US Patent 7546873 Low temperature barriers for use with in situ processes

A method of forming and maintaining a low temperature zone around at least a portion of a subsurface treatment area is described. The method includes reducing a temperature of heat transfer fluid with a refrigeration system. The heat transfer fluid is circulated through freeze well canisters and placed in a formation around at least a portion of the subsurface treatment area. An initial temperature of the heat transfer fluid supplied to a first freeze well canister is in a range from about −35 ° C. to about −55 ° C. At least one of the well canisters includes carbon steel. The heat transfer fluid is returned to the refrigeration system.