ELECTRIC LOG, ALSO REFERRED TO AS AN ELOG OR E-LOG
The Electric Log consists of the spontaneous potential (SP) curve, two resistivity curves of varying depths of investigation (the short-normal and long-normal resistivity curves), and the single-point resistance curve, along with a navigation package to determine borehole attitude. The electric log can aid in facies identification by assigning signature responses associated with gravel, sand and clay, to name a few. In freshwater formations of sedimentary material, the resistivity measured with the Electric Log is primarily a function of grain size, distribution, and sorting. Knowing this relationship helps in understanding basin characteristics and can be used in models along with supporting well design.
Spontaneous-Potential Log (SP):
This method records naturally occurring voltages when a borehole penetrate sands and clays, where a differences in ionic concentration and properties are found between the drilling fluid and the fluid within the pore space of the rock matrix. Spontaneous potential logs can evaluate lithology, water quality for specific ions, and indicate permeability.
Short-Normal and Long-Normal Resistivity Curves (RSN & RLN):
Placing a potential electrode at a known distance from the current electrode (which emits an oscillating DC current), a voltage can be measured resulting from the apparent resistivity of the surrounding rock. The longer the distance from the current electrode to the potential electrode, the greater the penetration of the galvanic current, hence the deeper into the formation the measured voltage represents.
There are two types of resistivity curves typically used in electric logging; those of shallow penetration and those of deep penetration. The shallow typically has a distance of 16 inches, and the deep is 64 inches from the current electrode. Suppose the invading drilling fluid (Rm) resistivity differs from the formation fluid (Rw). In that case, the resistivity measured by both electrodes will be influenced by the depth of that invading fluid. In typical groundwater environments, where Rw is very close to Rm, the apparent resistivity is more a function of grain size, sorting of grains, and distribution.
Single Point Resistance Curve (SPR):
A linear measurement of the resistance to the current flow from the excitation electrode at the borehole wall. Since this is a point measurement, it can resolve thin beds that the normal resistivity logs may have difficulty resolving due to their respective electrode spacing. A relative measurement in ohms can be highly influenced by borehole diameter and Rm.
Our resistivity tool comprises 9 separate measurements:
• SP Spontaneous Potential
• RSN Short Normal Resistivity (16-inch spacing)
• RLN Long Normal Resistivity (64-inch spacing)
• CSN Short Normal Conductivity
• CLN Long Normal Conductivity
• SPR Single Point Resistance (Point)
• GR Gamma Ray
• Inclination (borehole verticality)
• Direction (borehole direction)
Conventional Elog tools set the excitation current at a constant density. We designed the current electrode to provide a greater dynamic range by permitting current density to change with formation type while simultaneously measuring the potential at the short and long normal electrodes to derive formation resistivity.