Caesium formate is used in oil drilling because of its high density . Aqueous solutions of caesium formate ( HCOO-Cs + ) â '' made by reacting caesium hydroxide with formic acidâ '' were developed in the mid-1990s for use as oil well drilling and completion fluids . The function of caesium formate as a drilling fluid is to lubricate drill bits , to bring rock cuttings to the surface , and to maintain pressure on the formation during drilling of the well ; as completion fluid ( which refers to the emplacement of control hardware after drilling but prior to production ) is to maintain the pressure .