A process is provided for recovering hydrocarbons, such as heavy oils, from a subterranean reservoir. The process includes providing a subcritical or a supercritical aqueous fluid at a high temperature and high pressure to the underground hydrocarbon reservoir, injecting the aqueous fluid into the reservoir to heat the hydrocarbons in the reservoir, and recovering the heated hydrocarbons from the reservoir. In some cases, the supercritical fluid is also used to upgrade the hydrocarbons and/or facilitate the transportation of the hydrocarbons from the production field to another location, such as a refinery. Advantageously, isentropic expansion may be employed anywhere in the system for a more efficient and effective processes and systems.