Filtration control additive for invert emulsion drilling fluids and method of preparation

The method of drilling a well into a subsurface formation in which a water-in-oil emulsion drilling fluid containing a filtrate control additive is circulated into, through, and out of said well, said filtrate contial additive comprising:

• the pyrolyzed amine salts of humic acid obtained by intermixing and heating from about 185.degree. C. to about the decomposition temperature of the resultant pyrolysis products for from between about three hours to about sixteen hours:
• (a) a finely-divided humic acidcontaining material selected from the group consisting of leonardite and lignite;
• and (b) a primary amine having alkyl radicals ranging from ten to twenty carbon atoms, or a primary amine having one alkyl-substituted phenyl radical, the alkyl radical having ten to twenty carbon atoms, or a secondary amine having at least one alkyl radical of eight to twenty carbon atoms, the said pyrolyzed amine salts of humic acid having high thermal stability in such water-in-oil emulsion drilling fluid at a temperature of not less than about 230.degree. C., said method comprising the steps of:
• (1) introducing into said well said water-in-oil emulsion drilling fluid;
• and (2) circulating said drilling fluid into, through, and out of said well.