Registros gamma ray

Gamma Ray—Wireline

Scintillation Gamma Ray—33⁄8- and 111⁄16-in. Tools
Gamma Ray Correction for Hole Size and Barite Mud Weight

GR–1
(former GR-1)

Scintillation Gamma Ray 10.0

7.0

GR

5.0

3 3⁄8-in. tool, centered

3.0
111⁄16-in. tool, centered

2.0 Correction factor
3 3⁄8-in. tool, eccentered

1.0

111⁄16-in. tool, eccentered

0.7 0.5

0.3 0
© Schlumberger

510

15

20 t (g/cm )
2

25

30

35

40

Purpose This chart provides a correction factor for measured values of formation gamma ray (GR) in gAPI units. The corrected GR values can be used to determine shale volume corrections for calculating water saturation in shaly sands. Description The semilog chart has the t factor on the x-axis and the correction factor on the y-axis. Theinput parameter, t, in g/cm2, is calculated as follows: t= 2.54 d sonde ⎞ Wmud ⎛ 2.54 d h − ⎜ ⎟, ⎟ 8.345 ⎜ 2 2 ⎝ ⎠

Example Given: Find: Answer:

GR = 36 API units (gAPI), dh = 12 in., mud weight = 12 lbm/gal, tool OD = 33⁄8 in., and the tool is centered. Corrected GR value. 2.54 3.375 12 ⎛ 2.54 12 − ⎜ 8.345 ⎜ 2 2 ⎝

t=

( )

(

)⎞ = 15.8 g /cm2. 8 ⎟
⎟ ⎠

( )

(

)

Enter thechart at 15.8 on the x-axis and move upward to intersect the 33⁄8-in. centered curve. The corresponding correction factor is 1.6. 1.6 × 36 gAPI = 58 gAPI.

where Wmud = mud weight (lbm/gal) dh = diameter of wellbore (in.) dsonde = outside diameter (OD) of tool (in.).
25

Gamma Ray—Wireline

Scintillation Gamma Ray—33⁄8- and 111⁄16-in. Tools
Gamma Ray Correction for Barite Mud inVarious-Size Boreholes

GR-2
(former GR-2)

1.2 1.0

GR
0.8 Bmud 0.6

111⁄16-in. tool, centered

111⁄16-in. tool, eccentered

0.4
33⁄8-in. tool, centered

0.2
33⁄8-in. tool, eccentered

0 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Mud weight (lbm/gal)

1.2 1.0 0.8 Fbh 0.6 0.4 0.2 0 0
© Schlumberger

33⁄8-in. tool 111⁄16-in. tool

1

2

3

4

5 dh – dsonde (in.)

6

7

89

10

Purpose These charts are used to further correct the GR reading for various borehole sizes. Description Two components needed to complete correction of the GR reading are determined with these charts: barite mud factor (Bmud) and borehole function factor (Fbh). Example Given: Borehole diameter = 6.0 in., tool OD = 33⁄8 in., the tool is centered, mud weight = 12 lbm/gal, measured GR =36 gAPI. Corrected GR value. Enter the upper chart for Bmud versus mud weight at 12 lbm/gal on the x-axis. The intersection point with the 33⁄8-in. centered curve is Bmud < 0.15 on the y-axis. Determine (dh – dsonde) as 6 – 3.375 = 2.625 in. and enter

that value on the lower chart for Fbh versus (dh – dsonde) on the x-axis. Move upward to intersect the 3 3⁄8-in. curve, at which Fbh = 0.81.Determine the new value of t using the equation from Chart GR-1: t= 2.54 d sonde Wmud ⎛ 2.54 d h − ⎜ ⎜ 8.345 ⎝ 2 2 12 ⎛ 2.54 6 2.54 3.375 − ⎜ 8.345 ⎜ 2 2 ⎝

( )

(

)⎞ ⎟
⎟ ⎠

=

()

(

) ⎞ = 4.8 g /cm2. ⎟
⎟ ⎠

Find: Answer:

The correction factor determined from Chart GR-1 is 0.95. The complete correction factor is (Chart GR-1 correction factor) × [1 + (Bmud × Fbh)] = 1.12 × [1 +(0.15 × 0.81)] = 1.26. Corrected GR = 36 × 1.26 = 45.4 gAPI.

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Gamma Ray—Wireline

Scintillation Gamma Ray—33⁄8- and 111⁄16-in. Tools
Borehole Correction for Cased Hole

GR-3
(former GR-3)

Scintillation Gamma Ray 10.0

GR
7.0 5.0

3.0

33⁄8-in. tool 111⁄16-in. tool

2.0 Correction factor

1.0

0.7 0.5

0.3 0 5 10 15 20 t (g/cm2)
© Schlumberger

25

30

3540

Purpose This chart is used to compensate for the effects of the casing, cement sheath, and borehole fluid on the GR count rate. The correction brings the cased hole count rate in line with the measured openhole GR count rate. Description In small boreholes the count rate can be too large, and in larger boreholes the count rate can be too small. The chart is based on laboratory work and...