GAS RELEASE MECHANISMS FROM ENERGY WELLBORES AND SUBSEQUENT GROUNDWATER CONTAMINATION

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 of an energy well. 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. Both geoscientists and energy-industry personnel have noted that gas emissions recorded as surface casing vent releases frequently display a pulsing or periodic nature, which is of consequence if such emissions are to be monitored at ground surface. 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 and their periodic buoyant ascent behind the casing. Their displacement pressure is sufficient to overcome the capillary entry pressure posed by shallow aquifers and thus cause groundwater contamination. The transient nature of these pulses requires monitoring programs that reflect the non-steady state release of gas to the atmosphere (SCVF) and the shallow subsurface (GM).