- Wireline Logging
Wireline Logging consists of measuring and recording the physical properties of the rocks in
oil wells. "Logging" consists of introducing sensors in a boreholevia a "wireline", an electromechanical cable. Power and control is provided to the sensors inside the borehole, from a surface unit. The sensors send to the surface unit data representative of the physical parameters of the rock surrounding the borehole. The surface unit consists of computers to control and record data, power suppliesto provide power to the downhole sensors, a spoolof wireline cable, and a hoisting unit.
The sensors are lowered inside the borehole, to its bottom. Then, they are pulled up by spooling the cable using the hoist unit. As the sensors are moved up at a more or less constant speed, the sensors measure the properties of the rock, and send the data to the surface unit for recording, control and display.
A "wireline", in this context, is the name of the electromechanical cable that is used to provide power and control to the downhole sensors, and to send data to the surface. It also serves as the stress member that supports the weight of the "logging tools" or "logging instruments" that contain the sensors.
Wireline tool types
Typically the wireline tools are cylindrical in shape, usually from 1.5 to 5 inches in diameter. There are three types of wireline tools:
1. With sensors without excitation:There are units to measure spontaneous potential (SP), which is a voltage difference between a surface electrode and another electrode located in the downhole instrument, other instruments that measure the natural radiation from natural
isotopes of potassium, thorium, etc., to measure pressure and temperature, etc.
2. With sources of excitation and sensors:There are sensor systems consistent of a source of excitation and a sensor. In this type we find acoustic (also called sonic), electric, inductive, magnetic resonance, sensing systems, just to name a few.
3. Instruments that produce some mechanical work, or retrieve a sample of fluid or rock to the surface. :Devices to collect samples of rock, samples of fluid extracted from the rock, and some other mechanical devices.
A continuous measurement of formation properties with electrically powered instruments to infer properties and make decisions about drilling and production operations. The record of the measurements, typically a long strip of paper, is also called a log. Measurements include electrical properties (resistivity and conductivity at various frequencies), sonic properties, active and passive nuclear measurements, dimensional measurements of the wellbore, formation fluid sampling, formation pressure measurement, wireline-conveyed sidewall coring tools, and others. In wireline measurements, the logging tool (or probe) is lowered into the open wellbore on a multiple conductor, contra-helically armored wireline. Once lowered to the bottom of the interval of interest, the measurements are taken on the way out of the wellbore. This is done in an attempt to maintain tension on the cable (which stretches) as constant as possible for depth correlation purposes. (The exception to this practice is in certain hostile environments in which the tool electronics might not survive the temperatures on bottom for the amount of time it takes to lower the tool and then record measurements while pulling the tool up the hole. In this case, "down log" measurements might actually be conducted on the way into the well, and repeated on the way out if possible.) Most wireline measurements are recorded continuously even though the probe is moving. Certain fluid sampling and pressure-measuring tools require that the probe be stopped, increasing the chance that the probe or the cable might become stuck.
LWDtools take measurements in much the same way as wireline-logging tools, except that the measurements are taken by a self-contained tool near the bottom of the bottomhole assembly and are recorded downward (as the well is deepened) rather than upward from the bottom of the hole (as wireline logs are recorded).
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