Determining Water Salinity and Water-Filled Porosity of a Formation

Abstract

An illustrative method of determining water-filled porosity of a formation that includes acquiring a downhole temperature, acquiring a total porosity of the formation, acquiring a dielectric measurement of the formation, determining a water salinity based on the temperature, total porosity, and dielectric measurement. The method further includes determining a water-filled porosity of the formation based at least in part on the water salinity.

1 . A method of determining water-filled porosity of a formation, comprising:
 acquiring a downhole temperature;   acquiring a total porosity of the formation;   acquiring a dielectric measurement of the formation;   determining a water salinity based on the temperature, total porosity, and dielectric measurement; and   determining a water-filled porosity of the formation based at least in part on the water salinity.   
     
     
         2 . The method of  claim 1 , further comprising presenting a log of the water-filled porosity to a user. 
     
     
         3 . The method of  claim 1 , wherein said temperature, total porosity, and dielectric measurements are acquired as a function of depth or position along a borehole, and wherein said determining a water-filled porosity includes creating a log of said water-filled porosity as a function of depth or position along a borehole. 
     
     
         4 . The method of  claim 1 , wherein said determining a water salinity further comprises relating the real and imaginary components of water's dielectric constant. 
     
     
         5 . The method of  claim 1 , wherein determining the water salinity and determining the water-filled porosity are performed by a processor in real-time. 
     
     
         6 . The method of  claim 1 , further comprising determining a hydrocarbon porosity of the formation based on the water-filled porosity. 
     
     
         7 . The method of  claim 1 , wherein said determining a water salinity includes obtaining temperature-dependent function coefficients expressing a dependence of real and imaginary components of water's dielectric constant on salinity. 
     
     
         8 . The method of  claim 7 , wherein determining the function coefficients comprises implementing a piecewise polynomial fit. 
     
     
         9 . The method of  claim 1 , further comprising determining real and imaginary components of a refractive index of the formation from the dielectric measurement. 
     
     
         10 . The method of  claim 9 , further comprising calculating a constant based on the refractive index, the porosity, a dielectric constant of hydrocarbons, and a dielectric constant of the matrix. 
     
     
         11 . The method of  claim 1 , further comprising calculating a dielectric constant of water based at least in part on the water salinity calculating a refractive index of water 
     
     
         12 . The method of  claim 11 , further comprising calculating a refractive index of water based at least in part on the water salinity. 
     
     
         13 . The method of  claim 7 , further comprising storing the coefficients in a memory. 
     
     
         14 . A system for determining and logging water-filled porosity, comprising:
 a downhole tool assembly that acquires downhole temperature measurements, wherein the tool assembly includes:
 a dielectric measurement tool that acquires dielectric measurements of formations surrounding a borehole; 
 a porosity measurement tool that acquires total porosity measurements of said formations; and 
   a processor that determines and logs water-filled porosity of the formation as a function of the temperature, dielectric, and porosity measurements.   
     
     
         15 . The system of  claim 14 , further comprising a user interface coupled to the processor, wherein said user interface displays said log of water-filled porosity. 
     
     
         16 . The system of  claim 14 , wherein the processor further determines a water salinity of the formation as a function of the temperature, dielectric, and porosity measurements. 
     
     
         17 . The system of  claim 14 , wherein the processor further determines said water-filled porosity based at least in part on salinity and temperature. 
     
     
         18 . The system of  claim 14 , further comprising a memory that stores temperature dependent function coefficients expressing dependence of water's dielectric constant on salinity. 
     
     
         19 . The system of  claim 14 , wherein the dielectric measurement tool is a high frequency dielectric measurement tool (HFDT) operating at a frequency of 1 GHz or greater.