Drilling, completing or workover of a well, particularly an oil and/or gas well, is carried out using a fluid which includes a continuous phase including an ester of the formula (I): (R.sup.2).sub.p-Ph-(CH.sub.2).sub.m--COO-(AO).sub.n-R.sup.1 where R.sup.1, AO, n, m, Ph, R.sup.2 and p have defined meanings and particularly where the esters are alkyl or alkenyl benzoate esters. These esters are beneficial by having a range of viscosities, especially extending to low viscosities, and attractive toxicological and environmental profiles for use in drilling, completion and/or workover of wells.