An improved method for building a plug in a horizontal wellbore using a fluid pill pumped into the wellbore at the end of a fracturing treatment. The fluid pill includes a high concentration of an ultra lightweight proppant, such as a neutrally buoyant proppant or an ultra lightweight proppant mixture. The fluid pill is pumped down the wellbore until it almost reaches fractures within a zone of interest. The pumping is then ceased or reduced, allowing the fractures to partially close. The ultra lightweight proppant remains suspended within the fluid pill while stationary. The pumping is then resumed at a very slow rate or as a short pump burst, thus causing the proppant in the fluid pill to bridge off until a bridge plug is formed.