Spontaneous potential logging

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The Spontaneous Potential log, commonly called the self-potential log or SP log, is a measurement taken by oil industry well loggers to characterise rock formation properties. The log works by measuring small electric potentials (measured in millivolts) between depths in the borehole and a grounded voltage at the surface.

The change in voltage through the well bore is caused by a buildup of charge on the well bore walls. Clays and shales (which are composed predominantly of clays) will generate one charge and permeable formations such as sandstone will generate an opposite one. This build up of charge is, in turn, caused by differences in the salt content of the well bore fluid (drilling mud) and the formation water (connate water). Whether the mud contains more or less salt than the connate water will determine which way the SP curve will go. The amplitudes of the line made by the changing SP will vary from formation to formation and will not give a definitive answer to how permeable or the porosity of the formation that it is logging.

The SP tool is one of the simplest tools and is generally run as standard when logging a hole along with the gamma ray. SP data can be used to find

Where the permeable formations are
The boundaries of these formations
Correlation of formations when compared with data from other analogue wells
Values for the formation-water resistivity

The SP curve can be influenced by various factors both in the formation and introduced into the wellbore by the drilling process. These factors can cause the SP curve to be muted or even inverted depending on the situation.

Formation bed thickness
Resistivities in the formation bed and the adjacent formations
Resistivity and make up of the drilling mud
Wellbore diameter
The depth of invasion by the drilling mud into the formation

The drilling mud salinity will effect the strength of the electromotive forces (EMF) which give the SP deflections. IF the salinity of the mud is similar to the formation water then the SP curve is dampened, if it more saline then the curve is reversed while if it is less the deflection is broader. In rare cases the baseline of the SP can shift if the salinity of the mud changes part way down hole.

Mud invasion into the permeable formation can cause the deflections in the SP curve to be rounded off and to reduce the amplitude of thin beds.

A larger wellbore will cause, like a mud filtrate invasion, the deflections on the SP curve to be rounded off and decrease the amplitude opposite thin beds, while a smaller diameter wellbore has the opposite effect.