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