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