Wellbore fluid

A method of drilling a well comprising,

• attaching a cutting bit to a length of drill pipe,
• rotating said cutting bit,
• removing cuttings from around said bit with a drilling fluid wherein the drilling fluid is a biodegradable wellbore fluid which is an emulsion of a dispersed hydrophilic liquid in a continuous oil phase comprising at least 50% by weight of the oil phase of an n-alk-1-ene having from 12 to 20 carbon atoms or a linear polyunsaturated olefin having one of the double bonds in the 1-position and having from 12 to 22 carbon atoms, or a mixture thereof, wherein the oil phase contains less than 10% by weight of aromatic hydrocarbons, cycloparaffins, cycloalkenes, isoalkanes, no more than 35% by weight of branched chain alk-1-enes, and no more than 20% by volume of polyalphaolefins.

The use, as the continuous oil phase of a wellbore fluid, of an oil comprising an n-alk-1-ene having from 12 to 20 carbon atoms or a linear polyunsaturated olefin having one of the double bonds in the 1-position and having from 12 to 22 carbon atoms, or a mixture thereof.

A wellbore fluid comprising an oily phase characterized in that the wellbore fluid further comprises sources of one or more elements selected from the group consisting of nitrogen, phosphorous and potassium, added as nutrients for the purpose of enhancing biodegradation of said oil phase of the wellbore fluid.