and sulfuric acid and having dissolved therein from about 0 5 to about 5 0 percent by weight of an oxyalkylated acrylamido alkanesulfonic acid polymer and from about 2 5 to about 10 percent by weight of a fluorine containing salt selected from the group consisting of a fluoride salts such as ammonium fluoride and lithium as well as b acid fluorides as exemplified by ammonium acid fluoride nh 4 hf 2 and lithium acid fluoride lihf 2 the fluorine containing salt when added to the solution of the mineral acid reacts to form hydrogen fluoride and the corresponding ammonium or lithium chloride or sulfate it has been found that a highly beneficial effect is achieved when the acidic aqueous polymer solution employed in the secondary recovery process of this invention contains in addition to the mineral acid hydrofluoric acid and an oxyalkylated acrylamido alkanesulfonic acid polymer the ammonium or lithium ions derived from the fluorine containing salt in order to promote effective acidization of the formation with the hydrofluoric acid at a considerable distance from the bore hole the introduction of the acidic aqueous polymer solution into the formation can be preceded by the injection into the formation via the injection well of a slug of an aqueous solution containing from about 2 5 to about 10 weight percent of a fluorine containing salt selected from the group consisting of a fluorides such as ammonium and lithium fluoride and b the corresponding acid fluorides since these fluoride salts are not especially reactive with the sandstone.