Patent ID: 7514389

Claim:
A water based drilling fluid for use in drilling a subterranean well through a subterranean formation that swells in the presence of water, the drilling fluid comprising: an aqueous based continuous phase; a weight material selected from the group consisting of barite, hematite, iron oxide, calcium carbonate, magnesium carbonate, inorganic salts, calcium chloride, calcium bromide, magnesium chloride, zinc halides and combinations thereof; an optional shale hydration inhibition agent; and a shale encapsulator having the formula: in which x and y have a value so that the molecular weight of the cation is in the range of about 10,000 to about 200,000 AMU; A is selected from C 1 -C 6 ; alkyl, C 2 -C 6 ether or C 2 -C 6 amide; R, R′ and R″ are independently selectable C 1 -C 3 alkyl; and B − is a charge balancing anion; and wherein the shale hydration inhibition agent and the shale encapsulator are present in sufficient concentrations to reduce the swelling of the subterranean formation in the presence of water.