Below-grade stormwater storage tunnels have been used in many urban areas as means to provide relief to stormwater collection systems during intense rain events. Such events may also create operational issues to these tunnels, one of which is the uncontrolled release of entrapped air pockets in these systems through water-filled vertical shafts. Air release has been linked to severe geysering episodes, which is an occurrence of concern to designer and tunnel operators. Ongoing numerical investigations using CFD tools have been useful in estimating the severity of geysering occurrences. This work presents results from a CFD model developed in OpenFOAM that solves this type of turbulent, transient multiphase flows using the VOF and considering air compressibility. A hypothetical tunnel geometry is used along with different air pocket volumes, and the effect of air release is studied. Results indicate that uncontrolled air release can create significant vertical velocities in shafts both for air and water phases, which could explain the observed severity of geysering episodes. It is hoped that this investigation will contribute in improving shaft geometry so that the impacts of uncontrolled air pocket release can be reduced.