WELLOG STATIC SP (SSP)
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What causes Static SP or SSP?
SSP results from the contrast between the resistivity (actually the ionic activity) of the mud and the formation fluid.
SSP in millivolts is calculated as follows:
SSP = -(60.7 + 0.132 * T) * Log10 (Rmfe/Rwe)
Where T is temperature in degrees F.
SSP = -(64.9 + 0.238 * T) * Log10 (Rmfe/Rwe)
Where T is temperature in degrees C.
K is determined mathematically as (60.7 + 0.132 * T) or (64.9 + 0.0238 * T)
The above equations apply to predominantly NaCl formation fluids. The constants used apply to Na+ and Cl- transport properties.
The equations vary slightly.
Note also Rwe equals Rw in pure NaCl solutions at greater than 0.3 ohm-meters.
The equations above provide a workable method for determination of Rw from SSP values but only if the measurement is made properly. If the SP electrode is not isolated from other metallic elements and from stray currents, the measurement will not be valid.
View a chart that shows the effect of rw on sp.
View a chart that determines the Static SP from Rwe.
View a chart that shows SP correction.
View a chart that shows SP bed correction.
Reference: Well Logging For Physical Properties, Joseph R. Hearst, Philip H. Nelson, McGraw-Hill Book Company, 1985
Other references cited by the above:
MacInnes, 1961, Sato & Mooney, 1960, Pirson, 1977, Gondouin et al, 1957, Schlumberger, 1972
Birdwell Interpretation Manual (April 1973)
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