Method of recovering petroleum

Claim 1

A METHOD OF RECOVERING NATURALLY OCCURRING VISCOUS HYDROCARBON OIL FROM AN UNDERGROUND FORMATIION, SAID FORMATION HAVING IN COMMUNICATION THEREWITH AT LEAST ONE PRODUCING WELL AND AT LEAST ONE INJECTION WELL, SAID FORMATION HAVING A PERMEABLE FLOW CHANNEL IN THE LOWER PORTION THEREOF, SAID FLOW CHANNEL INCLUDING AT LEAST ONE FORMATION FRACTURE EXTENDING FROM ONE OF SAID WELLS TOWARD THE OTHER OF SAID WELLS AND SUBSTANTIALLY PARALLEL TO THE BOTTOM OF SAID FORMATION, COMPRISING INJECTING A LIQUEFIED NORMALLY GASEOUS SOLVENT FOR SAID OIL AT A PRESSURE ABOVE THE VAPOR PRESSURE OF SAID SOLVENT AT FORMATION TEMPERATURE INTO SAID INJECTION WELL AND THROUGH SAID FLOW CHANNEL, AND REDUCING THE PRESSURE OF SAID SOLVENT TO A VALUE LESS THAN THE VAPOR PRESSURE OF SAID SOLVENT AT FORMATION TEMPERATURE, WHEREBY THE VAPORS FROM SAID SOLVENT PASS UPWARD INTO THE OVERLYING FORMATION, CONTACTING SAID OIL AND RENDERING SAID OIL LESS VISCOUS ANBD INCREASING THE RATE OF DRAINAGE OF SAID OIL INTO SAID FLOW CHANNEL, THEREAFTER INCREASING THE PRESSURE OF SAID SOLVENT TO A VALUE ABOUT THE VAPOR PRESSURE OF SAID SOLVENT AT FORMATION TEMPERATURE, AND INCREASING THE FLOW OF SAID SOLVENT THROUGH SAID FLOW CHANNEL WHEREBY SAID SOLVENT DISOLVES AND CARRIES SAID OIL TO SAID PRODUCING WELL FROM WHICH SAID SOLVENT AND SAID OIL MAY BE RECOVERED.