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CO₂ geosequestration is an important contributor to United Nations Sustainable Development Goal 13, i.e., Climate Action, which states a global Net-Zero CO₂ emissions by 2050. A potential impact of CO₂ geosequestration in depleted oil and gas reservoirs is the variations in induced pressure across the caprocks, which can lead to significant local variations in CO₂ saturation. A detailed understanding of the relationship between the pressure gradient across the caprock and local CO₂ concentration is of utmost importance for assessing the potential of CO₂ geosequestration. Achieving this through experimental techniques is extremely difficult, and thus, we employ a coupled Computational Fluid Dynamics (CFD) and Finite Element Method (FEM) based solver to mimic sub-critical CO₂ injection in Opalinus Clay under various pressure gradients across the sample. The geomechanical and multiphase flow modelling utilising Darcy Law helps evaluate local variations in CO₂ concentration in Opalinus Clay. Well-validated numerical results indicate favourable sub-critical CO₂ geosequestration under a positive pressure gradient across Opalinus Clay. In the absence of a positive pressure gradient, a peak CO₂ concentration of 5% has been recorded, which increases substantially (above 90%) as the pressure gradient across the sample increases.