Method and structure for retorting oil shale in situ by cycling fluid flows

Claim 1

A RETORTING METHOD EMPOLYING IN SITU COMBUSTION OF A SUBTERRANEAN DEPOSIT OF OIL SHALE HAVING INTERCONNECTED FRACTURES, WHICH FRACTURES ARE ORIENTED PREDOMINANTLY VERTICALLY AND ALIGNED WITH INTERCONNECTION INTO A FRACTURE SYSTEM HAVING PERMEABILITY TO FLUIDS IN AT LEAST ONE DIRECTION BY WHICH FLUIDS CAN FLOW THROUGH SUCH OIL SHALE, COMPRISING THE STEPS OF: (A) PROVIDING WELL MEANS FOR CONVEYING FLUIDS BETWEEN THE EARTH''S SURFACE AND THE OIL SHALE WITH SAID MEANS PENETRATING THE OIL SHALE INTO FLUID COMMUNICATION WITH INTERCONNECTED FRACTURES IN THE FRACTURE SYSTEM AND REMOTE FROM ANY ATMOSPHERIC VENT THROUGH WHICH FRACTURE-CONTAINED PRESSURIZED FLUIDS SUFFER APPRECIABLE LEAKAGE AND SUBSTANTIAL LOSS. (B) SUPPLYING A HEATING GAS THROUGH THE WELL MEANS INTO THE INTERCONNECTED FRACTURES IN FLUID COMMUNICATION THEREWITH AND INITIATING COMBUSTION IN THE OIL SHALE ADJACENT THE WELL MEANS WITH THE HEATING GAS IN MAINTAINING COMBUSTION IN THE OIL SHALE INTRODUCED AT A FIRST PRESSURE IN THE FRACTURES GREATER THAN ATMOSPHERIC PRESSURE AND LESS THAN THE PRESSURE AT WHICH INDUCED FRACTURING OF THE OIL SHALE OCCURS, (C) TERMINATING THE SUPPLY OF SAID HEATING GAS INTO THE WELL MEANS ONLY WHEN THE INFLOW OF SAID HEATING GAS INTO THE IS SUBSTANTIALLY REDUCED IN RATE AT SAID FIRST PRESSURE UPON A PERIPHERAL PRESSURE SEAL BEING FORMED IN THE FRACTURES IN FLUID COMMUNICATION WITH THE WELL MEANS, THEREAFTER, (D) VENTING THE PRESSURIZED FLUIDS CONTAINED BY THE PERIPHERAL SEAL IN THE OIL SHALE THROUGH THE WELL MEANS AND RECOVERING HYDROCARBONS FROM SAID FLUIDS UNTIL THE OUTFLOW OF SAID FLUIDS IS SUBSTANTIALLY REDUCED IN RATE AT A SECOND PRESSURE LESS THAN SAID FIRST PRESSURE, (E) INJECTING THE HEATING GAS THROUGH THE WELL MEANS INTO THE OIL SHALE TO PROMOTE IN SITU COMBUSTION AT A THIRD PRESSURE GREATER THAN THE SECOND PRESSURE AND NOT GREATER THAN ABOUT THE FIRST PRESSURE UNTIL THE INFLOW OF SAID HEATING GAS SUBSTATIALLY CEASES AT SAID THIRD PRESSURE, AND, THEREAFTER, (F) REPEATING STEP (D).