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Gamma Ray and Spontaneous Potential

Schlumberger

Rweq Determination from ESSP
Clean formations

SP-1

This chart and nomograph calculate the equivalent formation water resistivity, R weq, from the static spontaneous potential, E SSP, measurement in clean formations. Enter the nomograph with ESSP in mV, turning through the reservoir temperature in °F or °C to define the R mfeq /R weqratio. From this value, pass through the R mfeq value to define R weq. For predominantly NaCl muds, determine R mfeq as follows: a. If R mf at 75°F (24°C) is greater than 0.1 ohm-m, correct R mf to formation temperature using Chart Gen-9, and use R mfeq = 0.85 R mf. b. If R mf at 75°F (24°C) is less than 0.1 ohm-m, use Chart SP-2 to derive a value of R mfeq at formation temperature.

Example: SSP =100 mV at 250°F R mf = 0.70 ohm-m at 100°F or 0.33 ohm-m at 250°F Therefore, R mfeq = 0.85 × 0.33 = 0.28 ohm-m at 250°F R weq = 0.025 ohm-m at 250°F E SSP = –K c log(R mfeq /R weq ) K C = 61 + 0.133 T°F K C = 65 + 0.24 T°C

Rweq (ohm-m) 0.001

SP

Rmfeq /Rweq 0.3 0.4 0.5 0.6 0.8 1 0.3 0.4 0.6 0.8 1 0.02 0.04 0.06 2 aw /amf or Rmfe /Rwe 3 4 5 6 8 10
F 0° F 50 0° 40 0°F C C 0° 30 25 00° C 0°F2 0° 20 15 0°C °F 10 100 C 50° 0°C

0.005

Rmfeq (ohm-m) 0.01

0.01

0.02

2

0.1 0.2

0.05

4 6

0.4 0.6 1 0.1

8 10 2 4 6 20 10 20 40 40 60 100 2.0 1.0 0.5 0.2

20 30 40 50 +50 0 –50 Formation temperature

–100

–150

–200

ESSP, static spontaneous potential (mV)
© Schlumberger

2-5

Gamma Ray and Spontaneous Potential

Schlumberger

Rw versus Rweq andFormation Temperature

SP-2 (English)

0.001 500°F 400°F 0.002 300°F 200°F 150°F 0.005 100°F 75°F 0.01 Saturation 0.02 R weq or R mfeq (ohm-m)

SP

0.05

0.1

0.2
500° F 400 °F 300 °F 200 °F 150 °F 100 °F 75° F

0.5

F 5° t7 la aC N

1.0

2.0 0.005

0.01

0.02 0.03

0.05

0.1

0.2

0.3

0.5

1.0

2

3

4 5

R w or Rmf (ohm-m)
© Schlumberger

Thesecharts convert equivalent water resistivity, R weq , from Chart SP-1 to actual water resistivity, R w. They may also be used to convert R mf to R mfeq in saline muds. Use the solid lines for predominantly NaCl waters. The dashed lines are approximate for “average” fresh formation waters (where effects of salts other than NaCl become significant). The dashed portions may also be used for gyp-base mudfiltrates.

Example: R weq = 0.025 ohm-m at 120°C From chart, R w = 0.031 ohm-m at 120°C Special procedures for muds containing Ca or Mg in solution are discussed in Reference 3. Lime-base muds usually have a negligible amount of Ca in solution; they may be treated as regular mud types.

2-6

Gamma Ray and Spontaneous Potential

Schlumberger

Rw versus Rweq and Formation Temperature

SP-2m(Metric)

0.001 250°C 200°C 0.002 150°C 100°C 75°C 0.005 50°C 25°C Saturation 0.02 R weq or R mfeq (ohm-m)

SP

0.01

0.05

0.1

0.2
250° C 200 °C 150 °C 100 ° 75° C C 50° C 25° C

0.5

C 5° t2 la aC N

1.0

2.0 0.005

0.01

0.02 0.03

0.05

0.1

0.2

0.3

0.5

1.0

2

3

4 5

R w or Rmf (ohm-m)
© Schlumberger

2-7

Gamma Ray and SpontaneousPotential

Schlumberger

SP Correction Charts
For representative cases

SP-3

No invasion R xo = 0.2 R t Rs =1 Rm
1.0
1 52 10 20

Invasion, di /dh = 5 R xo = R t
1.0
0.1 0.2 0.5 1 2 5

R xo = 5 R t
1.0
0.5 21 5 10

1.0

0.8

0.8

0.8

0.8

SP
ESP /ESPcor
0.6

0.6
50

0.5 1

0.6
10 20

0.6
20

0.4 0.2

0.4 0.2

2 5 10 20 50

0.4 0.2

0.4
50

100200

50

0.2
100

100 200

R t /Rm

Rxo /Rm

100

Rxo /Rm

200

Rxo /Rm

40 30 20 15 10 7.5

5

40 30 20 15 10 7.5

5

40 30 20 15 10 7.5 1.0

5

40 30 20 15 10 7.5 1.0
1 5 2

5

Rs =5 Rm

h/dh
1.0
5 10 2

h/dh
1.0
0.2 0.5 1 2

0.8
20

0.8

0.5 1

0.8

5 10 20

0.8
10 20

ESP /ESPcor

0.6
50

0.6

2 5

0.6

0.6

0.4...
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