Secondary recovery of petroleum

Claim 1

AN IMPROVED OIL RECOVERY PROCESS WHICH COMPRISES INJECTING WATER INTO A SUBSURFACE OIL-BEARING RESERVOIR THROUGH AT LEAST ONE INJECTION WELL UNTIL A MEAN RESERVOIR PRESSURE OF AT LEAST 1000 P.S.I. IS ATTAINED, INJECTING A BANK OF FROM ABOUT 0.03 TO ABOUT 0.5 PORE VOLUME, BASED ON RESERVOIR CONDITIONS, OF NORMALLY GASEOUS FLUID COMPRISING CARBON DIOXIDE INTO SAID RESERVOIR THROUGH SAID INJECTION WELL AT A PRESSURE IN EXCESS OF 1000 P.S.I., INJECTING AN AQUEOUS MEDIUM INTO SAID RESERVOIR THROUGH SAID INJECTION WELL TO DISPLACE SAID BANK OF FLUID TOWARD AT LEAST ONE PRODUCTION WELL, WITHDRAWING PRODUCED FLUIDS FROM SAID PRODUCTION WELL WHILE MAINTAINING A MEAN RESERVOIR PRESSURE OF AT LEAST 1000 P.S.I.; DISCONTINUING THE INJECTION OF SAID AQUEOUS MEDIUM WHEN THE PRODUCING FLUIDS NO LONGER CONTAIN SUBSTANTIAL QUANTITIES OF OIL AND CARBON DIOXIDE, AND THEREAFTER DEPRESSURING SAID RESERVOIR BY PUMPING DOWN SAID PRODUCTION WELL AND RECOVERING ADDITIONAL OIL FROM SAID PRODUCTION WELL.