests to determine the influences on growth. Scientific quality and data reliability are guaranteed by multiple pre-experiments (e.g. to verify sterility of sample bottles) and extra measurements (e.g. duplo samples, oxygen concentrations). It has been found that TSS is reduced through the systems by filters (from 35-55 mg/L in production water to <8-16 mg/L in injection water). Also, scales can potentially form (calcite before- and barite after the heat exchanger). Furthermore, biological activity in the water is initially low, but can grow over time to high levels (>500.000 microbial equivalents). Microbial growth is site-specific and correlatable to injectivity reduction. When there is microbial growth, corrosion inhibitor (CI) enhances this growth. Scale formation is possible, but has a limited likelyhood to cause significant injectivity problems. Furthermore, there is no reason for TSS to be problematic, but it can also not be excluded as possible source for injectivity reduction. Mitigation of these issues can be done by using more injection filters in a geothermal system (for further TSS reduction). Scaling inhibitor can be used preventive and a CI without nutrients (in the solvent) should be used, or a biocide should be injected simultaneously with the CI. This report is interesting for geothermal operators, researchers and anybody else interested in geothermal operations in general or more specifically in injectivity reduction.