A method to remove a metals impurity from a petroleum feedstock for use in a power generating process is provided. The method comprising the steps of mixing a heated feedstock with a heated water stream in a mixing device to produce a mixed stream; introducing the mixed stream to a supercritical water reactor in the absence of externally provided hydrogen and externally provided oxidizing agent to produce a reactor effluent comprising a refined petroleum portion; cooling the reactor effluent to produce a cooled stream; feeding the cooled stream to a rejecter configured to separate a sludge fraction to produce a de-sludged stream; reducing the pressure of the de-sludged stream to produce a depressurized product; separating the depressurized product to produce a gas phase product and a liquid product; separating the liquid product to produce a petroleum product, having a reduced asphaltene content, reduced concentration of metals impurity, and reduced sulfur.