Abstract Because of autogenous shrinkage of cement and the subsequent reduction in radial contact stress between the borehole wall and the cement, a micro-annulus may develop that creates a pathway by which natural gas may rise outside the casing. This gas may become evident as sustained casing pressure (SCP) in the US, or in Canada as surface casing vent flow (SCVF) or as gas migration (GM), i.e., that which occurs outside the casing strings. GM may emit at ground surface as a greenhouse gas or penetrate shallow aquifers causing groundwater contamination. Several observers have noted that gas emissions recorded as surface casing vent releases display a pulsing or periodic nature, which is of consequence if such emissions are to be monitored. We hypothesize that these pulses are due to the formation of Taylor bubbles, i.e., gas slugs, which are created by the coalescence of small bubbles. Their displacement pressure is sufficient to overcome the capillary entry pressure posed by shallow aquifers and thus cause groundwater contamination.