Anchor plug for open hole test tools

A method for drilling a borehole and providing a cement anchor plug in a soft or weak formation existing in the borehole below a zone to be tested, comprising

• preparing a universal fluid by mixing a drilling fluid and a cementitious component selected from blast furnace slag and a proton acceptor metal compound;
• drilling the borehole with the universal fluid and laying down a settable filter cake on the walls of the borehole;
• diluting at least part of the universal fluid with a water source selected from water, brine, seawater, water base drilling fluid, and water emulsion drilling fluid;
• forming a cementitious slurry from said at least part of the universal fluid by adding additional cementitious component and/or
• an activator wherein, when said cementitious component is said blast furnace slag, said activator is 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## 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;
• circulating the cementitious slurry to the soft or weak formation in the borehole below a zone to be tested, the cementitious slurry being in direct contact during and after placement with the universal fluid in the borehole;
• and
• allowing the cementitious slurry to solidify in-situ adjacent to the settable filter cake covering the soft or weak formation, to form an anchor plug in the wellbore.