Michael Holmes
Geological/Petroleum Engineering Consultant
Denver, Colorado
Most methods for calculating water resistivity (Rw) from logs rely either on the SP log or on extrapolation from known or assumed water–bearing formations. The method presented in this paper can be applied in hydrocarbon-bearing formations; estimates of water resistivity can be made even though no water-bearing zones exist. Confidence in the calculations improves if water-bearing zones can be identified.
Ratio methods of flushed zone resistivity (Rxo) to uninvaded zone resistivity (Rt) are frequently used to identify water layers. High values exist when water saturation (Sw) is unity, because the ratio of Sw to flushed zone saturation (Sxo) is a maximum. However, if a hydrocarbon-bearing interval has not been invaded by mud filtrate (low-permeability formation) then, again, the ratio Sw:Sxo (and hence
Rxo:Rt) reaches a maximum value. If mud filtrate resistivity (Rmf) is known, Rw can be calculated.
Computational procedures are as follows:
1. Read Rt and Rxo from logs throughout the interval (or R deep and R shallow
as approximations).
2. Calculate apparent water resistivity, Rwa, from
Rwa = Rt / F or Rdeep / F, where F = Formation Factor and
Rwa = Rw in water zone.