The present disclosure relates to systems and methods for wellbore pressure control with separated fluid columns. One embodiment is a composition comprising: a base fluid, a hectorite clay, a secondary clay comprising at least one clay selected from the group consisting of an attapulgite clay, a sepiolite clay, a palygorskite clay, and any combination thereof, and a plurality of fibers. Another embodiment is a method comprising: introducing a first fluid into a wellbore; introducing a barrier pill into the wellbore, wherein the barrier pill comprises: a base fluid, a hectorite clay, and a plurality of fibers; and introducing a second fluid into the wellbore. Another embodiment is a method comprising: introducing a first fluid into a wellbore; introducing a barrier pill into the wellbore, wherein the barrier pill comprises: a base fluid, a hectorite clay, and a secondary clay comprising at least one clay selected from the group consisting of an attapulgite clay, a sepiolite clay, a palygorskite clay, and any combination thereof; and introducing a second fluid into the wellbore.