Surface Nuclear Magnetic Resonance (SNMR) for groundwater and oil detection

 

SNMR is performed in the Earth's magnetic field

 
Surface Nuclear Magnetic Resonance (SNMR), also termed Magnetic Resonance Sounding (MRS) (cf. "Sondage par la Résonance Magnétique Protonique"), consists of the use of magnetic resonance of hygrogen nuclei (protons) to detect proton-containing liquids such as water and oil in the Earth's subsurface with the purpose of groundwater and oil investigation/exploration. It suffices to use a wire connected to a power supply to create a loop with a diameter of 100 meters, for instance, that will serve as the transmitting and receiving antenna to probe water or oil in the subsurface. Other geometrical configurations can also be used (e.g. square or figure-eight).
 
A typical SNMR or MRS survey starts will measurement of ambient electromagnetic noise (EM)*. Then, a pulse of electrical current is provided through the wire placed on the surface of the ground, thereby creating an electromagnetic field in the subsurface. The frequency of the applied field and therefore the frequency of the applied current has to be equal to the Larmor frequency of the proton in the Earth's magnetic field and to this purpose the latter is measured with a magnetometer.
 
The Larmor frequency is given by the equation ω=γ*Β, where γ is the gyromagnetic ratio of the hydrogen nucleus which is 42,57 MHz/T and where Β is the magnetic field strength which for the Earth is approximately 0,000050 T. Therefore, the Larmor frequency will be 2,1 KHz for the proton, thereby requiring to use AC current of 2,1 KHz.
 
Following the end of the pulse, the magnetic resonance signal is acquired. Three measurement results are obtained:
1) Amplitude (Eo), which depends on proton quantity and therefore water quantity.
2) Decay (relaxation) time (T2*), which correlates with pore size of the aquifer
3) Phase, which is used for determination of rock conductivity
 
(It is noted that an inversion step of the data with a linear equation is included.)
 
In magnetic resonance, the amplitude is proportional to the volume of the measured sample and also proportional to the square of the magnetic field. SNMR uses volumes in the order of the cubic meter and therefore is performed in the geomagnetic field. For other NMR applications such as NMR well logging (which is performed in cubic centimeter volumes) or MRI and NMR in chemistry labs the static magnetic field has to be increased.
 
 
A study related to groundwater detection to avoid a surface collapse risk was performed for a Mont Blanc glacier using proton NMR (ref., presentation). The researchers used sets of cable loops for both emission and reception with dimensions of 80 m * 80 m (262 ft * 262 ft). The study detected the presence of a water volume of 65 000 m3 +/- 10 000 m3 in the glacier.

 

 
*Note that the presence of power lines is considered to represent an additional energy source which can provide noise. To avoid this issue, the loop configuration is adapted or alternatively reference loops are used in order to measure noise derived from the power lines.
 
References
1. http://bit.ly/2oMCgpi - Legtchenko A., "Magnetic Resonance Imaging for Groundwater", John Wiley & Sons, 2013
 
 

 

Fig. 1: Surface Nuclear Magnetic Resonance (SNMR) in Nigeria. Image from:

http://www.ige-grenoble.fr/Resonance-Magnetique-Protonique

 

 

 

Nuclear Magnetic Resonance (NMR) well logging

 

Generation of a report (log) for a well by lowering a wire with an NMR instrument

NMR well logging was initially developed in the Earth's magnetic field before permanent magnets and electromagnets were used

 

Well logging or borehole logging consists of generating a detailed report (log) of the geological formations along the length of a well of borehole. This can be accomplished by visual inspection or by measurement of physical properties with instruments that are lowered with a wire in the well. The oil and gas industry perform well logging to obtain a record of rock properties. There exist different categories of well logging such as electrical, acoustic etc. NMR logging uses the magnetic resonance of hydrogen nuclei or protons to detect fluid type e.g. water or oil and additionally to determine rock porosity (pore size) and permeability. For instance, based on the duration of the T2 relaxation time, it is possible to determine clay bound water and capillary bound water i.e. water held by capillary forces onto the rock (ref. https://iopscience.iop.org/article/10.1088/1757-899X/579/1/012034/pdf).

 

An illustrative video is found at this link https://vimeo.com/grahamdavis/petrophysics-for-dummies-6-nmr.

 

 

Petrophysics For Dummies - 6. NMR from Graham Davis on Vimeo.

 

Figure 2: Video on NMR and petrophysics. The featured screen capture illustrates the T2 distribution for clay-water and capillary-bound water.

 

 

 

Some related information from Wikipedia is provided below.

 

(https://en.wikipedia.org/wiki/Well_logging) “The Schlumberger brothers from Alsace, France, who founded Schlumberger Ltd in France in 1926 are considered the inventors of electric well logging."

 

"On September 5, 1927, a crew working for Schlumberger lowered an electric sonde or tool down a well in Pechelbronn, Alsace, France creating the first well log."

 

"During World War II, the US Government gave a near wartime monopoly on open-hole logging to Schlumberger, and a monopoly on cased-hole logging to Lane-Wells [5]."

 

(https://en.wikipedia.org/wiki/Schlumberger) “Schlumberger Limited is the world's largest oilfield services company. Schlumberger has four principal executive offices located in Paris, Houston, London, and the Hague [6].”

 

"After the discovery of nuclear magnetic resonance by Bloch and Purcell in 1946, the nuclear magnetic resonance log using the Earth's field was developed in the early 1950's by Chevron and Schlumberger.”

 

Information from the PetroWiki is provided below. 

(https://petrowiki.org/Nuclear_magnetic_resonance_(NMR)_logging)

 

“In 1978, the Los Alamos Natl. Laboratory developed a logging tool that employed permanent magnets and used a pulsed radio frequency (RF) (pulse-echo) NMR method. 

 

Ultimately, two wireline tools using different magnet and coil configurations emerged from these efforts:

  • Numar’s mandrel device (MRIL)

  • Schlumberger’s skid (sometimes called "pad") design [combinable magnetic resonance (CMR) tool]

Commercial logging began with these tools in 1991 and 1995, respectively. These wireline-tool designs continue to evolve (see section on Tool Design in this chapter)."

 

https://petrowiki.org/PEH:Nuclear_Magnetic_Resonance_Applications_in_Petrophysics_and_Formation_Evaluation#Logging_Tools (Logging tools - click on Expand)

 

 

It is noted that “a variety of multiple-echo pulse sequences have been developed for different purposes [11]. In well logging and petrophysical studies, the most widely used is the Carr-Meiboom-Purcell-Gill (CMPG) sequence [13][14].

 

 

At the Schlumberger site (http://www.slb.com/) it is possible to consult a video that is representative of logging and the related principle of "Pulsar Multifunction Spectroscopy Service" at the link http://www.slb.com/services/characterization/petrophysics/wireline/pulsar-spectroscopy.aspx.