Nuclear Magnetic Resonance (NMR) Logging
BY MARIE WALKER
Outline
• Introduction• Principles of NMR• Interpretations of Raw Data• The NMR Log• NMR Tools• Advantages and Disadvantages of NMR Logging• Advances in NMR Logging• Future of NMR Logging• Summary
Introduction• Nuclear Magnetic Resonance (NMR) is the phenomenon
whereby a magnetic nuclei absorbs and emits energy in the presence of a magnetic field
• The first NMR Logging Tools were developed in the early 1960’s
• Plagued with problems so it was retired in the 1980’s • Re-emerged with the advent of pulsed tools• NMR allows for the calculation of:
Porosity Permeability Pore-space distributions Producible fluids and Irreducible Fluids Reservoir Quality Hydrocarbon Quality
Principles of NMR• All subatomic particles (neutrons, protons,
electrons) have the intrinsic property of spin• This spin corresponds to a small magnetic
moment• In the absence of a magnetic field the moments
are randomly aligned• When a static magnetic field, Bo is applied this
field acts as a turning force that aligns the nuclear spin axis of magnetic nuclei with the direction of the applied field
Principles of NMR con’t
• When a torque is applied to a spinning object, the axis of the object moves perpendicularly to the applied torque in motion called precession
• So the nucleus will precesses around Bo with a frequency called the Lamour frequency
• This proton can be in 2 energy states depending on the orientation of the axis
• The difference between the number of protons in each state gives the bulk magnetization which provides the signal measured by NMR devices
Principles of NMR con’t• 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
• As the pulse dies, the magnetisation caused buy the precession decreases as the spins return out of phase and the signal seen in the receiver decays
• This very rapid decay is referred to as free induction decay (FID)
Spin-Echo Detection• To re-phase the protons a 180° pulse can be
applied• This pulse re-energizes the spins until a peak
magnetization signal is achieved called a spin echo.
• A series of these 180° pulses is sent into the formation at a fixed time interval called TE
• The spin echo signal decreases with each pulse. • This decay of the series called a CPMG series
(Carr-Purcell-Meiboom-Gill) is referred to as transverse relaxation time, T2
Interpretations of Raw Data• Total Porosity
Initial amplitude of the decay curve is a measure of the amount of polarized hydrogen in the pore fluid
• Pore size distribution T2 is smaller at surface area of grains than in pore
space therefore smaller T2 values mean smaller grain sizes
• Producible porosity and Bulk Volume Irreducible Assuming that producible fluids reside in large
pores and non-producible in small pores, T2 distribution curves can give the values for producible porosity of a formation
• Permeability Based on scientific models that show
permeability increases with porosity combined with core data
• Properties of Reservoir Fluids Based on T1 and T2 times which indicates pore
sizes. Clay-bound water, capillary-bound water, movable water, brine, hydrocarbons can all be differentiated based on various pore sizes
Coates Model:
Perm= [total porosity/C)2(producible porosity/Bulk irreducible volume)]2
Sandy Interval
The NMR Log
Magnetic Resonance Imaging (MRIL) Logging• Introduced by Numar in 1991• Composed of a permanent magnet and antennae • Magnet generates a static magnetic field • Antennae sends bursts of radio-frequency energy
into the formation in the form of an oscillating magnetic field
• Antennae acts as a receiver for decaying echo signal
Schlumberger Combinable Magnetic Resonance (CRM) tool
• Introduced by Schlumberger in 1995• Uses a bowstring to press against borehole • Antennae sandwiched between two permanent
magnets • Creates a sensitive zone of about 6 by 1 inches in
the formation• Used for high resolution data and high-precision
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
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
Advances in NMR• Schlumberger MRX eXpert
Contains multiple antennae that enable multiple depths-of-investigation over a broad range and it has a magnetic field gradient
Eliminates the need for multiple passes through zones of interest
Suitable for carbonate formations and low-resistivity pay zones
Advances in NMR• High- Resolution NMR- allows for the evaluation of
producibility of thinly laminated beds• Lithology Independent NMR Total Porosity- NMR is
the most accurate tool for measuring the porosity of heterogeneous formations
• Density/ Magnetic-Resonance Method- combines density and NMR log to predict gas-bearing formation total porosities
• Multi-dimensional NMR Fluid Characterization- composes 2D and 3D maps used to visually identify fluids present in the reservoir on the basis of contrasts in relaxation time
Future Research• Imaging reservoirs the same way MR is used to
image the human body• Inferring rock wettability from NMR • Pressure/Volume/Temperature properties of
reservoirs• Define rock/pore-space connectivity and structure
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
Questions?
Sources• Coates, G. R., Xiao, L. and Prammer, M. G. - NMR
Logging Principles and Applications• Darling, Toby-Well Logging and Formation
Evaluation• David Allen et al- Trends in NMR Logging• David Allen et al- How to Use Borehole Nuclear
Magnetic Resonance• Ellis, Darwin V. and Julian M. Singer- Well-Logging
for Earth Scientists• Freedman, Robert- Advances in NMR Logging