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Use of Resistivity Logs as a Tool for Estimating Interparticle Porosity. 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. Setting the stage.
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Use of Resistivity Logs as a Tool for Estimating Interparticle Porosity
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
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
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
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
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.
A short derivation • Water saturation at some depth in from the wellbore, Si, is calculated from:Sin = Frz/Riwhere 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
A short derivation • Swn = Frw / Rt, Sxon = 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)
A short derivation • Swn = Frw / Rt • F = 1 / fm • Swn fm = rw / Rt • Swf = (Rw / Rt)1/2 • Sw = { [rw / rmf] / [Rt / Rxo] }1/(2(1 – .2)) f = (Rmf / Rxo)1/2 [(Rt Rmf) / (Rw Rxo)]1.25
Some pitfalls • Technique is sensitive to the ratio of Rmf to Rw • Environmental corrections are important • Micro-porosity or intra-particle porosity can be an issue
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