. . . "If it is desired to reduce the oxygen concentration of the injected gas to a predetermined value of at least 75 vol. %, air or an inert gas such as nitrogen or carbon dioxide or mixtures thereof is transported via line 42 through open valve 44, line 40 and open valve 38 into tubing 28 where it is comingled with the oxygen from cryogenic pump 20." . . . . . .