NMR logging, theory, NMR log application, geophysics, well logging

1. NUCLEAR MAGNETIC
RESONANCE LOGGING
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BY

2. What is NMR?..
• Response of nuclei in the presence of Magnetic field
• Nuclei having odd number of proton/neutron: in our case it is Hydrogen.
• Each proton is like spinning bar magnet
• These tiny bar magnet are aligned along in external magnet field applied
• When external field is switch off precession starts like gyroscope in gravity
field.
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3. • This precession has certain frequency known as Larmor
frequency and is given by
f = yH
where y is the gyromagnetic ratio of the proton ( y =
4.2576 x 10^3 Hz/G).
• The precession of the spins results in a varying
magnetic field which induces an AC voltage (at the
precession frequency).
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4. Principle of NML
• The alignment of these protons is called polarization but this does
not happen immediately it grows with a time constant called
longitudinal relaxation time T1
• After T1 an oscillating magnetic field is applied, sending pulses of
radio-frequency energy into the formation
• The initial pulse is perpendicular to Bo and aligns the spins in the
transverse direction in phase with one another
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5. • As the pulse dies, protons losses their energy/precession by coming
to the initial state.
• This exponential decay is sinusoidal , this is called relaxation(T2)
• This signal is seen in the receiver coil
• This very rapid decay is referred to as free induction decay (FID)
• Both during build up magnetisation and relaxation there is transfer of
energy. How this energy is transferred into the surrounding material
form the basis for NML
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6. • The quantities measured are signal amplitude and decay
• Signal amplitude is proportional to hydrogen nuclei present and
calibrated to give porosity
• However decay of NMR signal(Relaxation time) during each cycle gives
information about pore size
• Small pores short relaxation time Clay bound
• Large pores large relaxation time readily producible fluid
• Relaxation times and their distribution may be interpreted to know:
• Permeability
• Producible porosity
• Irreducible water saturation
• Capillary pressure curve
• Hydrocarbon identification
• Distinction b/w free fluid and capillary bound porosity
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7. NMR Logging Tool
• The modern Schlumberger NMR apparatus consists of two samarium
cobalt magnets, each about one foot (30 centimeters) long, that
project a magnetic field into the formation.
• An antenna located in a cylindrical well on the face of the apparatus
generates the oscillating magnetic field, which is also projected into
the rock
• The two magnetic fields are substantially perpendicular within the
volume of rock being examined, which extends about an inch (2.5 cm)
into the formation
• The antenna is used as both transmitter and receiver
• The transmitter operates at the Larmor frequency, which depends on
the strength of the permanent magnet on the tool. On a CMR, for
hydrogen, this is 3.8 MHz.
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8. Sandy
Interval
The NMR Log
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Porosity logs in mixed-lithology carbonates show the
advantage of NMR lithology-independent total porosity.
NMR total porosity is not sensitive to lithology and reads
correctly over the entire interval. The density tool reads
correctly only in intervals in which the assumed matrix density
(lithology) used to compute the density-log porosity matches
the actual matrix density of the formation.
Dolomite
Limestone

10. Advantages of NMR
• Only fluids are visible to NMR technology so
porosity measurement is independent of the
lithology
• Producible zones with high percentage of clay-
bound water can be identified
• A better measurement of permeability is
possible than traditional plots
• In-situ measurement of oil viscosity
• Differentiation of oil/gas zones
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11. Disadvantages of NMR Tools
• Any diamagnetic or paramagnetic ions present in
the formation can affect the tool response
• Expensive
• Slower logging speeds
• Slimhole tools are not available
• Shallow depth of penetration
• Permeability measurement is actually an empirical
measurement and should only be used to compare
to permeabilities
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12. Summary
• NMR Logging uses the energy given off from
hydrogen protons as they precess in a magnetic
field to infer measurements of a formation’s
porosity, permeability, pore space distribution,
etc.
• Logs can be used to interpret zones of high
porosity and producibility
• Main tools used are the Halliburton’s MRIL and
Schlumberger’s CMR tool
• Gives lithology independent porosities but is
more expensive than conventional tools
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13. Thank You!
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