<p>Carbon Capture and Storage (CCS) is critical for climate change mitigation, enabling secure storage of carbon dioxide (CO₂) in subsurface geological formations. Coupled hydraulic-geomechanical models are essential for predicting reservoir response to CO₂ injection, but require calibration using field observations that quantify mechanical perturbations. This study introduces a novel history matching workflow using downhole Distributed Fibre Optic Sensing (DFOS) to monitor reservoir expansion from injection onset, enabling real-time model adjustments and surpassing limitations of surface-based methods. The approach was validated through experimental injection of 16 tonnes of CO₂ into a shallow aquifer at CO2CRC's Otway International Test Centre, southeastern Australia. Sensitivity analysis demonstrated that Young's modulus is the dominant parameter controlling strain magnitude, Poisson's ratio governs spatial deformation distribution, and Biot's coefficient influences vertical strain patterns. These insights enabled targeted parameter adjustments during history matching: Young's modulus was increased by a factor of 6 to match observed strain magnitude, while Biot's coefficient was reduced to 0.7 to capture vertical strain distribution. Synthetic strain profiles generated from the calibrated model successfully matched DFOS data collected during injection, validating the approach and providing a robust basis for calibrating elastic rock properties. This method enables early-stage geomechanical model calibration using in situ strain measurements, offering significant advantages over surface uplift monitoring including immediate data availability and applicability to all geological settings. The calibrated model enhances confidence in predicting surface deformation and stress redistribution, which are essential for fault stability assessment and ensuring safe, permanent CO₂ storage. This approach strengthens CCS as a reliable, scalable climate solution.</p>