Expansion in the supply of intermittent renewable energy sources on the electricity grid can potentially benefit from implementation of large-scale compressed air energy storage in porous media systems (PM-CAES) such as aquifers and depleted hydrocarbon reservoirs. Despite a large government research program 30 years ago that included a test of air injection and production in an aquifer, and an abundance of literature on CAES mostly relevant to caverns, there remain fundamental questions about the hydrologic and energetic performance of PM-CAES. We have developed rigorous simulation capabilities for PM-CAES that include modeling the coupled wellbore-reservoir system. Through consideration of a prototypical PM-CAES wellbore-reservoir system representing a depleted hydrocarbon reservoir, we have simulated 100 daily cycles of PM-CAES. We find that (1) PM-CAES can store energy but that pervasive pressure gradients in PM-CAES result in spatially variable energy storage density in the reservoir, (2) the wellbore-reservoir storage component of PM-CAES is very efficient, (3) cap-rock and hydrologic seals along with proper sizing of the PM-CAES reservoir prevent excess pressure diffusion from being a problem, and (4) injection and production of air does not significantly mobilize residual liquid water in the reservoir.