Process for increasing oil recovery by miscible displacement

Claim 1

A PROCESS FOR THE RECOVERY OF PETROLEUM FROM A SUBSURFACE RESERVOIR WHICH COMPRISES DRILLING A WELLBORE THROUGH SUBSTANTIALLY THE ENTIRE OIL-BEARING ZONE, LIMITING COMMUNICATION BETWEEN SAID ZONE AND SAID WELLBORE TO SUBSTANTIALLY THE LOWER BOUNDARY LEVEL THEREOF, FORMING A HIGHLY CONDUCTIVE, HORIZONTALLY ORIENTED FRACTURE AT SAID LEVEL OF COMMUNICATION, INJECTING THROUGH SAID FRACTURE AND INTO THE RESERVOIR A CONDENSIBLE GAS, THE LIQUID PHASE OF WHICH EXHIBITS A SUBSTANTIAL DEGREE OF MISCIBILITY WITH THE RESERVOIR OIL, THEREAFTER INJECTING A RELATIVELY NONCONDENSIBLE GAS THROUGH SAID FRACTURE AND INTO THE RESERVIOR, THE VOLUME OF SAID NON-CONDENSIBLE GAS BEING SUFFICIENT TO RAISE THE RESERVOIR PRESSURE AT LEAST 50 P.S.I. ABOVE THE VAPOR PRESSURE OF THE CONDENSIBLE GAS, DISCONTINUING INJECTION OF SAID NON-CONDENSIBLE GAS AND THEREAFTER PRODUCING OIL FROM SAID RESERVOIR THROUGH SAID FRACTURE, WITH A CONTROLLED BACK PRESSURE SUFFICIENT TO PERMIT THE EFFECTS OF GRAVITY SEGREGATION TO BALANCE THE TENDENCY FOR THE CONDENSED LAYER OF MISCIBLE LIQUID TO FINGER THROUGH THE RESERVOIR OIL.