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 weq ratio. 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.
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
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. 0.005
Rmfeq /Rweq 0.3
0.3
0.4
0.4
0.5 0.6
0.6
0.8
0.8
1
1
Rmfeq (ohm-m) 0.01
0.01
0.02
0.02
0.04 0.06 aw /amf or Rmfe /Rwe
2
2
3
0.1
0.05
0.2
4
4
5 6
6
8
8
10
10
0.4 0.6
0.1
1
30
Formation temperature
40 50 +50
0
–50
F 0° F 50 0° 40 0°F C C 0° 30 25 00° C °F 2 00 0° 2 15 0°C °F 10 100 C 50° 0°C
