Well Logging in Simple Language

Well logging in simple language

Millions of dollars can be spent in drilling a well. Hence it is essential

to gather as much information as possible, at every stage to

determine if it makes good business sense to continue drilling and

complete the well. Logging a well is one very effective way, oil and

gas companies gather detailed information about the layers of rock

they drill through. This gives greater certainty of whether or not

hydrocarbons are present. If the well is not promising the process

can be stopped and the well abandoned before incurring a large cost

of completing the well.

Once the well is drilled through its target depth the drill pipe and the

drill bit are removed from the hole. A specialized crew and

equipment are brought on the location. The crew assembles the

probe which is several logging tools connected together. Depending

upon the number of logging tools in the assembly the string may be

from 72 to 120 feet in length.

Using diverge scientific techniques each tool performs a different

kind of measurement on the rock and the fluid properties within the

geological formations surrounding the well bore. The probe is

lowered down the well bore on a wire line until the top of the probe

is below the target depth. The process is carefully monitored by the

logging crew and the geologist. As the probe is raised back up the

hole, the various logging tools are activated by the computers on the

surface that produce a curve called a well log which represents the

geological properties of the layers below the surface.

Gamma Ray Tool

On the well log the Gamma ray tool measures any radiation emitted

by the rocks. Rocks with lower amount of radiations are called

cleaner rocks such as lime stone and sand stone. These are more

likely to contain hydrocarbons than the dirty rocks.

Notice that near the top of the log the line is relatively flat and the

reading is consistently high indicating dirty rock likely shale. But near

the bottom the logging fluctuates because the logging tool is

entering a sandy area which has little radiation and could contain oil

or gas.

Water Resistivity

This data is called the apparent water resistivity curve. It indicates

the potential for water in the rock formations surrounding the well

bore. At the top of the log the line is at the left which indicates that

more than the likely the formation is wet or shaley. As we move

down the log the measurements change drastically. This indicates

quickly changing mixture of shale or other water bearing formations

on the left. Two sands with the possible presence of hydrocarbons as

the curve moves to the right.

SP ( Spontaneous potential)

This next reading is called the spontaneous potential curve shows

geologists when the instrument passes from one rock formation to

the next. The spontaneous potential tool uses two sensors one at the

surface and one down the hole to compare the natural electrical

charges at the surface with the underground rock. Some layers of

rock have positive charge compared to the surface while some other

layers have a negative charge.

A shift to the right on the log indicates positive charge usually

associated with shale. A shift to the left indicates negative difference

usually associated with sand stone.

Resistivity tool

The center section of the log shows readings from the resistivity tool.

The overlapping lines are six resistivity tools each one measuring

from a different distance from the wellbore how well the rock

conducts electricity.

The left side of the grid shows less resistance to electricity while the

right side shows greater resistance. Oil and gas bearing rocks are

less conductive and therefore more resistive to electrical current.

Notice that for most of the vertical distance the rock has less

resistivity this is likely shale. Higher resistance is an indicator of

likely sand a layer that can hold hydrocarbon.

Neutron porosity Tool

The reading from neutron porosity tool which can count how many

hydrogen atoms are there in the surrounding rock can be seen in this

section of the log. A higher concentration of hydrogen indicates a

possible presence of oil. When the curve moves to the right this

indicates fewer number of hydrogen atoms so as we move down the

well bore we can see that the amount of hydrogen is generally

decreasing this means there is either very tight rock that cannot

hold oil or gas may be present.

.

Density porosity tool

Working in tandem with the neutron porosity tool is the density

porosity tool. It measures the space between electrons in the rock

also known as bulk density. Near the top of the rock there is a

greater amount of density but as we move down the wellbore the

log indicates that the rock is becoming less dense you notice that in

this section the density porosity curve and the neutron porosity

curves cross over. This is significant. Whereas the neutron indicates

either tight rock or gas, the density porosity curve tells us that rock is

more porous and therefore cannot be tight rock. This area indicates

a higher probability of natural gas.

Sonic Tool

The next curve represents the measurements taken by the sonic tool.

It shoots the sound waves and records the time taken to return.

Dense rock will have a faster travel time while porous rocks will be

slower.

Caliper Tool

Finally on the left edge of this section we see the reading from the

caliper tool which measures the variation in the size of the well bore.

These variations can affect the accuracy of the other tools on the

probe. So based on these measurements computer recalibrates the

measurements from the other tools as necessary.

These are the common logging tools used by oil and gas companies

to gather information n about the rocks and any fluid or gas

contained thousands of feet below the earth’s surface. There are

numerous other tools for specific logging situations.

Special Logging Tools

The more complicated the geology is the more diverse the set of

logging tools needs to be. None of these tools by themselves provide

sufficient information to make an informed assessment about the

well. But combined these tools offer a more comprehensive and

reliable perspective.

When trained geologists look at well logs such as this there is enough

information for them to literally envision what the subsurface looks

like Because most wells are logged, oil and gas companies can

research these logs to evaluate perspective drilling sites. When a

company plans to drill in a new area they can look at the logs of

nearby wells called control wells. Lining up the logs, adjusting for

variation in the surface altitude of each well, the geologists can get a

good idea of the rock structure and possible presence of

hydrocarbons across the area. This information is very helpful in

identifying promising well sites before drilling begins. With this

information oil and gas companies can drill new wells with greater

confidence and promise of success.

Reference: Based on [YOUTUBE]-Well_Logging, Courtesy : Subodh

Jain, DGM(W)

Amarnath Murthy, DGM(E&T)