Oil-in-water emulsion drilling fluid

Claim 1

AN OIL-IN-WATER EMULSION WELL DRILLIN FLUID COMPRISING A MINOR AMOUNT OF OIL DISPERSED IN A MAJOR AMOUNT OF WATER SUBSTANTIALLY SATURATED WITH CALCIUM SULFATE AND CONTAINING AT LEAST 1 PERCENT BY WEIGHT OF SODIUM CHLORIDE, SAID EMULSION BEING STABILIZED BY AT LEAST ONE SURFACTANT PRESENT IN AN AMOUNT OF AT LEAST 0.5 POUND PER BARREL AND SELECTED FROM THE GROUP CONSISTING OF: (A) PHENOL ADDUCTED WITH AT LEAST 28 MOLS OF AN OXYALKYLENE PER MOL OF PHENOL; (B) A MONOALKYLPHENOL ADDUCTED WITH 6 TO 50 MOLS OF AN OXYALKYLENE PER MOL OF MONOALKYLPHENOL, THE ALKYL GROUP OF THE LATTER HAVING FROM 4 TO 14 CARBON ATOMS; (C) A POLYALKYLPHENOL ADDUCTED WITH 12 TO 50 MOLS OF AN OXYALKYLENE PER MOL OF POLYALKYPHENOL, THE ALKYL GROUPS OF THE LATTER EACH HAVING AT LEAST ONE CARBON ATOM AND TOGETHER HAVING FROM 3 TO 28 CARBON ATOMS; (D) A MIXTURE OF A MONOALKYLPHENOL AND POLYALKYLPHENOL ADDUCTED WITH 2 TO 4 WEIGHTS OF OXYALKYLENE PER WEIGHT OF SAID MIXTURE, THE ALKYL GROUPS OF THE MONOALKYLPHENOL AND THE POLYALKYLPHENOL EACH HAVING 4 TO 14 CARBON ATOMS; (E) A SULFONATED ALKYLPHENOL-OXYALKYLENE ADDUCT CONTAINING 4 TO 30 MOLS OF OXYALKYLENE PER MOL OF ALKYLPHENOL, THE ALKYL GROUPS OF THE ALKYLPHENOL CONTAINING 6 TO 14 CARBON ATOMS; AND (F) A NAPHTHENIC ACID ADDUCTED WITH 15 TO 50 MOLS OF OXYALKYLENE PER MOL OF NAPHTHENIC ACID, THE LATTER HAVING A MOLECULAR WEIGHT IN THE RANGE OF 300 TO 350; SAID OXYALKYLENE ADDUCTED WITH THE SELECTED SURFACTANT BEING SELECTED FROM THE GROUP CONSISTING OF OXYETHYLENE AND OXYPROPYLENE AND MIXTURES THEREOF.