US Patent 7647971 Method of developing subsurface freeze zone

A method of lowering the temperature of a subsurface formation, e.g., comprising oil shale, includes injecting a cooling fluid under pressure into a wellbore. The wellbore is completed at or below a depth of the subsurface formation, and the wellbore includes a bore formed through the subsurface formation defining a diameter. The cooling fluid comprises a slurry having particles of frozen material. The cooling fluid is circulated across the formation in order to lower the temperature of at least a portion of the formation to a point that is at or below the freezing point of water.