Abstract: A downhole burner that economically generates hot inert gas and injects this gas into a subterranean formation. The burner has a nipple attached to a first tubing string that carries an oxidizer to the burner. The first tubing string also contains a coil tubing string that carries fuel to a movable nozzle inside the nipple. A small orifice in the nozzle increases the velocity of the fuel, and the nozzle extends out through a small orifice in a burner shroud that increases the velocity of the oxidizer flowing around the nozzle. The high velocity of the fuel and oxidizer keeps the flame front below the shroud and nozzle. A second tubing string supplies coolant gas around the burner shroud. Injection of an oxygen containing coolant gas provides a method for enhancing in situ combustion reactions through superheating the oxygen containing mix of coolant and exhaust gas.