Magnetics

Magnetics can severely impact the outcome of drilling a well and it's a complicated subject to deal with. Hence, a basic understanding of the principles governing magnetism is required, that helps the directional driller to deal with its effect.

The tools we most commonly use to acquire while-drilling surveys and even after-drilling surveys, make use of the Earth's magnetic field to provide a reference direction. But, unfortunately earth's magnetic field keeps on changing with time and our tools in addition to it, are further placed under other magnetic fields too. For this reason a basic knowledge of magnetism is required to understand the functions and limitations of magnetic tools; it'll also help in maximizing the performance of our tools.

What is Magnetism?

Magnetism is a form of energy, caused by motion of electrons in some materials.

The magnetic fields can be visually represented by imaginary lines of force drawn in space surrounding a magnet. These lines of force are believed to be caused by the way groups of atoms called domains or dipoles, in some materials, become aligned when the material is placed in a magnetic field.

Magnetic field is a region around a magnetic material or a moving electric charge within which force of magnetism acts.
Magnetic field strength is the force experienced by an object when it's placed in the magnetic field. It's measured in Tesla, Gamma, Gauss.



Magnetic Earth: Overview

We know that earth acts like a gigantic magnet and have its own magnetic poles which is different from geographic pole. The magnetic poles are located around one thousand miles away from the geographic poles. The angular separation of the geographic axis from the geomagnetic axis is called the magnetic declination angle. 

The declination angle changes slowly with time causing the actual position of the poles to move across the surface of the Earth. This gradual change caused by variation in the electric currents flowing through the molten outer core of Earth is known as the secular variation.

The position of the poles is also influenced by magnetic disturbances both inside and outside the Earth, such as solar activity. This causes the poles to wander, daily, in a roughly elliptical path around its average position, and can often be as much as 100 miles away from this position during times of magnetic disturbance. This phenomena is known as the diurnal variation.

The earth behaves as a magnet with magnetic flux lines flowing externally from the Negative (southern) pole to the Positive (northern) pole giving the lines of flux a direction of Magnetic North. The magnitude of the magnetic field varies with the distance from the poles.
Note: The earth’s positive pole is actually a magnetic SOUTH POLE.

Geomagnetic observatories are located all over the earth at different places, which record and monitor the changes in the earth's magnetic field regularly.

In directional drilling, we deal with the earth's magnetic field and the local magnetic field caused by any metal particles such as junk or the parts of drill string itself.



Magnetic Inclination (or) Dip Angle

It's the angle between the earth's magnetic flux lines and the horizontal at any point on the earth.
The range of dip angle is from −90° (at the South Magnetic Pole) to 0° at equator to +90° (at the North Magnetic Pole). Places on earth where dip angle is 90° are called as Dip Holes.




Components of Earth's Magnetic Field

The Earth's magnetic field is a vector quantity i.e, at each point in space it has a strength and a direction.

Magnetic Components

Component	Description
F	The total intensity of the magnetic field vector.
H	The horizontal intensity of the magnetic field vector.
Z	The vertical component of the magnetic field vector; By convention Z is positive downward.
X	The north component of the magnetic field; X is positive northward.
Y	The east component of the magnetic field; Y is positive eastward.
D	Magnetic declination, D is positive eastward of true North.
I	Magnetic Inclination, I is positive downward.

D and I are measured in degrees. All other elements are measured in nanotesla (nT).
1 nT = 10^-9 Tesla

D = Tan^-1(Y/X) = Magnetic Declination

I = Tan^-1(Z/H) = Magnetic Inclination

H = (X² + Y²)^1/2 = Horizontal Component

X = H CosD = True North Component

Y = H SinD = True East Component

F = (X² + Y² + Z²)^1/2 = Total Magnetic Field Intensity

On a Directional Well Plan we can find the values of Dip Angle, Magnetic Declination, Magnetic Field Strength. It's the responsibility of the directional drilling service providers to provide the updated values and these values are then once confirmed with the concerned persons prior to drilling the well.

PREVIOUS                                                      NEXT