A class of salts made from neutralization of formic acid with a metal hydroxide or oxide. Three alkali-metal formates are used in drilling, drill-in and completion fluids, (1) sodium formate, HCOO^-Na⁺, (2) potassium formate, HCOO^-K⁺ and (3) cesium formate, HCOO^-Cs⁺. Clear solutions of each can reach densities of 1. 32, 1. 58 and 2. 4 g/cm³, respectively. They are near neutral pH and meet HSE standards. Most formates can be mixed together over broad ranges of concentration or temperature without solubility or crystallization problems.