Method for measuring formation water salinity from within a borehole

1. A method, comprising

lowering a pulsed neutron tool into a borehole penetrating a formation;

irradiating the formation with neutrons;

measuring neutron induced gamma ray spectra with at least two gamma ray detectors comprising different sensitivities to borehole and formation gamma rays, wherein a first detector of the at least two gamma ray detectors is configured to measure a borehole signal and a second detector of the at least two gamma ray detectors is configured to measure a formation signal, wherein the first detector comprises back-shielding facing the formation and the second detector comprises back-shielding facing the borehole;

in a computer, generating a cross-plot forming a quadrilateral of at least one of:

ratios of spectroscopically determined yields of hydrogen (H) and chlorine (Cl) obtained from a database; and

near detector apparent salinity and far detector apparent salinity values determined from the ratios and obtained from the database, wherein said quadrilateral has four endpoints corresponding to: (1) low borehole salinity and low formation salinity, (2) low borehole salinity and high formation salinity, (3) high borehole salinity and low formation salinity, and (4) high borehole salinity and high formation salinity;

in the computer, deriving formation fluid apparent salinities from measurements obtained from the two gamma ray detectors in the pulsed neutron tool and positions of such measurements on the quadrilateral cross plot;

measuring water saturation of the formation in which the pulsed neutron tool is disposed; and

in the computer, computing formation water salinity from the formation fluid apparent salinity and water saturation.

2. The method of claim 1 , wherein the pulsed neutron tool is conveyed by a while-drilling conveyance, and the while-drilling conveyance obtains a borehole signal and a formation signal by rotating a drill collar containing the pulsed neutron tool.

3. The method of claim 1 , wherein the database is constructed using varying values of water saturation, hydrogen index of the formation fluid, borehole fluid salt type, or borehole fluid hydrogen index.

4. The method of claim 1 , further comprising determining formation water density; and computing a density porosity that accounts for the formation water salinity.

5. The method of claim 4 , wherein the computed density porosity is derived from a bulk density measurement performed with a gamma-gamma density sonde.

6. The method of claim 4 , wherein the computed density porosity is derived from a bulk density measurement performed with a borehole gravity sonde.

7. The method of claim 1 , further comprising determining formation water density, inputting the formation water density into a 3-dimensional reservoir model, and varying the 3-dimensional reservoir model to represent different times, thereby reflecting fluid movement and changing water salinity.

8. The method of claim 1 , further comprising computing borehole salinity from the apparent borehole fluid salinity and an oil holdup.

9. A method for determining formation water salinity, comprising:

in a computer, generating a cross-plot forming a quadrilateral of at least one of ratios of spectroscopically determined yields of hydrogen (H) and chlorine (Cl) obtained from a database, and near detector apparent salinity and far detector apparent salinity values determined from the ratios and obtained from the database, wherein said quadrilateral has four endpoints corresponding to: (1) low borehole salinity and low formation salinity, (2) low borehole salinity and high formation salinity, (3) high borehole salinity and low formation salinity, and (4) high borehole salinity and high formation salinity;

lowering a pulsed neutron tool in a borehole penetrating a formation;

irradiating the formation with neutrons;

measuring neutron induced gamma ray spectra with at least two gamma ray detectors located in the pulsed neutron tool, wherein a first detector of the at least two gamma ray detectors is configured to measure a borehole signal and a second detector of the at least two gamma ray detectors is configured to measure a formation signal, wherein the first detector comprises back-shielding facing the formation and the second detector comprises back-shielding facing the borehole; and

in the computer, deriving formation water salinities based on position of pulsed neutron measurements within the quadrilateral.

10. The method of claim 9 , wherein the pulsed neutron tool is a smaller diameter variant of a larger diameter pulsed neutron tool.

11. The method of claim 9 , wherein the pulsed neutron tool is conveyed by a while-drilling conveyance, and the while-drilling conveyance obtains a borehole signal and a formation signal by rotating a drill collar containing the pulsed neutron tool.

12. The method of claim 9 , further comprising:

determining formation water density; and

computing a density porosity that accounts for the formation water salinity.

13. The method of claim 9 , wherein the database is constructed using varying values of water saturation, hydrogen index of the formation fluid, borehole fluid salt type, or borehole fluid hydrogen index.

14. The method of claim 9 , wherein the database is further constructed using varying values of borehole fluid oil holdup.

15. The method of claim 9 , wherein the database is represented as sets of coefficients of polynomials.

16. The method of claim 9 , further comprising:

determining formation water density;

inputting the formation water density into a 3-dimensional reservoir mode; and

varying the 3-dimensional reservoir model to represent different times, reflecting fluid movement and changing water salinity.

17. The method of claim 9 , wherein formation water salinity is included in a history matching phase of creating reservoir simulation input.