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Geological storage of CO₂ requires multiphase flow models coupled with key hydrogeologic features to accurately predict the long‐term consequences. The prediction uncertainty during geological CO₂ storage requires a computationally efficient and practically useful framework. This paper presents a comparative study between ensemble‐based filtering algorithms (En‐As) and calibration‐constrained null‐space Monte Carlo (NSMC) methods. For the En‐As, we use the ensemble Kalman filter (EnKF), ensemble smoother (ES), ES with multiple data assimilation (ES‐MDA), and EnKF and ES with the pilot point method. For the NSMC calibrated models with various parameterization, schemes are tested and single and multiple NSMC (M‐NSMC) methods are used. A synthetic case with two layers was developed to mimic an actual CO₂ injection pilot test where one injection and two observation wells are located …