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Formation Water Resistivity (Rw) From Resistivity Ratio

 

The following relationships have been established experimentally and form the basis for quantitative log analysis;

Equation 1 (1)
Equation 2 (2)
where;
F is Formation Factor
Rw is the resistivity of the formation water
Ro is the resistivity of a reservoir rock fully saturated with brine,
Rmf is the resistivity of the mud filtrate
Rxo is the resistivity of the filtrate saturated reservoir

Combining and rearranging equations (1) and (2) we can solve for water resistivity Rw;



Both Ro and Rxo measurements should be corrected for borehole environmental effects using appropriate service company charts. Rmf and the measured temperature are listed in the log header.

Rmf  is corrected to formation temperature using;

for Fahrenheit, or,

for Celcius.
where;
T 1 is the mudfiltrate surface temperature read from the log header,
R1 is the measured mud filtrate resistivity from the log header,
T2 is formation temperature,
R2 is mud filtrate resistivity at formation temperature.

Formation water salinity is estimated from Rw using (for example) Schlumberger Chart Gen-9.



Example Calculation:
Rmf (Surface) 0.101 Ohmm AT 27oC
Rmf (BHT) 0.046 Ohmm AT 84.2o
Ro / Rxo 0.69
Rw 0.032 Ohmm
Brine salinity 95,000 ppm NaCl equivalent


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