Determination of residual oil in a formation

Abstract

A method for determining the amount of residual oil remaining in an oil bearing formation after primary production which includes the sequential steps of: A. logging the formation to obtain logging data measurements of the relative quantities of residual oil and formation water present in the formation; B. injecting a sufficient amount of an oil miscible solution through the bore hole into the formation to displace substantially all of the residual oil in the formation surrounding the bore hole; C. injecting a sufficient amount of water into the formation through the bore hole to displace substantially all of the oil miscible solution thereby rendering the formation being tested substantially 100 per cent water saturated; and D. logging the formation for a second time to obtain logging data measurements which, when compared with the logging data measurements of the initial log indicate the amount of residual oil present in the formation.Iadd., wherein said logging data measurements are obtained by employing a reservoir property logging apparatus selected from the group consisting of resistivity logging apparatus, sonic logging apparatus and density logging apparatus.Iaddend..

Having thus described the invention, we claim: 
     
       1. A method for determining the residual oil in an oil bearing formation penetrated by the bore hole of a well comprising the steps of: a. logging said formation to obtain logging data measurements of the relative quantities of residual oil and formation water present in the formation;   b. injecting into said formation, through said borehole, a sufficient amount of an oil miscible solution, to displace substantially all of the residual oil in said formation surrounding said borehole to a distance exceeding the radius of investigation of the logging means providing the logging data;   c. injecting into said formation through said borehole a sufficient amount of water to displace substantially all of said oil miscible solution and cause said formation being tested to be substantially 100 percent water saturated; and   d. logging for a second time said formation to obtain logging data measurements; and,   e. comparing the logging data measurements of said second log with the logging data measurements of said first log to determine the amount of residual oil in said formation.Iadd., wherein said logging data measurements are obtained by employing a reservoir property logging means selected from the group consisting of resistivity log means, sonic log means and density log means.Iaddend..  .[.2. The method of claim 1 wherein said logging data measurements are obtained by employing a reservoir property logging means selected from the group consisting of resistivity   
     
     
        log means, sonic log means and density log means..]. 3. The method of claim 1 wherein said oil miscible solution is an aqueous solution containing from about 0.5 to 60 weight percent of a surface active agent. 
     
     
         . The method of claim 3 wherein said surface active agent is an overbased 
     
     
        pale oil extract sulfonate. 5. The method of claim 1 wherein said oil miscible solution is a mixture of liquified low molecular weight hydrocarbon and water, said hydrocarbon being present in an amount of from about 3 to 15 percent of the reservoir hydrocarbon pore space and said water being in an amount such that the movement velocity of said water in said reservoir is greater than the movement velocity of said hydrocarbon 
     
     
        in said reservoir. 6. The method of claim 1 wherein said oil miscible solution is a micellar solution and from about 3 to 20 percent pore volume 
     
     
        of said micellar solution is injected into said formation. 7. The method of claim 1 wherein said oil miscible solution is microemulsion forming solution and said microemulsion forming solution is an aqueous solution containing from about 15 to 60 weight percent of a polar organic compound and from about 5 to 40 percent by weight, based on the total weight of 
     
     
        said aqueous solution, of a surface active agent. 8. The method of claim 7 which includes the step of injecting a minor amount of an oil component miscible with the reservoir oil into said formation prior to injection of said microemulsion forming solution.