Abstract: The disclosure is directed to a composition for controlling wellbore fluid and gas invasion including methods for using said composition. The composition of the invention is a flowable slurry preferably comprising about 70 to 30% by weight of magnesium oxychloride and about 70 to 30% by weight of water. The composition is a flowable slurry at composition temperatures below about 60° C. and sets to form a substantially solid mass upon reaching a composition temperature of about 60-68° C. The phase transition is rapid, thereby limiting or avoiding formation of gel states which contribute to incomplete wellbore seals. The rapid set of the slurry further avoids invasive contamination of earthen formations surrounding the wellbore. The composition has a near-linear relationship between the time required for the phase transition to occur and the composition temperature at which the phase transition occurs.