Valves for autonomous actuation of downhole tools

An autonomous valve includes a fluid-sensitive actuator operable to automatically actuate the valve in response to being exposed to a target fluid. The target fluid may be a fluid whose presence is generally undesirable in a production fluid, such as water, and the valve may be actuated to stop flow from an interval within a wellbore that is no longer expected to produce hydrocarbon-bearing fluids. The valve further includes a protective layer disposed between the fluid inlet and the fluid-sensitive actuator, the protective layer being insoluble in the target fluid. The fluid-sensitive actuator may include a swellable piston that swells to actuate the valve in the presence of the target fluid, a soluble plug that isolates a hydraulic, closing piston until the soluble plug is dissolved into the target fluid, and an electromechanical actuator that closes the valve in response to detecting the target fluid using an electronic sensor.