Spontaneous potential logging is a method of determining the composition of underground materials that are being drilled into, usually in the petroleum exploration field. The process of spontaneous potential logging involves recording the direct current electrical voltage that exists between a water-based fluid like drilling mud that is pumped down a borehole, and the walls of the borehole itself. This data is often referred to as a self potential log (SP log), and is a recording of differences in electrical potential in the range of millivolts between a sub-surface layer and the voltage potential by grounding at the top of the drilling site. Common uses for spontaneous potential logging aside from oil exploration include determining the lithology or rock characteristics of a drilled hole in mineral exploration, and underground water quality for municipal purposes.
The use of spontaneous potential logging is considered to be one of the earliest of methods in drilling exploration for determining the nature of underground terrain. It relies on the natural static electric charge that the ground itself holds. This charge is separated into potential ranges by spaces in porous rock or through conduction by salt-based fluids, and a water-based fluid must be introduced into the borehole to make a connection to the native charge that can then be channeled to the surface. Generally, the higher the electrical potential that is detected, the more permeable the sub-surface layer is, but the magnitude of deflection also depends on the salinity content of the drilling mud being used and that of formation water that occurs naturally within the underground rock layers.
Well logging relies on both the properties of the drilling mud to be ionic in nature, or have electrically-charged atoms, as well as there to be clay or mineral content in the borehole for spontaneous potential logging to take place. The ionic charge in the drilling mud is used to conduct a signal back to the surface. The presence of some level of clay or minerals in the hole are necessary, as their crystalline structure allows for a semipermeable structure to form. This structure impedes the diffusion of ions in the underground strata so that a natural charge state is maintained.
While the procedure for conducting spontaneous potential logging can be fairly routine, the interpretation of the data can be difficult. This is because, under certain drilling conditions such as in underground aquifers, where shale, clay and sand beds meet and merge, the data can be interpreted differently. Freshwater sites in particular are known to produce widely-varying readings based in some part on variations in the type of drilling mud used and its salinity in relation to the groundwater itself. A reading of negative SP potential is usually recorded in oil well exploration, but in freshwater wells the result is usually a positive SP reading indicating the presence of sand layers. The spontaneous potential log can also be one of zero if both the drilling mud and groundwater have the same electrical potential, which can lead to confusion about the true nature of sub-surface terrain.
In petroleum production, the use of spontaneous potential logging is more reliable, as the process relies on salinity for good readings. With freshwater, sodium chloride needs to be present in both the drilling mud or borehole fluid used as well as in the natural formation water, but, ideally, the salinity of the formation water should be considerably higher. The method of using spontaneous potential logging to gain an understanding of underground features is, therefore, best limited to regions where the sand or shale that are present have a high salinity content.