Fluid loss reducer for high temperature high pressure water based-mud application

A method of lubricating a drilling bit during the drilling of a well which comprises circulating a water-based drilling mud in the vicinity of the drilling bit wherein the water-based drilling mud is comprised of

• (1) from 50 to 90% of the aqueous phase,
• (2) from 0.01 to 0.5% of pH control agent,
• (3) from 0.1 to 5% of viscosifiers,
• (4) from 0.01 to 30% of salts,
• (5) from 0.1 to 3% of emulsifiers,
• (6) from 4 to 60% of weighting agents,
• (7) from 0 to 15% of clays, and
• (8) from 0.1 to 20% of an oil soluble polymer in form of a gel as a fluid loss reducer, said percentages being based on the weight of the water-based drilling mud, wherein the oil soluble polymer in the form of a gel is prepared by a process comprising the steps of dissolving at least one polymer in a hydrocarbon oil to form a clear solution or a gel, adding an emulsifier to the solution or the gel, and keeping the mixture under conditions of agitation until a clear creamy mixture is obtained;
• wherein said oil soluble polymer fluid loss control agent contains up to 50 percent water based on the weight of the control agent;
• wherein the emulsifier is present at a level which is within the range of 3 percent to 30 percent based upon the weight of the control agent;
• and wherein the polymer is prepared from monomers selected from the group consisting of styrene, substituted styrene, alkyl acrylate, substituted alkyl acrylate, alkyl methacrylate, substituted alkyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, N-alkylacrylamide, N-alkylmethacrylamide, isoprene, butadiene, ethylene, vinyl acetate, and vinyl esters of versatic acids containing from 9 to 19 carbon atoms.