Drilling fluid

A drilling fluid and method of using same. The drilling fluid comprises water as a continuous phase, a divalent salt, preferably calcium chloride and a quantity of polyacrylamide copolymer. The said copolymer may comprise partially hydrolysed polyacrylamide and trimethylaminoethyl acrylate with a cationic charge density of about 5% to 10%. The copolymer has a mole weight about 800 000 to 1 000 000. The amounts of calcium chloride and acrylamide copolymer are sufficient to produce a rate of drill penetration approaching that achieved using a synthetic oil based drilling fluid, whilst preventing screen blinding. The acrylamide copolymer includes an ester group comprising an amide group and a cationic group, where the cationic group has a charge in the range +1 to +4. The continuous phase may also include a monovalent salt effective to increase gas hydrate suppression and decrease density when compared to a fluid comprising only the divalent salt.