Gen

Basic Material

Estimation of Rmf and Rmc

Schlumberger

Gen-7

1-3

Direct measurements of filtrate and mudcake samples are pre-ferred. When not available, filtrate resistivity, Rmf, and mudcakeresistivity, Rmc, may be estimated from one of the followingmethods.

Method 1

Lowe and Dunlap (Reference 36)

For freshwater muds with mud resistivity, Rm, in the range from0.1 to 2.0 ohm-m at 75°F [24°C], and measured values of Rm

and mud density, ρm, in pounds per gallon:

Method 2

Overton and Lipson (Reference 1)

For drilling muds with mud resistivity, Rm, in the range from 0.1to 10.0 ohm-m at 75°F [24°C], where Km is given as a functionof mud weight in the table below:

Example: Rm = 3.5 ohm-m at 75°F [24°C]

Mud weight = 12 lbm/gal [1440 kg/m3]

Therefore, Km = 0.584

Rmf = (0.584)(3.5)1.07= 2.23 ohm-m at 75°F

Rmc = 0.69(2.23)(3.5/2.23)2.65= 5.07 ohm-m at 75°F

The calculated value of Rmf is more reliable than that of Rmc.

Method 3

A statistical approximation, for predominantly NaCl muds, is Rmc = 1.5 Rm, and Rmf = 0.75 Rm.

R K R

R RR

R

mf m m

mc mfm

mf

=

=

( )

. ( )

.

.

1 07

2 65

0 69

log . .R

Rmf

mm

= −0 396 0 0475 ρ

Mud Weight

lbm/gal kg/m3 K m

10 1200 0.84711 1320 0.70812 1440 0.58413 1560 0.48814 1680 0.41216 1920 0.38018 2160 0.350