Spontaneous Potential

The SP is a passive measurement of very small electrical voltages resulting from electrical currents in the borehole caused by the differences in the salinities (resistivities) of the formation connate water (Rw) and the drilling mud filtrate (Rmf), and by the presence of ion selective shale beds. The voltage changes are measured by a downhole electrode relative to a surface ground. The unit of measurement is millivolts (mV)

Unlike other logging tools which are displayed on a specific scale with a specified reference value, the SP has no specified origin and values used for computation are referenced to deflection from the nearby shale baseline established by the interpreter.

Interpretation Goals:

1. Correlation of formations from well to well
2. Gross lithology 
3. Formation water resistivity (Rw) estimation 
4. Shale/clay content estimation 
5. Qualitative indication of permeability 
6. Identification of depositional environments 

FORMATION WATER RESISTIVITY (Rw) ESTIMATION FROM SP

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

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

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)

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).

•  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).

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)

 

GROSS LITHOLOGY

Reservoirs are shown as deflections (either positive or negative) from a relatively stable (and arbitrary) shale baseline. The direction of the deflection is determined by the relative salinities (resistivities) of the formation water (Rw) and the mud filtrate (Rmf), and is not directly related to formation porosity or permeability.
As a rule of thumb the following relationships are true:
If Rmf > Rw, then the SP deflection is negative.
If Rmf = Rw, then the SP deflection is zero.
If Rmf < Rw, then the SP deflection is positive.

CORRELATION OF FORMATIONS

SP Curves are scanned for similarities in shape and magnitude.