Abstract: A process is described for replacing at least a portion of the liquid within the annular volume of a casing system within a wellbore with a second liquid. The second liquid is preselected to provide a measure of control of the pressure within the annular volume as the fluid within the volume is being heated.

Abstract: A method and apparatus are described for creating a fluid seal in a subterranean well structure having a fluid seal defect. The method comprises introducing a meltable repair material proximate a structure in a subterranean well which has a fluid seal defect or enhanced seal capacity is required or it is desired to temporarily or permanently hydraulically isolate a portion the well or strengthen the structural integrity of well tubulars or tubular hangers. Exothermic reactant materials are located proximate the meltable repair material. The exothermic reactant material is ignited or an exothermic reaction otherwise initiated which supplies heat to and melts the meltable repair material into a molten mass. The molten mass flows and solidifies across the structure and the fluid seal defect to effect a fluid seal in the subterranean well structure or the structural integrity is enhanced.