A process for upgrading a petroleum-based composition comprising: combining a supercritical water stream with a pressurized, heated petroleum-based composition in a mixing device to create a combined feed stream, introducing the combined feed stream into an upgrading reactor system comprising one or more downflow supercritical upgrading reactors operating at a temperature greater than a critical temperature of water and a pressure greater than a critical pressure of water, where the downflow supercritical upgrading reactor comprise at least one catalyst layer, where the at least one catalyst layer is a heterogeneous porous metal containing catalyst having a void volume ratio, where the heterogeneous porous metal containing catalyst includes one or more components selected from the group consisting of transition metals and precious metals; where the transition metals comprise one or more metal containing components comprising metals selected from the group consisting of Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, and combinations thereof; where the precious metals comprise one or more metal containing components comprising metals selected from the group consisting of Au, Ag, Pt, Ro, Rh, Os, and combinations thereof; and where the downflow supercritical upgrading reactor includes at one or more purging fluid inlet disposed on one or more side locations of the downflow supercritical upgrading reactor proximate the catalyst layer; passing the combined feed stream through the at least one catalyst layer, where light hydrocarbons in the combined feed stream at least partially flow through the at least one catalyst layer while heavy hydrocarbons in the combined feed stream are at least partially sifted in the voids of the at least one catalyst layer; at least partially converting the sifted heavy hydrocarbons to light hydrocarbons in the catalyst layer in the presence of the supercritical water; injecting purging fluid through the purging fluid inlets to contact the first catalyst layer, a second catalyst layer, when present, or both to reduce plugging; and passing upgraded product comprising light hydrocarbons and the converted light hydrocarbons out of the downflow supercritical upgrading reactor. combining a supercritical water stream with a pressurized, heated petroleum-based composition in a mixing device to create a combined feed stream, introducing the combined feed stream into an upgrading reactor system comprising one or more downflow supercritical upgrading reactors operating at a temperature greater than a critical temperature of water and a pressure greater than a critical pressure of water, where the downflow supercritical upgrading reactor comprise at least one catalyst layer, where the at least one catalyst layer is a heterogeneous porous metal containing catalyst having a void volume ratio, where the heterogeneous porous metal containing catalyst includes one or more components selected from the group consisting of transition metals and precious metals; where the transition metals comprise one or more metal containing components comprising metals selected from the group consisting of Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, and combinations thereof; where the precious metals comprise one or more metal containing components comprising metals selected from the group consisting of Au, Ag, Pt, Ro, Rh, Os, and combinations thereof; and where the downflow supercritical upgrading reactor includes at one or more purging fluid inlet disposed on one or more side locations of the downflow supercritical upgrading reactor proximate the catalyst layer; where the downflow supercritical upgrading reactor comprise at least one catalyst layer, where the at least one catalyst layer is a heterogeneous porous metal containing catalyst having a void volume ratio, where the heterogeneous porous metal containing catalyst includes one or more components selected from the group consisting of transition metals and precious metals; where the transition metals comprise one or more metal containing components comprising metals selected from the group consisting of Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, and combinations thereof; where the precious metals comprise one or more metal containing components comprising metals selected from the group consisting of Au, Ag, Pt, Ro, Rh, Os, and combinations thereof; and where the downflow supercritical upgrading reactor includes at one or more purging fluid inlet disposed on one or more side locations of the downflow supercritical upgrading reactor proximate the catalyst layer; passing the combined feed stream through the at least one catalyst layer, where light hydrocarbons in the combined feed stream at least partially flow through the at least one catalyst layer while heavy hydrocarbons in the combined feed stream are at least partially sifted in the voids of the at least one catalyst layer; at least partially converting the sifted heavy hydrocarbons to light hydrocarbons in the catalyst layer in the presence of the supercritical water; injecting purging fluid through the purging fluid inlets to contact the first catalyst layer, a second catalyst layer, when present, or both to reduce plugging; and passing upgraded product comprising light hydrocarbons and the converted light hydrocarbons out of the downflow supercritical upgrading reactor.