1.  Field of the Invention
The present invention relates generally to methods and systems for providing a barrier for production of hydrocarbons, hydrogen, and/or other products from various subsurface formations such as hydrocarbon containing formations.  Embodiments relate to the formation of double barrier around at least a portion of a treatment area. 
2.  Description of Related Art
In situ processes may be used to treat subsurface formations.  During some in situ processes, fluids may be introduced or generated in the formation.  Introduced or generated fluids may need to be contained in a treatment area to minimize or eliminate impact of the in situ process on adjacent areas.  During some in situ processes, a barrier may be formed around all or a portion of the treatment area to inhibit migration fluids out of or into the treatment area. 
A low temperature zone may be used to isolate selected areas of subsurface formation for many purposes.  In some systems, ground is frozen to inhibit migration of fluids from a treatment area during soil remediation.  U.S. Pat.  No.  4,860,544 to Krieg et al.,  U.S. Pat.  No.  4,974,425 to Krieg et al.;  U.S. Pat.  No.  5,507,149 to Dash et al.,  U.S. Pat.  No.  6,796,139 to Briley et al.;  and U.S. Pat.  No.  6,854,929 to Vinegar et al.,  each of which is incorporated by reference as if fully set forth herein, describe systems for freezing ground. 
To form a low temperature barrier, spaced apart wellbores may be formed in the formation where the barrier is to be formed.  Piping may be placed in the wellbores.  A low temperature heat transfer fluid may be circulated through the piping to reduce the temperature adjacent to the wellbores.  The low temperature zone around the wellbores may expand outward.  Eventually the low temperature zones produced by two adjacent wellbores merge.  The temperature of the low temperature zones may be sufficiently low to freeze formation fluid so that a substantially impermeable barrier is formed.  The wellbore spacing may be from about 1 m to 3 m or more. 
Wellbore spacing may be a function of a number of factors, including formation composition and properties, formation fluid and properties, time available for forming the barrier, and temperature and properties of the low temperature heat transfer fluid.  In general, a very cold temperature of the low temperature heat transfer fluid allows for a larger spacing and/or for quicker formation of the barrier.  A very cold temperature may be −20° C. or less.