A method for hydrocarbon conversion in a supercritical reactor of an intimately mixed stream produced by a sequential mixer upstream of the supercritical reactor, the method comprising the steps of: introducing a heated hydrocarbon stream to a hydrocarbon inlet of the sequential mixer, the hydrocarbon inlet parallel to a traversing axis of the sequential mixer, wherein the heated hydrocarbon stream has a pressure greater than the critical pressure of water and a temperature between 30°C and 150°C, where the traversing axis extends through the center of a body of the sequential mixer from the hydrocarbon inlet to a mixed stream outlet, the body being physically connected to the hydrocarbon inlet; injecting a supercritical fluid through a plurality of fluid ports of the sequential mixer, wherein the plurality of fluid ports are arranged in a port alignment arrayed along the traversing axis and having a flow restrictor installed between each of the plurality of fluid ports, wherein each fluid port of the plurality of fluid ports has a port volumetric flow rate, wherein the supercritical fluid has a pressure greater than the critical pressure of water and a temperature greater than the critical temperature of water; allowing the heated hydrocarbon stream and supercritical fluid to mix in the sequential mixer to produce an intimately mixed stream, wherein the volumetric ratio of supercritical fluid to hydrocarbons in an in-mixer stream increases as the in-mixer stream passes each of the fluid ports of the plurality of fluid ports and the in-mixer stream receives a further injection of supercritical fluid, wherein the intimately mixed stream comprises hydrocarbons and supercritical fluid, wherein the intimately mixed stream has a mixed temperature; transferring the intimately mixed stream to a supercritical reactor, the supercritical reactor maintained at a pressure above the critical pressure of water and a temperature above the critical temperature of water, wherein the hydrocarbons in the intimately mixed stream undergo hydrocarbon conversion reactions in the supercritical reactor to produce an effluent stream; cooling the effluent stream in a product exchanger to produce a cooled effluent, the product exchanger configured to cool the effluent stream to a temperature below the critical temperature of water to produce the cooled effluent; depressurizing the cooled effluent in a pressure reducer to produce a product stream, the pressure reducer configured to reduce the pressure of the cooled effluent to a pressure below the critical pressure of water to produce the product stream; separating the product stream in a gas-liquid product separator to produce a vapor phase product and a liquid phase product; separating the liquid phase product in a liquid product separator to produce an upgraded hydrocarbon stream and a recovered fluid stream. introducing a heated hydrocarbon stream to a hydrocarbon inlet of the sequential mixer, the hydrocarbon inlet parallel to a traversing axis of the sequential mixer, wherein the heated hydrocarbon stream has a pressure greater than the critical pressure of water and a temperature between 30°C and 150°C, where the traversing axis extends through the center of a body of the sequential mixer from the hydrocarbon inlet to a mixed stream outlet, the body being physically connected to the hydrocarbon inlet; injecting a supercritical fluid through a plurality of fluid ports of the sequential mixer, wherein the plurality of fluid ports are arranged in a port alignment arrayed along the traversing axis and having a flow restrictor installed between each of the plurality of fluid ports, wherein each fluid port of the plurality of fluid ports has a port volumetric flow rate, wherein the supercritical fluid has a pressure greater than the critical pressure of water and a temperature greater than the critical temperature of water; allowing the heated hydrocarbon stream and supercritical fluid to mix in the sequential mixer to produce an intimately mixed stream, wherein the volumetric ratio of supercritical fluid to hydrocarbons in an in-mixer stream increases as the in-mixer stream passes each of the fluid ports of the plurality of fluid ports and the in-mixer stream receives a further injection of supercritical fluid, wherein the intimately mixed stream comprises hydrocarbons and supercritical fluid, wherein the intimately mixed stream has a mixed temperature; transferring the intimately mixed stream to a supercritical reactor, the supercritical reactor maintained at a pressure above the critical pressure of water and a temperature above the critical temperature of water, wherein the hydrocarbons in the intimately mixed stream undergo hydrocarbon conversion reactions in the supercritical reactor to produce an effluent stream; cooling the effluent stream in a product exchanger to produce a cooled effluent, the product exchanger configured to cool the effluent stream to a temperature below the critical temperature of water to produce the cooled effluent; depressurizing the cooled effluent in a pressure reducer to produce a product stream, the pressure reducer configured to reduce the pressure of the cooled effluent to a pressure below the critical pressure of water to produce the product stream; separating the product stream in a gas-liquid product separator to produce a vapor phase product and a liquid phase product; separating the liquid phase product in a liquid product separator to produce an upgraded hydrocarbon stream and a recovered fluid stream.