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Use of Resistivity Logs as a Tool for Estimating Interparticle Porosity

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

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  1. Use of Resistivity Logs as a Tool for Estimating Interparticle Porosity

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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.

  7. 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

  8. 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)

  9. 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

  10. An example from the Clearfork

  11. An example from the Clearfork

  12. An example from the Clearfork

  13. An example from the Clearfork

  14. An example from the Clearfork

  15. An example from the Clearfork

  16. An example from the Clearfork

  17. An example from the Clearfork

  18. An example from the Clearfork

  19. An example from the Clearfork

  20. 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

  21. 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

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