Downhole fluid flow control system and method having autonomous closure

A fluid flow control system includes a flow control assembly having a fluid flow path for a flowing fluid. First and second flow control components are disposed in parallel in the fluid flow path, the first having a positive flowrate response to decreasing fluid viscosity and the second having a negative flowrate response to decreasing fluid viscosity. A valve is disposed in the fluid flow path in downstream series with the first and second flow control components. The valve has first and second inlet paths for fluid from the first and second flow control components, respectively, such that decreasing the fluid viscosity of the flowing fluid increases a ratio of the flowrate through the first control component to the flowrate through the second flow control component and such that when the ratio reaches a predetermined level, the valve autonomously shifts from an open position to a closed position.