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In the context of a horizontal well, displacement is the distance between the wellhead and the top of a vertical line from the bottom hole location to the wellhead elevation at the surface.
In the context of processes, displacement is the set of actions used to flow a fluid or solids out of a well and replace it with another fluid system.
In the context of volume, displacement is the volume of a wellbore occupied by fluid. When the swept volume varies from the calculated displacement, part of the wellbore may not be actively swept.