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.