Method of using gelled fluids with defined specific gravity

A method of preventing ingress of an unwanted subsurface fluid into a subsurface apparatus which comprises introducing into the subsurface apparatus a barrier fluid wherein the barrier fluid, at subsurface conditions, has a specific gravity which is within 0.01 of the specific gravity of the unwanted subsurface fluid, the barrier fluid having been prepared by first preparing a base fluid having a specific gravity less than the unwanted subsurface fluid and then introducing to the base fluid a gellant comprising a viscosifying polymer and crosslinking agent to form a gelled barrier fluid, wherein the subsurface apparatus is selected from the group consisting of spools, manifolds, distribution units, pipes, pipelines, control systems, control modules, completion test trees and blowout preventors.