There are many different types of well logs. Some of the logs that are used to interpret the rocks in a well are discussed below. Other types of logs measure temperatures, the flow rate of oil and gas that is being produced in the well, and the quality of cement used to bond production pipe (which is actually called casing) to the surrounding rock. Today, there are even cameras that can be lowered into wells to make videos of the inside of the casing and determine what types of fluids are flowing out of perforation holes shot into the casing.
GR (gamma ray) logs measure radioactivity to determine what types of rocks are present in the well. Because shales contain radioactive elements, they emit lots of gamma rays. On the other hand, clean sandstones emit very few gamma rays.
SP (spontaneous potential) logs indicate the permemabilities of rocks in the well by measuring the amount of electrical current generated between the drilling fluid and the formation water that is held in pore spaces of the reservoir rock. Porous sandstones with high permeabilities tend to generate more electricity than impermeable shales. Thus, SP logs are often used to tell sandstones from shales.
Resistivity logs determine what types of fluids are present in the reservoir rocks by measuring how effective these rocks are at conducting electricity. Because fresh water and oil are poor conductors of electricity they have high resistivities. By contrast, most formation waters are salty enough that they conduct electricity with ease. Thus, formation waters generally have low resistivities. There are many different types of resistivity logs, which results in a confusing array of acronyms.
BHC (borehole compensated) logs, also called sonic logs, determine porosity by measuring how fast sound waves travel through rocks in the well. In general, sound waves travel faster through high-density shales than through lower-density sandstones.
FDC (formation density compensated) logs, also called density logs, determine porosity by measuring the density of the rocks. Because these logs overestimate the porosity of rocks that contain gas they result in "crossover" of the log curves when paired with Neutron logs (described under CNL logs below).
CNL (compensated neutron) logs, also called neutron logs, determine porosity by assuming that the reservoir pore spaces are filled with either water or oil and then measuring the amount of hydrogen atoms (neutrons) in the pores. Because these logs underestimate the porosity of rocks that contain gas they result in "crossover" of the log curves when paired with FDC logs (described above).
NMR (nuclear magnetic resonance) logs may be the well logs of the future. These logs measure the magnetic response of fluids present in the pore spaces of the reservoir rocks. In so doing, these logs measure both porosity and permeability, as well as the types of fluids present in the pore spaces.
Dipmeter logs determine the orientations of sandstone and shale beds in the well, as well as the orientations of faults and fractures in these rocks. The original dipmeters did this by measuring the resisitivity of rocks on at least four sides of the well hole. Modern dipmeters actually make a detailed image of the rocks on all sides of the well hole. Borehole scanners do this with sonic (sound) waves, whereas FMS (formation microscanner) and FMI (formation micro-imager) logs do this by measuring the resisitivity. These modern, essentially 3D logs are known as image logs since they provide a 360° image of the bore hole that can show bedding features, faults and fractures, and even sedimentary structures, in addition to providng basic dipmeter data on the orientations of bedding.
LWD and MWD versus Wireline Tools
Wireline refers to the logging technique in which after a well has finished drilling and reached TD (total depth), the logging tool is lowered down the hole the hole on a cable (i.e., the wireline). As the tool is brought to the surface ,it measures data (gamma ray, resistivity, etc.) from which the log for the well is constructed.
LWD and MWD are acronyms for "Logging While Drilling" and "Measurement While Drilling" and refer to the technique of placing the logging tool somewhere behind the drill bit so that it can record data during the actual drilling. Depending on how far the tool sits behind the bit, the data can be measured, more or less, in real time to create Realtime Logs at the surface. After the tool is pulled from the hole, data can then be downloaded from the tool itself to create what are called Memory Logs, which are higher resolution and more reliable than the Realtime logs.
Acronyms of Resistivity Logs
There are many different types of resisitivity logs, which differ primarily in how far into the rocks they measure the resisitivity. Because drilling fluids tend to force their way into the surrounding rock, resisitivity logs with shallow depths of investigation are unable to see beyond an "invasion zone" to determine the true formation water resisitivy of permeable rocks. Instead, these logs measure the lower resisitivity of the contaminated zone. Thus, by pairing logs with deep and shallow depths of investigation, it is possible to measure permeability by looking at the resisitivity diffences between the logs. The acronyms of some of the more popular resisitivity logs are listed below.