Geologic storage of CO2 can be a viable technology for reducing atmospheric emissions of greenhouse gases only if it can be demonstrated that leakage from proposed storage reservoirs and associated hazards are small or can be mitigated. Risk assessment must evaluate potential leakage scenarios and develop a rational, mechanistic understanding of CO2 behavior during leakage. Flow of CO2 may be subject to positive feedbacks that could amplify leakage risks and hazards, placing a premium on identifying and avoiding adverse conditions and mechanisms. A scenario that is unfavorable in terms of leakage behavior is formation of a secondary CO2 accumulation at shallow depth. This paper develops a detailed numerical simulation model to investigate CO2 discharge from a secondary accumulation, and evaluates the role of different thermodynamic and hydrogeologic conditions. Our simulations demonstrate self-enhancing as well as self-limiting feedbacks. Condensation of gaseous CO2, 3-phase flow of aqueous phase -- liquid CO2 -- gaseous CO2, and cooling from Joule-Thomson expansion and boiling of liquid CO2 are found to play important roles in the behavior of a CO2 leakage system. We find no evidence that a subsurface accumulation of CO2 at ambient temperatures could give rise to a high-energy discharge, a so-called "pneumatic eruption."