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How to estimate Porosity from Flushed-Zone Rxo Resistivity


Porosity can be estimated from the log curve of flushed zone resistivity Rxo. Since the drilling mud invades the zone closest to the borehole wall, it displaces most of the hydrocarbons, so the local hydrocarbon saturation in such small zone can drop easily to around 20% or less.


The Archie equation can be applied then to this invaded zone —for now let's assume that the rock matrix is clean, non-clayey, and non conductive—. Replacing the deep, true far field uninvaded resistivity Rt with the flushed zone resistivity Rxo, and replacing the formation water resistivity Rw with the mud filtrate resistivity Rmf (which has to be corrected to the depth temperature), and solving for the porosity φ:


Equation to estimate porosity in clean reservoirs from micro-resistivity



The former equation is appealing. Provided that a good quality curve for Rxo is available in a good quality and stable borehole —occasionally MSFL yields good results—, and Rmf is accurately measured and converted to the depth temperature, the estimator for porosity is useful. This φ estimate is independent of the classics density porosity, neutron porosity, and sonic porosity. It is based only on resistivity and provides yet another in situ estimate, valuable either for validation or even estimation on its own right.


The figure below ↓ shows the GeolOil Panel to estimate porosity on clean reservoirs from Rxo


GeolOil Window Panel to estimate porosity from flushed zone resistivity Rxo



How can we deal with clayey matrices with excess of conductivity? Certainly the Archie equation is not suitable for such cases. In this article —first published in this geoloil.com web-page on April 2020— we introduce a correction to deal with shaly reservoirs. Just take the Indonesia equation for water saturation. After following the same algebraic steps, an approximation to estimate the effective porosity φe with a correction for shale content is:


Equation to estimate effective porosity with Vshale correction through the Indonesia formula


The former equation provides a correction for shale content, and may work for moderately shaly rocks (say VSH ≤ 30%). However, it requires knowledge of an additional parameter, the shale resistivity Rsh. If a nearby pure shale can not be found around the study zone —and also we must assume that such shale has a similar nature than the dispersed shales inside the target rock—, the formula is of little use.


One workaround is the use instead the Fertl equation for water saturation as a coarse approximation (it does not depend on Rsh). After following the same algebraic steps, it is found:


Equation to estimate effective porosity with Vshale correction through the Fertl formula



Both former equations can be blended into a unique estimate. Use w=1 if Rshale can be estimated with reasonable accuracy. Use w=0 if Rshale is not reliable at all, and use intermediate values around w=0.5 for a general, more robust estimate under uncertainty:


Equation to estimate porosity from Rxo, by using the Indonesia and Fertl formulas       ∎

REMARKS: The techniques described 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 and the value of Rmf. Occasionally good results are seen with curves 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 shallow resistivity curves as replacements for Rxo. They are not close enough to the flushed zone to work. However, shallow resistivity may work if the mud invasion spans for several feet. In those cases Sxo=0.80 might be a coarse approximartion, and Rmf may represent accurately enough the conductive (equivalent to water Rw) fluid resistivity in the zone.

  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 value for the parameters Sxo=0.80, may be not close enough. Rather than a constant, it may well behave better as curve itself. Feel free to change it and evaluate the results.

  5. In most of the cases, take the porosity estimate as qualitative and look for trends. In some cases the calculated porosity curve matches nicely with regular porosity estimates and core data, which validates the computations.

light bulb idea One robust feature of the estimated in-situ micro-resistivity porosity, is that its value is independent of the fluids nature in the pore space —it only uses resistivity data—. This is specially useful when working with gas reservoirs, for which carefully adjustments have to be made to neutron porosity and density porosity to avoid porosity under-estimation and over-estimation.

The figure below ↓ shows comparisons of porosities from core, density, neutron porosity, sonic, and micro-resistivity porosity


Well log plot with comparisons of porosity estimates from core, density porosity, neutron porosity, sonic porosity, and micro-resistivity porosity

NOTE: The GLOG file work-flow for this log is available for download with the set of optional interpretation examples.




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