Compositions for controlling wellbore fluid and gas invasion including methods for using the compositions. The compositions are initially flowable slurries preferably comprising about 70 to 30% by weight of magnesium oxychloride and about 70 to 30% by weight of water. The compositions undergo a rapid phase transition to form a substantially solid mass with a near-linear relationship between the time required for the phase transition to occur and the composition temperature at which the phase transition occurs thereby permitting accurate determination of the set time. The method includes use of the composition to control the loss of fluids and gases from the wellbore.