Surface casing vent flows and gas migration can occur at some oil or gas wells due to imperfections within well seals. These imperfections may result from drilling or cementing conditions, or from methods practiced at older wells not constructed to present day standards.
Surface Casing Vent Flow, or SCVF, involves the movement of gas between the surface casing and the next string of casing upward out of the surface casing vent assembly. In rare instances, a liquid could also follow the same flow path.
Gas Migration, or GM, is the flow of gas outside the surface casing of a well. When GM occurs, there is potential for migration to enter the groundwater and this may cause changes to groundwater chemistry in the vicinity of the well.