SayPro Application of the methodologies previously indicated for different sites, assuring an exhaustive coverage of the different European sites’ peculiarities. The methodologies should be applied in different laboratories, considering different site samples. Microbial activity should also be observed during different testing periods, possibly up to six months, and in different batch/reactor operation volumes. Different levels of experiment together with modelling will be necessary to reproduce underground storage conditions and enable upscaling to the reservoir scale.