US Patent 7493956 Subsurface safety valve with closure provided by the flowing medium

Devices and methods for selectively closing and opening flow of production fluid (oil, gas, or water) through the flowbore of a production string by chemically altering the phase of fluid media flowing through the production tubing to form a substantially solid plug or blockage. The phase of the fluid media is altered by selectively injecting a predetermined catalyst into a portion of the flowbore wherein it is desired to create a plug or blockage. When it is desired to remove the blockage, a dissolving agent is selectively added to the flowbore via a reversing sub to reverse the phase of the blockage from solid to liquid/gas. As the blockage is dissolved, flow is reestablished within the flowbore.