Patent ID: 7638466

Claim:
A method of drilling a borehole in a subterranean formation at a temperature in the range of about 40° F. to about 250° F., comprising: providing or using in the drilling an invert emulsion drilling fluid comprising a continuous oil phase and having a plastic viscosity in the range of about 10 cP to about 60 cP and a yield point in the range of about 5 lb/100 ft 2 to about 40 lb/100 ft 2 when measured at about 122° F. (50° C.) and a yield point in the range of about 5 lb/100 ft 2 to about 75 lb/100 ft 2 when measured at about 40° F. (4° C.); and adding to the drilling fluid before or during the drilling at least one non-ionic surfactant thinner, wherein the surfactant is the reaction product of at least one oxide, selected from the group consisting of ethylene oxide, propylene oxide and butylene oxide, with C 10-22 carboxylic acids or C 10-22 carboxylic acid derivatives containing at least one double bond in position 9, 10, 13, or 14 having structural units of the general formula (I): where R 1 is a hydrogen atom, or an OH group, or a group OR 2 , and where R 2 is an alkyl group of about 1 to about 18 carbon atoms, or an alkenyl group of about 2 to about 18 carbon atoms, or a group of the formula (II): and where R 3 is a hydrogen atom, or an alkyl group of about 1 to about 21 carbon atoms, or an alkylene group of about 2 to about 21 carbon atoms; such that the drilling fluid maintains during the drilling a plastic viscosity in the range of about 10 cP to about 60 cP and a yield point in the range of about 5 lb/100 ft 2 to about 40 lb/100 ft 2 when measured at about 122° F. (50° C.) and a yield point in the range of about 5 lb/100 ft 2 to about 75 lb/100 ft 2 when measured at about 40° F. (4° C.).