Method and apparatus for lithology-independent well log analysis of formation water saturation

A system for analyzing water saturation of a formation surrounding a wellbore, comprising:

• a computer memory, for storing thermal and epithermal neutron counting rate measurements and the pulsed neutron capture log measurements;
• a computer, programmed to perform the steps of:
• retrieving the thermal and epithermal neutron counting rate measurements and the pulsed neutron capture log measurements for each of the plurality of depths from the computer memory;
• for each of the plurality of depths:
• selecting an initial porosity value;
• using the initial porosity value, determining a first matrix diffusion length value at the depth from the retrieved thermal and epithermal neutron counting rate measurements;
• using the initial porosity value, determining a second matrix diffusion length value at the depth from the retrieved pulsed neutron log measurements;
• comparing the first and second matrix diffusion lengths to determine a difference therebetween;
• responsive to the difference between the first and second matrix diffusion lengths being below a limit, adjusting the initial porosity value and repeating the determining steps;
• responsive to the difference between the first and second matrix diffusion lengths exceeding the limit, storing the adjusted initial porosity value as an actual porosity value;
• and
• determining the water saturation at the depth by solving a system of equations corresponding to the actual porosity value, the retrieved thermal neutron counting rate measurements and the retrieved pulsed neutron capture measurements at the first depth;
• and
• an output device, coupled to the computer, for displaying the water saturation at each of the plurality of depths.

A computer-readable memory having a storage medium configured so that, when read and used by a computer, the computer is directed to determine the water saturation of a formation surrounding a wellbore, wherein the computer has a memory for storing digital data corresponding to thermal and epithermal neutron counting rate measurements from the formation at a plurality of depths, and for storing digital data corresponding to pulsed neutron capture log measurements from the formation at a plurality of depths, and wherein the computer is so directed by a plurality of operations comprising:

• storing the thermal and epithermal neutron counting rate measurements and the pulsed neutron capture log measurements in computer memory;
• retrieving the thermal and epithermal neutron counting rate measurements and the pulsed neutron capture log measurements for a first depth of the plurality of depths from the computer memory;
• selecting an initial porosity value;
• using the initial porosity value, determining a first matrix diffusion length value at the first depth from the retrieved thermal and epithermal neutron counting rate measurements;
• using the initial porosity value, determining a second matrix diffusion length value at the first depth from the retrieved pulsed neutron log measurements;
• comparing the first and second matrix diffusion lengths to one another;
• responsive to the first and second matrix diffusion lengths differing from one another by less than a limit, adjusting the initial porosity value and repeating the determining steps;
• responsive to the first and second matrix diffusion lengths differing from one another by more than the limit, storing the adjusted initial porosity value as an actual porosity value;
• and
• determining the water saturation at the first depth by operating a computer to solve a system of equations corresponding to the retrieved thermal neutron counting rate measurements and the retrieved pulsed neutron capture measurements at the first depth.