Nuclear magnetic resonance (NMR) logging is a type of
well logging that uses the
NMR response of a
formation
Formation may refer to:
Linguistics
* Back-formation, the process of creating a new lexeme by removing or affixes
* Word formation, the creation of a new word by adding affixes
Mathematics and science
* Cave formation or speleothem, a secondar ...
to directly determine its
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
and
permeability, providing a continuous record along the length of the
borehole.
[Gluyas, J. & Swarbrick, R. (2004) Petroleum Geoscience. Publ. Blackwell Publishing][Nuclear Magnetic Resonance Imaging – Technology of the 21st century. Kenyon, Kleinberg, Straley, Gubelin, and Morris. Oilfield Review. http://eps.mcgill.ca/~courses/c550/Literature/NMR-21st-century.pdf ]
Background
NMR logging exploits the large
magnetic moment
In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...
of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
, which is abundant in rocks in the form of
water. The NMR signal amplitude is proportional to the quantity of hydrogen nuclei present in a formation and can be calibrated to give a value for porosity that is free from lithology effects. Uniquely, a
petrophysicist can also analyse the rate of decay of the NMR signal amplitude to obtain information on the permeability of the formation - a crucial quantity in
hydrocarbon exploration
Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology.
Exploration methods
Vis ...
.
Relationship of NMR signal to pore size
The most important mechanism affecting NMR relaxation is grain-surface relaxation.
Molecules in fluids are in constant
Brownian motion, diffusing about the pore space and bouncing off the grain surfaces. Upon interaction with the grain surface, hydrogen protons can transfer some nuclear spin energy to the grain (contributing to
T1 relaxation
T1, T01, T.1 or T-1 may refer to:
Biology
* The first of the thoracic vertebrae in the vertebral column
* Thoracic spinal nerve 1, a nerve emerging from the vertebrae
* Cyclin T1, a human gene
* GalNAc-T1, a human gene
* Ribonuclease T1, a fung ...
) or irreversibley dephase (contributing to
T2 relaxation
T, or t, is the twentieth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''tee'' (pronounced ), plural ''tees''. It is deri ...
). Therefore the speed of relaxation most significantly depends on how often the hydrogen nuclei collide with the grain surface and this is controlled by the surface-to-volume ratio of the pore in which the nuclei are located. Collisions are less frequent in larger pores, resulting in a slower decay of the NMR signal amplitude and allowing a petrophysicist to understand the distribution of pore sizes.
See also
*
Nuclear magnetic resonance
*
Nuclear magnetic resonance in porous media Nuclear magnetic resonance (NMR) in porous materials covers the application of using NMR as a tool to study the structure of porous media and various processes occurring in them. This technique allows the determination of characteristics such as the ...
*
Logging while drilling
*
SNMR
Surface nuclear magnetic resonance (SNMR), also known as magnetic resonance Sounding (MRS), is a geophysical technique specially designed for hydrogeology. It is based on the principle of nuclear magnetic resonance (NMR) and measurements can be use ...
References
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Well logging