Methanogenic archaea are able to convert CO2 and H2 into CH4. This metabolic activity is of uttermost importance to understand the feasibility to store green hydrogen in gas reservoir by transforming it into more stable methane. The goal of this work was to study the impact of several reservoir conditions on the rate of conversion: temperature, pressure, gas/water/rock composition. In the best cases, high degrees of conversion were achieved after only few days. However, other sources of methane were noticed during the experiment probably due to the stimulation of other metabolic processes. Moreover, a strong rock/archaea interaction was underlined.