Editing Caesium (section)

===Petroleum exploration===<!-- linked from "Formate" article -->
The largest present-day use of nonradioactive caesium is in [[formate|caesium formate]] [[drilling fluid]]s for the [[extractive oil industry]].<ref name="USGS"/> Aqueous solutions of caesium formate (HCOO<sup>−</sup>Cs<sup>+</sup>)—made by reacting caesium hydroxide with [[formic acid]]—were developed in the mid-1990s for use as oil well drilling and [[completion (oil and gas wells)|completion fluids]]. The function of 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. Completion fluids assist the emplacement of control hardware after drilling but prior to production by maintaining the pressure.<ref name="USGS"/>

The high density of the caesium formate brine (up to 2.3&nbsp;g/cm<sup>3</sup>, or 19.2&nbsp;pounds per gallon),<ref name="Down">{{cite conference |conference=IADC/SPE Drilling Conference |date=February 2006 |location=Miami, Florida, USASociety of Petroleum Engineers |first1=J. D. |last1=Downs |first2=M. |last2=Blaszczynski |first3=J. |last3=Turner |first4=M. |last4=Harris |doi=10.2118/99068-MS |url=http://www.spe.org/elibinfo/eLibrary_Papers/spe/2006/06DC/SPE-99068-MS/SPE-99068-MS.htm |archive-url=https://web.archive.org/web/20071012122901/http://spe.org/elibinfo/eLibrary_Papers/spe/2006/06DC/SPE-99068-MS/SPE-99068-MS.htm |archive-date=12 October 2007 |title=Drilling and Completing Difficult HP/HT Wells With the Aid of Cesium Formate Brines-A Performance Review}}</ref> coupled with the relatively benign nature of most caesium compounds, reduces the requirement for toxic high-density suspended solids in the drilling fluid—a significant technological, engineering and environmental advantage. Unlike the components of many other heavy liquids, caesium formate is relatively environment-friendly.<ref name="Down"/> Caesium formate brine can be blended with potassium and sodium formates to decrease the density of the fluids to that of water (1.0&nbsp;g/cm<sup>3</sup>, or 8.3&nbsp;pounds per gallon). Furthermore, it is biodegradable and may be recycled, which is important in view of its high cost (about $4,000&nbsp;per [[barrel (volume)#Oil barrel|barrel]] in 2001).<ref>{{cite journal |last=Flatern |first=Rick |date=2001 |title=Keeping cool in the HPHT environment |journal=Offshore Engineer |issue=February |pages=33–37}}</ref> Alkali formates are safe to handle and do not damage the producing formation or downhole metals as corrosive alternative, high-density brines (such as [[zinc bromide]] {{Chem|ZnBr|2}} solutions) sometimes do; they also require less cleanup and reduce disposal costs.<ref name="USGS"/>