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Jeffrey A. Kane, BEG, 2003
Use of Resistivity Logs as a Tool for Estimating
Interparticle Porosity
Jeffrey A. Kane, BEG, 2003
Outline of talk
• A little bit about why
• How can we do it?– A short derivation
• An example from the Clearfork
• Some caveats
• Finish it up
Jeffrey A. Kane, BEG, 2003
Setting the stage
• We can model permeability from interparticle porosity and petrophysical class
• The only tool in current usage to get to interparticle porosity is the sonic
• Sonics are, unfortunately, an uncommon log
Jeffrey A. Kane, BEG, 2003
Setting the stage
• The resistivity log tends to respond to separate vug porosity in much the same fashion as the sonic tool– It tends to “ignore” the larger pore bodies
we refer to as separate vugs
Jeffrey A. Kane, BEG, 2003
How can we do it?
• We will assume that the interparticle portion of the porosity will behave as an “Archie type rock”.– This means that we can assume for the
interparticle porosity that the porosity exponent (m) and the saturation exponent (n) are both close to 2
Jeffrey A. Kane, BEG, 2003
How can we do it?
• We will also assume that the ratio water saturation provides us with a valid water saturation estimation in the interparticle porosity.
Jeffrey A. Kane, BEG, 2003
A short derivation
• Water saturation at some depth in from the wellbore, Si, is calculated from:
Sin = Frz/Ri
where Si is the water saturation, S, at some depth of investigation, i, Ri is the associated
formation resistivity at that depth, F is the formation factor, and rz is the apparent water
resistivity at that depth of investigation
Jeffrey A. Kane, BEG, 2003
A short derivation
• Swn = Frw / Rt, Sxo
n = Frmf / Rxo
• (Sw / Sxo)n = [rw / rmf] / [Rt / Rxo]
• Sxo = Swr
• (Sw / Swr)n = [rw / rmf] / [Rt / Rxo]
• Sw(1 - r)n = [rw / rmf] / [Rt / Rxo]
Sw = { [rw / rmf] / [Rt / Rxo] }1/((1 – r)n)
Jeffrey A. Kane, BEG, 2003
A short derivation
• Swn = Frw / Rt
• F = 1 / m
• Swn m = rw / Rt
• Sw = (Rw / Rt)
1/2
• Sw = { [rw / rmf] / [Rt / Rxo] }1/(2(1 – .2))
= (Rmf / Rxo)1/2 [(Rt Rmf) / (Rw Rxo)]
1.25
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
An example from the Clearfork
Jeffrey A. Kane, BEG, 2003
• Technique is sensitive to the ratio of Rmf to Rw
Some pitfalls
• Environmental corrections are important
• Micro-porosity or intra-particle porosity can be an issue
Jeffrey A. Kane, BEG, 2003
To finish up
• We have described a way to enhance the use of resistivity logs in characterizing porosity
• It allows us to not only estimate permeability, but allows us to look into water saturation variations
• A good understanding of the rocks is critical to making this work