Use of Naturally-occurring Tracers to Monitor Two-phase Conditions in the Coso Egs Project

A dramatic decrease in the ratio of chloride to boron was observed in the liquid discharge of a well proposed for EGS development in the Coso geothermal field. The decrease appears to be related to the transformation of some feed zones in the well from liquid-dominated to vapor-dominated. High concentrations of boron are transported to the wellbore in the steam, where it fractionates to the liquid phase flowing in from liquid-dominated feed zones. The high-boron steam is created when the reservoir liquid in some of the feed zones boils with a steam fraction greater than 90%. Combination of boron from both phases into the liquid phase results in the Cl;B ratio dropping from 40 to as low as 20.