Title: Use of Resistivity Logs as a Tool for Estimating Interparticle Porosity
1Use of Resistivity Logs as a Tool for Estimating
Interparticle Porosity
2Outline 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
3Setting 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
4Setting 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
5How 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
6How 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.
7A short derivation
- Water saturation at some depth in from the
wellbore, Si, is calculated fromSin
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
8A 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)
9A short derivation
- F 1 / fm
- Swn fm rw / Rt
- Sw f (Rw / Rt)1/2
- Sw rw / rmf / Rt / Rxo 1/(2(1 .2))
f (Rmf / Rxo)1/2 (Rt Rmf) / (Rw Rxo)1.25
10An example from the Clearfork
11An example from the Clearfork
12An example from the Clearfork
13An example from the Clearfork
14An example from the Clearfork
15An example from the Clearfork
16An example from the Clearfork
17An example from the Clearfork
18An example from the Clearfork
19An example from the Clearfork
20Some 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
21To 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