the present invention relates to a method and to a fluid for drilling or workover in a well running through a porous permeable formation , wherein a water - based well fluid circulates in said well . in the method , at most 3 % by weight of a composition obtained from fat , or an oil , and an alcohol , is added to the fluid in addition to the conventional constituents for such a fluid , the chain lengths of the acid and alcohol parts being chosen such that the ester thus obtained has sufficient dispersion in water , compatibility with said constituents , does not form an emulsion with the reservoir oil and adsorbs sufficiently on the porous formation .