GeolOil - The SW ratio method: Water Saturation from Flushed-Zone Rxo Resistivity

PROOF: (For the interested reader) The former equation is derived by applying the Archie equation for clean reservoirs —non shaly, non conductive matrix—, both in the non invaded zone (far beyond the zone invaded by the drilling mud), and in the zone invaded by the mud (close to the borehole wall):

⚠ REMARKS: Both the original Archie SW ratio equation, and our extension introduced above with a correction for VSH are appealing. However, they require good quality data and a stable borehole to work, otherwise the results may be misleading:

1. The results are very sensitive to the quality of the Rxo log curve —also to Rw and Rt—. We occasionally have had good results with curves with the mnemonic MSFL (Micro Spherical Focused Log), but not always. Other common curve mnemonics to try from service companies are: RXO, RXOZ, MCFL, SFLCC, and MGL.
   
   
2. Discard regular short resistivity curves as replacements for Rxo. They are not close enough to the flushed zone to work.
   
   
3. The mud filtrate resistivity Rmf needs to be measured accurately, and corrected to the reservoir temperature. Don't expect that a value of Rmf measured on one well can be applied to another well.
   
   
4. The assumed default values for the parameters β=1/5, and θ=0.30, may be not close enough. Rather than constants, they may well behave better as curve themselves. Feel free to change them and evaluate the results.
   
   
5. In most of the cases, take the water saturation values computed as qualitative and look for trends. In few cases the calculated SW curve matches nicely with regular SW estimates and core data, which validates the SW computations. ∎