As a clean energy carrier, hydrogen (H2) is considered as an indispensable part of energy transition roadmap. To meet the increasing energy demand, extremely large storage capacity is required. Previous studies focus on the underground H2 storage in conventional depleted gas reservoirs, salt caverns and saline aquifers. With the development of shale reservoirs, more and more depleted shale gas reservoirs may be good candidates for H2 storage. In this work, we analyze the potential of H2 storage in depleted gas reservoirs using Monte Carlo simulations. The competitive adsorption of methane-hydrogen (C1-H2) system under nanoscale condition is investigated, including the effect of pore size, temperature, pressure, boundary material, and fluid composition. Our results show that C1 is preferentially adsorbed in C1-H2 system. C1 forms the adsorption layer near the boundary surface while H2 molecules are freely distributed in the pore. The fluid distribution indicates that H2 can be easily produced during H2 recovery process, which contributes to H2 storage in depleted shale gas reservoirs. In addition, the effect of water (H2O) on C1-H2 competitive adsorption is analyzed. The strong interactions between H2O and boundary atoms force C1 molecules away from the adsorbed region. If we consider C1 as cushion gas, 50% of C1 is enough for H2 storage in depleted shale gas reservoirs. This work provides foundation for H2 storage in depleted shale gas reservoirs at a molecular level.