A number of operations are performed in the completion and maintenance of subterranean wells that require the introduction of fluids into the well and the production formation for specific purposes.  For example, subsequent to gravel packing, completion fluids are introduced to the well to displace the fluid used during the gravel packing procedure.  When the gravel packing and completion fluid introduction operations are completed, it is, of course, necessary to remove the wash tube of the gravel packing apparatus and, in particular, the tubular work string carrying such apparatus, and substantial quantities of completion fluid are normally contained in the removed apparatus.  It is, therefore, desirable to prevent the loss of such costly fluid by flow into the formation upon the removal of the tubular work string and the associated gravel packing apparatus from the well. 
There is, therefore, a need for a valve which may be conveniently inserted into the well casing in an open position above a production formation so that a wash tube of a gravel packing apparatus may be readily inserted through the open valve to extend to a position adjacent the production formation.  Such valve should be automatically closeable by the withdrawal of the wash string from the well.  Means for the reliable reopening of the valve for production or further work must also be provided.