SP response equation:
SP = Spontaneous Potential (from the log)
K = temperature-dependent factor (K=61+ 0.133*T; T in °F)
Rmfe = equivalent mud filtrate resistivity
Rwe = equivalent formation water resistivity
K = temperature-dependent factor (K=61+ 0.133*T; T in °F)
Rmfe = equivalent mud filtrate resistivity
Rwe = equivalent formation water resistivity
The magnitude of the SP is measured from the shale baseline near the zone of interest. The baseline is usually assumed to have a value of zero. "Equivalent" resistivities are required to correct for the non-linear relationship between resistivity and ionic activity which exists at high NaCl concentrations, and when significant amounts of divalent (non-NaCl) ions are present.
A good estimate of Rw (at formation temperature) can be obtained from the following equation:
where Rmf is corrected to formation temperature
DETERMINING Rw FROM SP (using Schlumberger chartbooks)
- Identify a zone on the logs which is clean, wet, and permeable.
- Read the SP value at the depth of maximum deflection.
SP = _______ mV at __________ feet.
- Calculate formation temperature (FT) at the depth of the SP value. (Use Schlumberger chart Gen-2 with total depth and maximum temperature from the log heading.)
Total depth (TD) = ______ feet
Formation depth (FD) = ______ feet
Bottom hole temperature (BHT) = ______ °F
Formation temperature (FT) = ______ °F
Annual Mean Surface Temperature (AMST) = ______°F
Formation depth (FD) = ______ feet
Bottom hole temperature (BHT) = ______ °F
Formation temperature (FT) = ______ °F
Annual Mean Surface Temperature (AMST) = ______°F
The following equation can also be used:
- Convert Rmf from surface temperature to formation temperature (use Schlumberger chart Gen-6 with Rmf at measured temperature from the log heading).
Rmf = _______ohm-m @ ________°F (measured temperature)
Rmf = _______ohm-m @ ________°F (formation temperature)
Rmf = _______ohm-m @ ________°F (formation temperature)
The following equation (Arps equation) can also be used:
RFM = fluid resistivity at formation temperature TFM (in °F).
RTk = known resistivity at a known temperature, Tk.
Tk = known temperature (in °F).
RTk = known resistivity at a known temperature, Tk.
Tk = known temperature (in °F).
- Convert Rmf at formation temperature to Rmfeq using one of the following:
a. If Rmf @ 75 °F > 0.1 ohm-m, use Rmfeq = 0.85•Rmf.
b. If Rmf @ 75 °F < 0.1 ohm-m, use Schlumberger chart SP-2.
(a and b are included on Chart SP-1 of the Schlumberger chartbook).
b. If Rmf @ 75 °F < 0.1 ohm-m, use Schlumberger chart SP-2.
(a and b are included on Chart SP-1 of the Schlumberger chartbook).
Rmfeq = ______ohm-m @ ________°F (formation temperature).
- Using SP, formation temperature, and Rmfeq, enter Schlumberger chart SP-1 to find Rweq.
Rweq = _______ohm-m @ _________°F (formation temperature).
The following equation can also be used:
- Convert Rweq to Rw using Schlumberger chart SP-2.
Rw = _______ohm-m @ ________°F (formation temperature).
DETERMINING Rw FROM SP (using Western Atlas chartbooks)
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