Sweep efficiency in miscible fluid floods

Claim 1

IN A PROCESS FOR SECONDARY RECOVERY OF HYDROCARBONS FROM A SUBTERRRANEAN POROUS FORMATION BY EMPLOYING A MISCIBLE FLUID FLOOD, AND WHEREIN GRAVITY SEGREGATION OF AN INJECTED HYDROCARBON SOLVENT OCCURS IN THE FORMATION, CAUSING THE SOLVENT TO PENETRATE MORE RAPIDLY THROUGH THE UPPER PORTION OF PAY ZONE OF SAID FORMATION THUS RESULTING IN PREMATURE BREAKTHROUHG TO AT LEAST ONE PRODUCING WELL PENETRATING SAID FORMATION, THE STEPS COMPRISING; (A) INJECTING A HYDROCARBON SOLVENT VI A SAID INPUT WELL INTO THE LOWER PORTION OF SAID PAY ZONE; (B) INJECTING A HYDROCARBON SOVLENT CONTAINING A VISCOSIFYING AGENT VIA SAID INPUT WELL INTO A SUFFICIENT AMOUNT OF THE UPPER PORTION OF SAID PAY ZONE TO PREVENT PREMATURE BREAKTHROUGH CAUSED BY GRAVITY SEGREGATION, THE VISCOSITY OF THE FLUID INTRODUCED INTO SAID UPPER PORTION BEING GREATER THAN THE VISCOSITY OF THE SOLVENT INTRODUCED INTO SAID LOWER PORTION; (C) INJECTING A NORMALLY GASEOUS HYDROCARBON INTO SAID INPUT WELL BEHIND BOTH OF SAID HYDROCARBON SOLVENTS; (D) AND PRODUCING THE HYDROCARBONS IN SAID FORMATION FROM SAID AT LEAST ONE PRODUCING WELL, THEREBY REDUCING SOLVENT MOBILITY IN SAID UPPER PORITION AND RESULTING IN IMPROVED VERTICAL SWEEP EFFICIENCY.