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)*