Natural logging potential is a method of determining underground material composition in petroleum and mineral exploration. It involves recording electrical voltage between drilling mud and well walls. The data is used to determine lithology, groundwater quality, and subsurface permeability. The method relies on the natural static electric charge of the soil and requires a water-based fluid to create a link with the native charge. Data interpretation can be difficult, especially in freshwater sites. The method is more reliable in oil production and should be limited to regions with high salinity content.
Natural logging potential is a method of determining the composition of underground materials that are being drilled, usually in the field of petroleum exploration. The process of recording spontaneous potential involves recording the direct current electrical voltage that exists between a water-based fluid such as drilling mud being pumped down a well and the walls of the well itself. This data is often referred to as a self-contained potential log (log SP) and is a record of electrical potential differences in the millivolt range between a subsurface layer and the grounded voltage potential at the top of the site. drilling. Common uses for potential spontaneous logging in addition to petroleum exploration include determining the lithology or rock characteristics of a hole drilled in mineral exploration and groundwater quality for municipal purposes.
The use of potential spontaneous logging is considered to be one of the first methods in drilling exploration to determine the nature of the underground terrain. It is based on the natural static electric charge that the soil itself contains. This charge is separated into potential gaps by gaps in the porous rock or through the conduction of salt-based fluids, and a water-based fluid must be introduced into the well to create a link with the native charge which can then be channeled to the surface. . Generally, the higher the electrical potential detected, the more permeable the subsurface layer, but the extent of the deflection also depends on the salinity content of the drilling mud used and on that of the formation water which occurs naturally at the interior of underground rock layers.
Well logging is based both on the properties of the drilling mud of being ionic in nature, or having electrically charged atoms, and on the presence of clay or mineral content in the well for potential spontaneous logging. The ionic charge in the drilling mud is used to carry a signal back to the surface. A certain level of clay or minerals in the hole is required, as their crystalline structure allows for the formation of a semi-permeable structure. This structure prevents the diffusion of ions into the underground layers in order to maintain a natural state of charge.
While the procedure for conducting spontaneous potential registration can be fairly routine, data interpretation can be difficult. This is because, under certain drilling conditions such as in underground aquifers, where beds of shale, clay and sand meet and merge, the data can be interpreted differently. In particular, freshwater sites are known to produce highly variable readings based in part on variations in the type of drilling mud used and its salinity in relation to the groundwater itself. A negative SP potential reading 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 register may even be one of zero if both the drilling mud and groundwater have the same electrical potential, which can lead to confusion as to the true nature of the ground below the surface.
In oil production, the use of spontaneous potential logging is more reliable, as the process relies on salinity to get good readings. With soft water, sodium chloride should be present in both the drilling mud or drilling fluid used and in the natural formation water, but ideally the formation water salinity should be considerably higher. The method of using potential natural logging to gain an understanding of subsurface characteristics is, therefore, best limited to regions where the sand or shale present has a high salinity content.
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