PorositySchlumberger

3-6

Por

Environmental Corrections to Formation Density Log,Litho-Density* Log and Sidewall Neutron Porosity Log Por-15a

Under some circumstances, the FDC* Compensated Formation Densitylog and Litho-Density log must be corrected for borehole size, and theSNP sidewall neutron log must be corrected for mudcake thickness.These charts provide those corrections.

For the FDC log, enter the chart with borehole diameter, dh. Go tothe apparent formation density, r b (FDC log density reading), and read,in ordinate, the correction to be added to the FDC log density reading.

Gas-filled holes

Mud-filled holes

0.06

0.05

0.04

0.03

0.02

0.01

10 11 12 13 14 15

150225 250 275 300 325 350 375

69

dh, borehole diameter (mm)FDC Borehole Correction

dh, borehole diameter (in.)

g/cm

3 to

be

adde

dto

FD

C de

nsity

2.62.42.2

2.62.42.2

Apparentformationdensity

0.040.030.020.01

00.010.020.03

5 0 5 10 15 20 25

125 0 125 250 375 500 625

(dh 200)(b m) in metric unitsLitho-Density Borehole Correction

(dh 8)(b m) in English units

g/cm

3 to

be

adde

dto

LD

T de

nsity

SNP Mudcake Correction

0 5 10 15 20 25 30 35 40

0 5 10 15 20 25 30 35 40

SNPcor (p.u.)

SNP (p.u.)

0

10

20

0141234

Mud

cake

thick

ness

(bi

t size

minu

sca

liper

read

ing)

(mm)

(in.)

*Mark of Schlumberger Schlumberger

Example: dh = 12 in.r b = 2.20 g/cm3 (mud-filled borehole)

Therefore, correction = 0.02 g/cm3

r bcor = 2.20 + 0.02 = 2.22 g/cm3

For the LDT log, enter the chart abscissa with theproduct of the borehole diameter, dh, less 8 in. [200 mm]and the LDT density reading, r b, less mud density, r m.Read, in ordinate, the correction to be added to the Litho-Density bulk density reading.

Example: dh = 325 mmr b = 2.45 g/cm3

r m = 1.05 g/cm3

giving (dh 200)( r b r m) =(325 200)(2.45 1.05) = 175

Therefore, correction = 0.014 g/cm3

r bcor = 2.45 + 0.014 = 2.464 g/cm3

Note: If the borehole diameter from the FDC or LDTcaliper is less than bit size, use the bit size in the abovecharts.

For the SNP log, enter the bottom of the chart with theSNP apparent porosity, f SNP; go vertically to the bit sizeminus caliper reading value; then, follow the diagonalcurves to the top edge of the chart to obtain the correctedSNP apparent porosity.

Example: f SNP = 13 p.u.Caliper = 758 in.Bit size = 778 in.

giving Bit size caliper = 778 738 = 14 in.Therefore, f SNPcor = 11.3 p.u.

Note: The full borehole diameter reduction shown on theSNP caliper is used as mudcake thickness, since the SNPbackup shoe usually cuts through the mudcake.