Process for using biodegradable drilling muds

Process for using biodegradable drilling muds, comprising introducing a biodegradable surface sludge which (1) is stable up to about 100° C., (2) has filtrate-reducing power which is adjustable from 2 to 13 cm3 and (3) has an electrical stability of greater than or equal to 1000 volts, into a well at the start of drilling, and

• replacing the surface sludge when the temperature of the terrain passed through reaches 100° C. with a bottom sludge which is stable at temperatures above 200° C., such that the volume of the surface sludge represents from 60 to 80% of the total volume of sludge used,
• wherein the surface sludge is a reverse emulsion having an oil/water ratio by volume ranging from 95/5 to 70/30, and an oil phase containing (1) a majority of unrefined plant or animal natural oils comprising 90% by weight of triglycerides, and (2) a minority of a diluent consisting of esters of these natural oils, and
• the bottom sludge is chosen from the group consisting of mineral and organic oils which are stable at temperatures of 250° C. and above, and non-toxic muds whose cuttings can be returned and stored onshore on account of their low volume.