A system to selectively remove metal compounds and sulfur from a petroleum feedstock includes a mixing zone configured to mix a pre-heated water stream and a pre-heated petroleum feedstock to form a mixed stream, a first supercritical water reactor configured to allow conversion reactions to occur to produce an upgraded stream, a carbon dispersal zone configured to mix carbon with a make-up water stream to produce a carbon dispersed water stream, such that the carbon is dispersed in the carbon dispersed water stream, a make-up mixing zone configured to combine the upgraded stream and the carbon dispersed water stream to produce a diluted carbon dispersed stream, the carbon is operable to trap metals present in the upgraded stream, and a second supercritical water reactor configured to allow conversion reactions to occur to produce a carbon dispersed effluent stream.