<p>At the Mont Terri Rock Laboratory in the Opalinus Clay, the in-situ Bitumen-Nitrate-Clay interaction experiment investigates the influence of oxidising components that could leach from radioactive waste on the reducing capacity of claystones at a near-neutral pH. Future tests are planned to examine the additional impact of an increased pH, mimicking the degradation of concrete-based engineered barriers used in radioactive disposal systems. However, substantial clogging phenomena due to mineral precipitation must be avoided in the experimental setup to ensure the continuous circulation of fluids in water lines and their contact in the borehole with the surrounding clay through filter screens. In this work, geochemical modelling was performed for prognostic purposes to assess the potential precipitation of minerals and formation of solid solutions at an increased pH, while also considering cation exchange with the surrounding clay. The modelling results indicated precipitation of negligible amounts of calcite (at relatively low <i>p</i>CO₂) and cementitious phases, which should be insufficient to cause significant clogging. However, substantial brucite precipitation was predicted above pH ~ 10.4. Cation exchange enhanced brucite precipitation, as magnesium from the clay was exchanged primarily with potassium from alkaline solutions. Such brucite precipitation (or calcite at higher <i>p</i>CO₂) could clog small-diameter installations near the clay since it has fast kinetics and only little oversaturation is needed for crystal nucleation. Furthermore, the geochemical model was designed to be easily adapted to investigate other clogging-related aspects in radioactive waste repositories and serves as a basis for upcoming laboratory experiments to verify clogging phenomena.</p>