Cement plug for well abandonment

A method for preventing zonal communication or migration of fluids, in a well to be abandoned, by placement of a plug, comprising:

• preparing a cementitious slurry comprising:
• (a) a cementitious component selected from blast furnace slag or a proton acceptor metal compound;
• and
• (b) an activator wherein, when said cementitious component is said blast furnace slag, said activator is optionally an alkaline agent, and when said cementitious component is said metal compound, said activator is a phosphorus acid or one of a polymer component of the formula:
• ##STR13## wherein A is ##STR14## or a mixture of ##STR15## and wherein R is H or a 1-10 carbon atom alkyl radical and the ratio of m to n is within the range of 0:1 to 100:1;
• (c) a water source selected from water, brine, seawater, water base drilling fluid, and water emulsion drilling fluid;
• drilling the borehole with a universal fluid containing a minor quantity of the cementitious slurry, and depositing a settable filter cake on the wall of the borehole;
• sequentially displacing the slurry in the borehole to various depths, beginning with the lowest depth, and following with successively higher depths, said various depths being chosen to prevent zonal communication or migration of fluids in the well;
• and
• allowing the filter cake to set and the slurry to solidify and form plugs at the various selected depths.