A tank contains both Zeolite and a hydrate in a gas chamber formed beneath a
piston
in the sample tank. Out of safety considerations, we avoid using source cylinders
of nitrogen whose pressures exceed 4000 psi. Thus, the gas chamber of the sample
tank is initially pressurized by the source cylinder to no more than 4000 psi of
nitrogen at room temperature at the surface. Nitrogen gas is sorbed onto the zeolite
at room temperature. As the tank is heated by being lowered downhole, nitrogen
desorbs from the zeolite and the gas pressure increases. However, once this tank
reaches a temperature high enough to release the hydrate's water of hydration,
the released water is preferentially sorbed by zeolite, displacing sorbed nitrogen,
and causing the pressure in the gas volume to increase even further. Because well
temperatures are not high enough to desorb water from zeolite, any water sorbed
onto a Zeolite sorption site will permanently block released nitrogen from resorbing
at that site. The process of lowering the tank downhole provides the necessary
heating to make the entire process occur. Thus, if returned to the surface at room
temperature with the original gas-chamber volume, the tank's pressure would not
fall back to the original pressure (e.g., 4000 psi) but would be at a substantially
higher pressure (e.g., 6000 psi or more depending on the amount of Zeolite used
and gaseous nitrogen gas released).
