WELL LOGING TECHNIQUES

What Is Well Logging?

Well logging is the process of recording various physical, chemical, electrical, or other properties of the rock/fluid mixtures penetrated by drilling a well into the earth's mantle. A log is a record of a voyage, similar to a ship's log. In this case, the ship is a measuring instrument of some kind, and the trip is taken into and out of the wellbore.

Thus, analysis of log data is required. The art and science of log analysis is mainly directed at reducing a large volume of data to more manageable results, and reducing the possible error in the assumptions and in the results based on them. When log analysis is combined with other physical measurements on the rocks, such as core analysis or petrographic data, the work is called petrophysics or petrophysical analysis. The results of the analysis are called petrophysical properties or mappable reservoir properties. The petrophysical analysis is said to be “calibrated” when the porosity, fluid saturation, and permeability results compare favourably with core analysis data. Further confirmation of petrophysical properties is obtained by production tests of the reservoir intervals.

Well logging chronicles the depths, subsurface formations and events encountered while drilling. Well logs can include visual observations or be made by instruments lowered into the well during drilling.

Engineers and drillers use well logs to measure depths of formation tops, thickness of formations, porosity, water saturation, temperature, types of formations encountered, presence of oil and/or gas, estimated permeability, reservoir pressures and formation dip -- ultimately determining whether a well is commercially viable or not and whether casing, cementing and completion should be run on a well. It's not only a journal of what is perforated below the surface, but also a predictor of success.

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.

Methods of Well Logging

Mud Logs refers to the drilling mud, or drilling fluid, used to provide buoyancy to the drill, as well as remove cuttings from the well. Information from a mud logger supplements the driller's log, cuttings log and evaluation log, and is used along with logs of nearby wells to determine the commerciality of a well. Additionally, mud logs monitor the wellbore to help prevent blowouts.

For many years, well logging tools were lowered into the well at regular intervals while drilling to retrieve data. With the advent of directional drilling, well logging had to develop to be able to log a well that was no longer vertical.

Logging While Drilling and Measurement-While-Drilling (or MWD) place the logging tools on the end of the drilling column. That way, drillers can use the information immediately to determine the direction and future of the well.

Well logs of today use Computer-Generated Logs to immediately interpret information gathered while drilling. In addition to keeping measurements, these sophisticated logs can notify drillers of a potential hazard and transmit data via satellite to computers offsite.

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.