A well barrier can include an inner mandrel, a flow passage, and a releasably secured plug. The plug blocks fluid flow through the flow passage, and includes a shoulder that prevents the plug from displacing completely through the inner mandrel. A method of treating a subterranean well can include treating a deeper zone, setting a well barrier in the well between the deeper zone and a shallower zone, then treating the shallower zone, and then applying a pressure differential from the deeper to the shallower zone, thereby displacing a plug out of the well barrier. A well treatment system can include a well barrier with a plug releasably secured to an inner mandrel. The plug is released by application of a pressure differential in a longitudinal direction, and fluid communication is unblocked by application of a pressure differential in an opposite longitudinal direction.