GeolOil - How to estimate Effective Porosity from Rt Resistivity in water zones

⚠ REMARKS: The described technique require good parameter estimates, specially for Rw formation water resistivity:

1. Formation water resistivity Rw is probably the most important parameter in the equations. If feasible, compute the equivalent NaCl ionic salinity from reliable water analyses. Then convert salinity to Rw at reservoir temperature to get a quality estimate for Rw. It does not make much sense to apply a Pickett-Plot on the water body which already is using a porosity estimate. That would be a "dead-lock" circular computation.
   
   
2. The porosity cementation exponent m is also quite influential for the porosity estimation. Some eolian sandstones may have a low m value, like m=1.5, which yields different results tham the typical m=1.8 used as default for some sandstones. Likewise, carbonates can have large values of m. Values like m=2.4 or more are common for carbonates rocks. Again, avoid a circular computation of m as commented before.
   
   
3. If the drilling mud is not too much salty, deep induction resistivity maybe robust against washouts and poor boreholes. That means that the estimated porosity in water bodies, will be also robust against bad boreholes and rugosity. Such quality estimate on poor boreholes can not be usually calculated from sources like bulk density, sonic transit time, and even neutron porosity.
   
   
4. The porosity estimates are only valid for water bodies, like aquifers and water pocket compartments. For pay zones, the porosity estimates will severely underestimate the correct porosity. In the log plot shown above, the red PhiE_Rt curve yields a porosity which is 50% or more, smaller than the correct porosity. Thus, an unusable porosity curve for hydrocarbon zones. The assumption that SW ≅ 1 no longer holds.
   
   
5. Notice that the rock has a low Vshale, and normal porosities in the range 8%-16%. However, the irreducible water saturation SWirr is quite high, which was measured through laboratory capillary pressure tests. This is because the permeability is low due to a small pore throat size, which drives the connate water saturation. Rocks with a same porosity can have different pore throat diameters, and hence different permeabilities.
   
   
6. The computed estimated porosity is already the effective porosity. No further corrections for clay content are needed. ∎

Sugestion: It is always a good idea to search for water pockets and compute the effective porosity filtered for those water zones (even if they are thin of few feet). That would provide another reference computation for porosity, to validate and compare against classic estimates from bulk density, sonic transit time, and neutron porosity.