Hypothesis: CO2 geological storage (CGS) involves different mechanisms which can store millions of tonnes of CO2 per year in depleted hydrocarbon reservoirs and deep saline aquifers. But their storage capacity is influenced by the presence of different carboxylic compounds in the reservoir. These molecules strongly affect the water wetness of the rock, which has a dramatic impact on storage capacities and containment security. However, precise understanding of how these carboxylic acids influence the rock's CO2-wettability is lacking. Experiments: We thus systematically analysed these relationships as a function of pressure, temperature, storage depth and organic acid concentrations. A particular focus was on identifying organic acid concentration thresholds above which storage efficiency may get influenced significantly. Findings: These thresholds (defined for structural trapping as a water contact angle theta > 90 degrees; and for capillary trapping when primary drainage is unaffected, i.e. 0 > 50) were very low for structural trapping (similar to 10(-3)-10(-7) M organic acid concentration C-organic) and extremely low for capillary trapping (10(-7) M to below 10(-1)0 M C-organic). Since minute organic acid concentrations are always present in deep saline aquifers and certainly in depleted hydrocarbon reservoirs, significantly lower storage capacities and containment security than previously thought can be predicted in carbonate reservoirs, and reservoir-scale models and evaluation schemes need to account for these effects to de-risk CGS projects. (C) 2018 Elsevier Inc. All rights reserved.