Disclosed are apparatus and method of closing a conduit against fluid flow in at least one longitudinal sense, and selectively disengaging and reengaging the closure mechanism without removing the closure mechanism from the conduit. In a particular embodiment, a valve assembly may be selectively anchored and sealed to a well packer, or disengaged therefrom. The valve assembly is manipulated by an operating tool to which the valve assembly is anchored in the well when the valve assembly is disengaged from the packer.