Enhanced oil recovery

Abstract

A method for enhancing the recovery of oil from underground formations is disclosed. A gas mixture which contains greater than 50% by volume carbon dioxide, the remainder being an inert gas, is injected in the underground formation to lower the oil viscosity and surface tension and increase the oil swelling. Preferably, the gas mixture contains greater than 60% by volume carbon dioxide with a gas mixture containing greater than 70% by volume carbon dioxide preferred.

Having thus described the invention, what we claim is:  
     
         1 . A method for enhancing the recovery of oil from an underground formation comprising injecting into said oil a gas mixture comprising at least 50% by volume carbon dioxide.  
     
     
         2 . The method as claimed in  claim 1  wherein the remainder of said gas mixture is selected from the group consisting of an inert gas, mixtures of inert gases, hydrocarbons, steam, air or mixtures of these.  
     
     
         3 . The method as claimed in  claim 2  wherein said inert gas is selected from the group consisting of nitrogen, helium and argon.  
     
     
         4 . The method as claimed in  claim 1  wherein said gas mixture comprises greater than 60% by volume carbon dioxide.  
     
     
         5 . The method as claimed in  claim 1  wherein said gas mixture comprises greater than 70% carbon dioxide by volume.  
     
     
         6 . The method as claimed in  claim 1  wherein said gas mixture is injected into said oil at the well head.  
     
     
         7 . The method as claimed in  claim 1  wherein said gas mixture is injected into a single production well head.  
     
     
         8 . The method as claimed in  claim 7  wherein said injection is cyclical.  
     
     
         9 . The method as claimed in  claim 7  wherein said gas mixture is injected into an injection well different from said production well head.  
     
     
         10 . The method as claimed in  claim 9  wherein said gas mixture is injected in an alternating pattern with a driving fluid.  
     
     
         11 . The method as claimed in  claim 10  wherein said driving fluid is selected from the group consisting of steam, water, nitrogen, carbon dioxide and air.  
     
     
         12 . The method as claimed in  claim 9  wherein there are a plurality of different injection wells.  
     
     
         13 . The method as claimed in  claim 1  wherein said gas mixture is injected into said oil at a pressure ranging from about 100 psi to about 20,000 psi.  
     
     
         14 . The method as claimed in  claim 1  wherein said carbon dioxide is obtained from a power plant or co-generation plant or from in situ combustion through the injection of air, oxygen enriched air, or pure oxygen.  
     
     
         15 . The method as claimed in  claim 1  wherein said nitrogen is obtained from an air separation plant.  
     
     
         16 . The method as claimed in  claim 1  wherein after injection of said gas mixture said underground formation is sealed for at least one day.  
     
     
         17 . The method as claimed in  claim 16  wherein said sealed underground formation is opened and a flood of a material selected from the group consisting of carbon dioxide, nitrogen, water or brine is driven through said injection point.  
     
     
         18 . A method for enhancing the recovery of oil from an underground formation comprising injecting into said oil a gas mixture comprising carbon dioxide and nitrogen wherein said carbon dioxide is present in said gas mixture in an amount of at least 50% by volume.  
     
     
         19 . The method as claimed in  claim 18  wherein said carbon dioxide is present in said gas mixture in an amount greater than 60% by volume.  
     
     
         20 . The method as claimed in  claim 18  wherein said carbon dioxide is present in said gas mixture in an amount greater than 70% by volume.  
     
     
         21 . The method as claimed in  claim 18  wherein said carbon dioxide is present in said gas mixture in an amount greater than 50% by volume and less than 99% by volume.  
     
     
         22 . The method as claimed in  claim 18  wherein said gas mixture is injected into said formation at a pressure of about 100 psi to about 20,000 psi.  
     
     
         23 . The method as claimed in  claim 18  wherein steam is additionally injected into said underground formation.  
     
     
         24 . A method for enhancing the recovery of oil from an underground formation having at least one production well comprising injecting into said underground formation in at least two distinct injection points a gas mixture comprising carbon dioxide and nitrogen wherein said carbon dioxide is present in said gas mixture in an amount of at least 50% by volume at the first injection point and further wherein the volume percentage of nitrogen in said mixture is higher at said second injection point than said first injection point, and said first injection point is closer to said at least one production well than said second injection point.  
     
     
         25 . The method as claimed in  claim 24  wherein the volume percentage of nitrogen in said gas mixture increases at each injection point further from said production well.  
     
     
         26 . The method as claimed in  claim 24  wherein the viscosity and surface tension of said oil at said second injection point is higher than the surface tension and viscosity at said first injection point.  
     
     
         27 . The method as claimed in  claim 24  wherein said underground formation is sealed for at least one day.  
     
     
         28 . The method as claimed in  claim 27  wherein said underground formation is opened and flooded with a material selected from the group consisting of carbon dioxide, nitrogen, water and brine to improve oil recovery.  
     
     
         29 . A method for lowering the viscosity and surface tension of oil in an underground formation comprising injecting the said oil a gas mixture comprising at least 50% by volume carbon dioxide.  
     
     
         30 . The method as claimed in  claim 29  wherein the remainder of said gas mixture is selected from the group consisting of an inert gas, mixtures of inert gases, hydrocarbons, steam, air or mixtures of these.  
     
     
         31 . The method as claimed in  claim 29  wherein said inert gas is selected from the group consisting of nitrogen, helium and argon.  
     
     
         32 . The method as claimed in  claim 29  wherein said gas mixture comprises at least 60% by volume carbon dioxide.  
     
     
         33 . The method as claimed in  claim 29  wherein said carbon dioxide comprises at least 70% by volume of said gas mixture.  
     
     
         34 . The method as claimed in  claim 29  wherein said gas mixture comprises about 50% to about 99% by volume carbon dioxide and about 1% to about 49% nitrogen by volume.  
     
     
         35 . The method as claimed in  claim 29  wherein said gas mixture is injected into said underground formation at a pressure of about 100 psi to about 20,000 psi.  
     
     
         36 . The method as claimed in  claim 29  wherein said hydrocarbons are derived from associated gas produced during the production of oil.  
     
     
         37 . The method as claimed in  claim 29  wherein said hydrocarbons are present in said mixture in an amount ranging from about 1% to about 50%.