A screen section has a valve assembly to control flow through it. The valve is open and has a closure spring held compressed by a shape memory material that responds to the presence of a specific well fluid or fluids so that its property changes to allow the spring to deliver the stored potential energy to the valve member to close it when the specific well fluid or fluids are detected. The preferred material is a shape memory polymer that, for example, is sensitive to the presence of water or methane and gets softer to release the potential energy source to operate the downhole tool.