A method to remove a metals impurity from a petroleum feedstock for use in a power generating process, the method comprising the steps of: mixing a heated feedstock with a heated water stream in a mixing device to produce a mixed stream, the heated feedstock comprising the metals impurity, wherein the heated feedstock is heated to a feedstock temperature of 150°C and a feedstock pressure greater than the critical pressure of water, wherein the heated water stream is heated to a water temperature above the critical temperature of water and a water pressure above the critical pressure of water, wherein the mixed stream comprises an asphaltene and resin portion, a hydrocarbon portion, and a supercritical water portion; 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, the reactor effluent comprising a refined petroleum portion and an amount of solid coke, wherein a demetallization reaction is operable to convert the metals impurity to a converted metal, wherein a set of conversion reactions is operable to refine the hydrocarbon portion in the presence of the supercritical water portion to produce the refined petroleum portion; cooling the reactor effluent in a cooling device to produce a cooled stream; feeding the cooled stream to a rejecter, the rejecter configured to separate a sludge fraction from the cooled stream to produce a de-sludged stream, the rejecter having a rejecter temperature, the sludge fraction comprising the asphaltene and resin portion and the converted metals; reducing the pressure of the de-sludged stream in a depressurizing device to produce a depressurized product; separating the depressurized product in a gas-liquid separator to produce a gas phase product and a liquid product; separating the liquid product in an oil-water separator to produce a petroleum product and a water product, the petroleum product having a liquid yield, the petroleum product having a reduced asphaltene content, reduced concentration of metals impurity, and reduced sulfur as compared to the petroleum feedstock. mixing a heated feedstock with a heated water stream in a mixing device to produce a mixed stream, the heated feedstock comprising the metals impurity, wherein the heated feedstock is heated to a feedstock temperature of 150°C and a feedstock pressure greater than the critical pressure of water, wherein the heated water stream is heated to a water temperature above the critical temperature of water and a water pressure above the critical pressure of water, wherein the mixed stream comprises an asphaltene and resin portion, a hydrocarbon portion, and a supercritical water portion; 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, the reactor effluent comprising a refined petroleum portion and an amount of solid coke, wherein a demetallization reaction is operable to convert the metals impurity to a converted metal, wherein a set of conversion reactions is operable to refine the hydrocarbon portion in the presence of the supercritical water portion to produce the refined petroleum portion; cooling the reactor effluent in a cooling device to produce a cooled stream; feeding the cooled stream to a rejecter, the rejecter configured to separate a sludge fraction from the cooled stream to produce a de-sludged stream, the rejecter having a rejecter temperature, the sludge fraction comprising the asphaltene and resin portion and the converted metals; reducing the pressure of the de-sludged stream in a depressurizing device to produce a depressurized product; separating the depressurized product in a gas-liquid separator to produce a gas phase product and a liquid product; separating the liquid product in an oil-water separator to produce a petroleum product and a water product, the petroleum product having a liquid yield, the petroleum product having a reduced asphaltene content, reduced concentration of metals impurity, and reduced sulfur as compared to the petroleum feedstock.