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Logging or Well logging is a technique
used in the Oil and Gas Industry for recording the properties of the rocks and
their fluids in the well to find the potential hydrocarbon zones in the
geological formations within the earth’s crust.

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It is a technique of making petro
physical measurements in the sub-surface formations through the drilled bore
hole to determine both the physical and chemical properties of the rocks and
the fluids they contain. It is also called as Formation Evaluation.

These physico-chemical measurements
are then interpreted to locate and quantify the potential depth zones where oil
and gas can be found.

There are different LOGGING TOOLS used
to measure the electrical, acoustic, radioactive, electromagnetic, and other properties
of the rocks and fluids containing the rock cuttings.

Well Logging is very useful as it
provides detailed geological information of drilled holes in a very cost
effective manner.

Logging or Formation Evaluation is carried out by:




Mud Logging:

Here the rock cuttings, brought to the
surface by the circulating drilling fluid / mud, are examined thoroughly to find out any
traces of subsurface hydrocarbon gases & oil when the well is being drilled.

Core Logging:

Here a cylindrical shaped rock sample
(core) is taken out from a particular depth and examined for various properties
like porosity, permeability, fluid saturation, etc and recorded in the well

This activity of core logging is carried out in between the drilling
operations by pulling out the drilling assembly and inserting a special
assembly which comprises a core bit & a core barrel to remove a core sample
for analysis. Here the drilling operation is stopped to remove the core. So
this core logging involves extra rig time and hence core logging increases the
exploration cost.


The cores removed at different depths are numbered and kept
in  a sequential way as per the depth
from where they have been removed and can be used in case of any future
analysis if required.

Electro logging
or Wire Line Logging:

Here a logging tool called as
“Sonde” is attached at the end of a cable or wireline & lowered
into a well to measure the rock and fluid properties like electrical,
electromagnetic, acoustic, radioactive properties of the formation. The
measured data is then transferred through the cable and recorded &
processed in a logging unit on the surface.

All the informations and measurements
obtained through different logging techniques are recorded and interpreted to locate the potential
depth zones where hydrocarbon oil and gas can be found out.

Description of Natural Gamma Ray

It is one type of Electro Logging or
Wire line Logging method based on the principle that different types of rocks
emit different amounts and different spectra of natural gamma radiation.

Usually shales emit more gamma rays
than other sedimentary rocks, such as sandstones &
lime stones. This
is because of the presence of radioactive potassium in the clay content of
shales and the cation exchange capacity of the clay causes them to absorb uranium and thorium. This
difference in radioactivity between shales and sandstones/limestone rocks
allows the gamma ray logging tool to distinguish between shales and non-shales.

This gamma ray logging, like other
types of Electro Logging, is done by lowering the probe, called as Sonde, down
the drill hole and recording the variation in gamma radiation at different depths.
The gamma ray log is plotted in a graph with low radioactivity to the left and
high radioactivity to the right.

Accordingly, the shales kick to the
right showing maximum radioactivity while sandstones and limestones kick to the
left showing minimum radioactivity. Gamma radiation is usually recorded in API units.

A typical Gamma Ray response is as

An advantage of the gamma log over
other types of well logs is that it works through the steel and cement walls of
cased boreholes where other logging methods do not respond properly.

Although concrete and steel absorb
some of the gamma radiation, enough travels through the steel and cement to
allow qualitative determinations of radioactivity. Typical application of Gamma
Ray Logging includes Indication of lithology, determination of reservoir
thickness, correlation between wells and estimation of Shale volume in the well
being drilled.

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