Method for producing petroleum by underground combustion

Claim 1

IN A PROCESS FOR SUBJECTING A TAR SAND DEPOSIT TO A REVERSE COMBUSION OPERATION, SAID DEPOSIT BEING PENETRATED BY A PRODUCING WELL AND AN INJECTION WELL AND WHEREIN THE PRESSURE REQUIRED TO INJECT A GAS INTO AT LEAST ANY TWO ZONES OF SAID DEPOSIT DIFFERS MORE THAN ABOUT 50 P.S.I., THE IMPROVEMENT WHICH COMPRISES: IGNITING THE FACE OF SAID DEPOSIT ADJACENT SAID PRODUCING WELL, SUPPLYING AN OXYGEN-CONTAINING GAS TO THE RESULTING IGNITED PORTION VIA SAID PRODUCING WELL SO AS TO PROPAGATE A COMBUSTION FRONT INTO SAID DEPOSIT IN THE SAME DIRECTION AS SAID GAS IS FLOWING THERETHROUGH, CONTINUING INTRODUCTION OF SAID GAS VIA SAID PRODUCING WELL UNTIL A PRESSURE DROP OF FROM ABOUT 50 TO ABOUT 100 P.S.I. IS OBSERVED BETWEEN SAID WELLS, INDICATING THAT CONDENSATION PRODUCTS ARE BEGINNING TO REDUCE THE PERMEABILITY OF SAID DEPOSIT, MAKING A GAS PERMEABILITY PROFILE OF THAT PORTION OF SAID DEPOSIT CONTAINING SAID TWO ZONES. REPEATING THE ABOVE CYCLE UNTIL THE PERMEABILITY OF SAID DEPOSIT INCLUDING SAID ZONES IS SUCH THAT THE PRESSURE REQUIRED TO INJECT A GAS INTO ANY LEVEL IN SAID DEPOSIT DOES NOT VARY MORE THAN ABOUT 50 P.S.I. THEREAFTER INJECTING AN OXYGEN-CONTAINING GAS STREAM INTO SAID DEPOSIT VIA SAID INJECTION WELL SO THAT SAID FRONT MOVES THROUGH SAID DEPOSIT IN A DIRECTION OPPOSITE TO THE FLOW OF SAID STREAM AND TOWARD SAID INJECTION WELL, AND RECOVERING HYDROCARBON PRODUCTS FROM SAID DEPOSIT VIA SAID PRODUCING WELL.