A method for building a plug in a horizontal wellbore using a fluid pill containing a suspended well treatment agent. The well treatment agent contains an ultra lightweight (ULW) deformable core and a viscosifying polymer and crosslinking agent coated onto the core. The fluid pill is pumped into the wellbore at the end of a fracturing treatment. The fluid pill is displaced by a displacement fluid and the fluid pill transforms to a thickened gel. The thickened gel is formed by the in-situ reaction of the viscosifying polymer and crosslinking which become disassociated from the ULW deformable core. The gelled fluid containing the ULW deformable core assists in the bridging of the ULW deformable core and forming the bridge plug.