Neutron method for determining residual oil-phase fluid concentration

Abstract

A method for determining the concentration of oil-phase fluid in an earth formation containing indigenous oil-phase fluid and aqueous liquid. A zone in the formation is irradiated with neutrons when the zone is filled with indigenous oil-phase and aqueous liquid. The thermal neutron capture rate response of the zone is measured with respect to the first irradiation. Substantially all indigenous oil-phase is removed from the zone and the zone is filled with only an aqueous liquid substantially equivalent in compositions to the indigeneous aqueous liquid. The zone is irradiated with neutrons a second time and the thermal neutron capture rate response of the zone is measured with respect to the second irradiation.

We claim as our invention: 
     
       1. In a method for determining the concentration of indigenous oil-phase fluid in an earth formation containing indigenous oil-phase fluid and aqueous liquid, the process comprising the steps of: irradiating with a pulse of neutrons a zone within said formation when said zone is filled with said indigenous oil-phase fluid and aqueous liquid;   measuring the thermal neutron capture rate response of the zone to said irradiation;   removing substantially all indigenous oil-phase fluid from the zone;   filling said zone with an aqueous liquid substantially equivalent in composition to said indigenous aqueous liquid;   irradiating the zone with a pulse of neutrons a second time;   measuring the thermal neutron capture rate response of the zone to said second irradiation; and   determining the quantity of the indigenous oil-phase fluid saturation times porosity from the difference between said measured first and said second responses.   
     
     
       2. The method of claim 1 including the step of flooding said earth formation with an aqueous liquid prior to first irradiating said zone so as to displace any gas present in said formation away from the zone being irradiated. 
     
     
       3. The method of claim 1 including the step of producing said well until its oil content is at least as low as waterflood residual prior to irradiating said zone a first time. 
     
     
       4. The method of claim 3 wherein the step of producing said well includes the step of injecting sufficient water into said well so as to reduce the residual oil level. 
     
     
       5. The method of claim 4 wherein the steps of injecting sufficient water includes injecting salt water sufficient to exceed the radius of investigation of the zone. 
     
     
       6. The method of claim 1 including the step of producing therefrom a supply of the indigenous oil-phase fluid and aqueous liquid within said zone; and separating said aqueous liquid from said indigenous oil-phase fluid; and   employing said separated aqueous liquid in the step of filling said zone with an aqueous liquid. .Iadd. 7. A method of measuring water saturation in a subsurface zone penetrated by a well bore which comprises the steps of:   a. running a logging instrument in said well bore to measure the water saturation of the rock in said zone adjacent said well bore,   b. injecting a driving fluid through said well bore into said zone to displace oil out of the rock immediately surrounding the well bore, said driving fluid easily driven through the rock by a second driving fluid having the same logging characteristics as the water in the rock at the time of Step (a),   c. injecting said second driving fluid through said well bore into said zone to drive said first driving fluid from the zone immediately adjacent the well bore, and   d. after Step (c), repeating Step (a). .Iaddend. .Iadd. 8. A method as defined in claim 7 in which Step (a) is the running of a thermal neutron decay time log. .Iaddend..Iadd. 9. A method as defined in claim 7 in which said first driving fluid is a micellar solution having mutual solubility in oil and in water. .Iaddend..Iadd. 10. A method as defined in claim 9 which comprises the step of injecting a water slug before Step (c). .Iaddend..Iadd. 11. A method as defined in claim 10 which includes the step of injecting water after Step (c). .Iaddend..Iadd. 12. A method as defined in claim 7 in which before Step (a) the well is conditioned to produce as near as possible the same fluid saturation adjacent the bore as that present in the rock formation remote from the well bore. .Iaddend..Iadd. 13. A method as defined in claim 7 when used in a zone having gas saturation, the method including the step of running logs to determine the gas saturation, such logs to be run after Step (a) and before Step (b). .Iaddend..Iadd. 14. A method of measuring water saturation in a subsurface zone penetrated by a well bore which comprises the steps of:   1. measuring the water saturation of the rock in said zone adjacent said well bore,   2. thereafter reducing to zero the oil saturation of the rock immediately surrounding the well bore,   3. filling the pore space of the rock adjacent the well bore with a fluid having the same logging characteristics as the water in the rock at the time of Step (1),   4. after Step 3 again measuring the water saturation of the rock in said zone adjacent said well bore. .Iaddend. .Iadd. 15. A method for determining the residual oil in an oil bearing formation penetrated by the borehole 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% 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. .Iaddend.