A method and system for enhancing petroleum production are provided, in which a fracturing operation can be conducted in a formation through a string and then petroleum is displaced from the fractured formation by selectively injecting fluid into selected fractures in the formation while other non-selected fractures remain without fluid injection. The injected fluid flows out into the fractured formation and enhances recovery from the non-selected fractures. Petroleum is selectively collected from the non-selected fractures.