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. The electric log can be used to correlate the types of formation materials (e.g., sand, clay, hard rock). In fresh water formations of sedimentary material, the resistivity measured with Electric Log is primarily a function of grain size, grain distribution and sorting.
Spontaneous-Potential Log (SP). This method records potentials or voltages developed between the borehole fluid and the surrounding rock and formation fluids. Spontaneous potential logs can be used in the evaluation of lithology, water quality and permeability.
Short-Normal and Long-Normal Resistivity Curves (RSN & RLN). There are two types of resistivity curves typically used in electric logging; those of shallow penetration and those of deep penetration. The depth of penetration is related to the distance between the current electrode and the potential electrode on the resistivity sonde. The short-normal or 16-inch resistivity curve may be influenced by invasion of the drilling fluid into the formation. The long-normal or 64-inch resistivity curve shows the resistivity of the same zone further away from the borehole, beyond the invaded zone. In permeable soil or rock, the porosity affects the depth of invasion and the dissolved ion content of water in the pores determines the resistivity of the material. The two curves are plotted side by side for comparison.
Single Point Resistance Curve (SPR) is a linear measurement of resistance of the formation at the borehole wall. This method is used to show thin beds that the normal resistivity logs have difficulty resolving due to their respective electrode spacing.
Our primary resistivity tool comprises (5) separate measurements:
- SP Spontaneous Potential
- RSN Short Normal Resistivity
(16 in. Normal)
- RLN Long Normal Resistivity
(64 in. Normal)
- SPR Single Point Resistance
- GR Gamma Ray
Conventional Elog tools set the current electrode at a constant rate. Our Elog’s current electrode provides a greater dynamic range of measurement by permitting current density to change with formation type. Clays, sands, and gravels are very different in grain size, sorting and distribution. Our method of a dynamic current renders these rock types clearer.
Click on thumbnail for a larger image of the Electric
PDF of Electric Log
PDF of Electric Log Tool Specifications