Abstract: An aqueous displacement fluid composition is used as a spacer fluid composition to change one type of drilling mud for another type of drilling mud or in cementing operations downhole, to clean downhole hardware in wellbores. The method comprises introducing into a wellbore an aqueous displacement fluid composition, such as a spacer fluid, to displace downhole wellbore fluids, having an improved rate of emulsification and a high capacity for emulsifying hydrocarbons, and aqueous fluid compositions. The aqueous displacement fluid composition introduced into wellbores contain mixtures of:a) water;b) a polyoxyalkylene monool surfactant represented by the formula: ##STR1## wherein R.sub.1 is a hydrocarbyl group, R.sub.2 is a C.sub.1 -C.sub.3 alkyl group, and the weight average of w+x is at least about 5; andc) a polyoxyalkylene monool surfactant represented by the formula: ##STR2## wherein R.sub.1 is a hydrocarbyl group, R.sub.2 is a C.sub.1 -C.sub.

Abstract: Well drilling, workover and completion fluids containing various water soluble condensation products of phenolic materials, formaldehyde and sulfite salts. Such condensation products may be further modified by including during the condensation reaction one or more of urea, melamine, salicylic acid, benzoic acid, phthalic acid, adipic acid, succinic acid, glutaric acid, maleic acid or the anhydrides of said acids. The drilling, workover or completion fluids may also contain clay-dispersants.