Page 157 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

146 Reservoir Engineering

where -K = -(0.133T + 61)(T in OF)
a, = chemical activity of formation water
a, = chemical activity of drilling mud filtrate
Since the chemical activity of a solution cannot be used, it must be converted
to its equivalent electrical resistivity. Chemical activity of a fluid is approximately
equal to the inverse of its equivalent electrical resistivity. Conversion to equivalent
resistivities makes the equation:

E, = -Klogh (5-95)
R,,

Since Ec is equal to the maximum SP deflection recorded on a log (SSP),
Equation 5-95 can be rewritten to read

(5-96)

where SSP is the static (or maximum) spontaneous potential recorded opposite
a permeable formation. Since the purpose of an SP log is to find Rwq and then
Rw, if we know SSP we can solve Equation 5-96 for Rwq:

(5-97)

Once R,, is known, it is converted to R, using the chart shown in Figure 5-71
[199]. SSP can come directly from the log if the bed is thick and the SP curve
reaches a ‘constant value and develops a “flat top.” If the curve is pointed or
rounded, it must be corrected for bed thickness.
The shape and amplitude of the SP are affected by:

1. Thickness and resistivity of the permeable bed (RJ.
2. Diameter of invasion and resistivity of flushed zone (RJ
3. Resistivity of the adjacent shales (R,).
4. Resistivity of the mud (R,).
5. Borehole diameter (4)

All of these must be accounted for when examining the SP, and any necessary
corrections should be made. To find the magnitude of the SP, take the maximum
deflection from the average shale value (shale baseline) to the most negative
value. (Figure 5-72 shows a curve that needs correction and one that does not.)
Bed thickness corrections can be made from Figure 5 73 and should always
increase the magnitude of the SP.
Another use for the SP log is finding permeable zones. Any negative deflec-
tion of the curve indicates a potentially permeable zone. The magnitude of the
deflection has no relation to the amount of permeability (in millidarcies); it
merely indicates that the rock has ionic permeability. No quantitative information
on this parameter can be derived from the SP. Figure 574 shows an example
of permeable and impermeable zones on an SP log.