Blast furnace slag transition fluid

A method of drilling and cementing comprising:

• drilling a borehole with a drill string utilizing a drilling fluid having a density of about 12 lbs/gal, said drilling fluid comprising water, sodium chloride, bentonite, carboxymethyl starch, partially hydrolyzed polyacrylamide and CMC thus producing a used drilling fluid further comprising drill solids;
• removing said drill string and disposing a pipe within said borehole, a portion of said used drilling fluid being disposed in said pipe and in an annulus surrounding said pipe;
• passing a flowing stream of a portion of said used drilling fluid to a mixing zone and introducing sufficient blast furnace slag to give a lead cementitious slurry having a density of about 15.3 lbs/gal;
• introducing said lead cementitious slurry into said pipe;
• passing another portion of said used drilling fluid as a flowing stream to a mixing zone and introducing blast furnace slag to said flowing stream to produce a transition fluid having a density of about 13 lb/gal;
• introducing said transition fluid into said pipe in direct fluid contact with said lead cementitious slurry, said lead cementitious slurry and said transition fluid each having incorporated therein an activator system comprising 2 to 6 lbs/bbl of caustic soda and 2 to 20 lbs/bbl of soda ash;
• thereafter introducing a Portland cement tail cementitious slurry into said pipe in direct fluid contact with said transition fluid, said tail cementitious slurry having a density of about 16.4 lbs/gal;
• and
• displacing said lead cementitious slurry to a position in an annulus surrounding said pipe adjacent an upper portion of said pipe and displacing said tail cementitious slurry into said annulus adjacent a lower portion of said pipe with a displacement fluid selected from seawater and drilling fluid.