Well Logging is a technique to collect data from the borehole to determine subsurface formations' physical properties. Also known as wireline logging, it is an indirect method of evaluation, as there is no direct contact with the formations. The technique allows us to determine the reservoir properties of rocks present beneath the surface of the Earth. Well logging is a key technique used in the exploration of minerals and O&G.
A well log is typically a record of the formations or events that are encountered during drilling. Sometimes it is also referred to as borehole logging.
Well logs are recognised as legal papers, which captures the activities - a well from the drilling stages to its completion.
A well log can be prepared by two different processes, i.e. by visual inspection of drill cuttings or specially-designed logging equipment, which can be lowered into the borehole to record various events.
The well log, which is prepared by the visual inspection of rock chip samples, is known as Geological Log, whereas the log prepared by using logging units is known as geophysical well logs.
The technique is primarily used for the evaluation of geological formation. The evaluation has wide applications in groundwater, hydrocarbon, and mineral exploration.
During the geophysical well logging, the logging tool is inserted into the borehole with the help of a wireline. A typical logging tool consists of a transmitter and a receiver, which is used to transmit and receive the signals.
Depending on the type of logging tool, relevant energy is transmitted to the formation by the transmitter. Upon reflection from the formation, the energy is recorded back at the receiver of the tool. The reflected energy carries useful information about the formation with them. It must be noted that this is an indirect technique of formation evaluation, which means the technique doesn't record any physical property of the formation on a direct measure basis.
The information collected at the receiver is then transmitted to the surface with cables, which can be decoded in software for further interpretation by geologists and petrophysicists.
The logging tools are designed to bring the various type of information required for formation evaluation by geoscientists. This may range from physical to chemical and structural information about the subsurface formations. Some of the major information that can be revealed from the well logging techniques are:
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Characteristics of Rock: It means one can determine the porosity, permeability, and saturation of rocks, to evaluate the volumetric space, where the hydrocarbons or minerals are accumulated.
Composition of Rocks: It means geologists can determine the type of rock present at a specific depth. Either, it's shale, sandstone or limestone.
Saturation: The petrophysicists can find the amount of saturation, which means the void space volume that is accommodated by liquids.
The trajectory of the Borehole: The technique can also tell us about the size, shape and trajectory of the borehole.
The integrity of Rocks: In addition to all the above advantages, the technique is well versed to identify cavings and other structural weaknesses along the wall of the borehole, which may lead to various potential borehole problems.
Conrad Schlumberger first started the science of well logging in 1927 as part of his work related to the Earth's resistivity measurements for surface exploration. Later on, the advancement in technology and more precise results made the technology more popular. The demand rose rapidly internationally.
Various types of well logs can be recorded with the help of logging tools, depending on the scope of requirement. There is a huge list of types of well logs that are used in the industry today. We will briefly discuss some most prominent logs, which are frequently used in the industry by experts.
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Well Logging has a wide range of applications from mining to oil and gas exploration. The technique is also used for groundwater exploration. The method has majorly following applications: