A separate heat source, such an a skin effect heater, is added to a subterranean heat exchanger and oxygen combustion device for upgrading viscous crude oil feeds. An insulated wire is integrated with combustion oxygen supply piping to form the skin effect heater operating in conjunction with the heat exchanger. The separately controlled heater output allows a reduction of the heat from the combustion device reduced quantities of combustion products. The improvement down-sizes or eliminates excess combustion product removal facilitates and also preheats the oxygen supply. The skin-effect heater and heat exchanger combination adds heat gradually and increases feed residence time at the desired upgrading temperature and pressure conditions. Alternative embodiments can be used to treat heavy crudes within a producing well.