nuclear magnetic resonance imaging-technology for the 21st century

8/10/2019 Nuclear Magnetic Resonance Imaging-Technology for the 21st Century

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19Autumn 1995

Although well logging has made major advances over the last 70 years, several important reservoir proper-

ties are still not measured in a continuous log. Among these are producibility, irreducible water saturation

and residual oil saturation. Nuclear magnetic resonance (NMR) logging has long promised to measure these,

yet it is only recently that technological developments backed up by sound research into the physics behind

the measurements show signs of fulfilling that promise.

Nuclear Magnetic Resonance Imaging

Technology for the 21st Century

Bill KenyonRobert KleinbergChristian StraleyRidgefield, Connecticut, USA

Greg GubelinChris MorrissSugar Land, Texas, USA

For help in preparation of this article, thanks to AustinBoyd and Billie-Dean Gibson, Schlumberger Wireline &Testing, Sugar Land, Texas, USA.

In this article, CMR (Combinable Magnetic Resonancetool), ELAN (Elemental Log Analysis), Litho-Density (pho-

toelectric density log) and NML (Nuclear Magnetism Log-ging tool) are marks of Schlumberger. MRIL (MagneticResonance Imager Log) is a mark of NUMAR Corporation.

For nearly 70 years, the oil industry hasrelied on logging tools to reveal the proper-ties of the subsurface. The arsenal of wire-line measurements has grown to allowunprecedented understanding of hydrocar-bon reservoirs, but problems persist: a con-tinuous log of permeability remains elusive,pay zones are bypassed and oil is left in theground. A reliable nuclear magnetic reso-nance (NMR) measurement may change allthat. This article reviews the physics andinterpretation of NMR techniques, andexamines field examples where NMR log-

ging has been successful.

Some Basics

Nuclear magnetic resonance refers to aphysical principleresponse of nuclei to amagnetic field. Many nuclei have a mag-netic momentthey behave like spinningbar magnets (next page, left). These spin-ning magnetic nuclei can interact withexternally applied magnetic fields, produc-ing measurable signals.


2/15

1. Murphy DP: NMR Logging and Core AnalysisSimplified, World Oil216, no. 4 (April 1995): 65-70.

2. For examples of laboratory T2 core measurementsenabling direct comparison to log measurements andto laboratory T1 core measurements:

Straley C, Rossini D, Vinegar H, Tutunjian P and Mor-riss C: Core Analysis by Low Field NMR, Proceed-ings of the 1994 International Symposium of the Soci-ety of Core Analysts, Stavanger, Norway, September12-14, 1994, paper SCA-9404.

Kleinberg RL, Straley C, Kenyon WE, Akkurt R andFarooqui SA: Nuclear Magnetic Resonance of Rocks:T1 versus T2 , paper SPE 26470, presented at the 68thSPE Annual Technical Conference and Exhibition,Houston, Texas, USA, October 3-6, 1993.

decay of the NMR signal during each mea-surement cyclecalled the relaxationtimegenerates the most excitement amongthe petrophysical community.

Relaxation times depend on pore sizes(right). For example, small pores shortenrelaxation timesthe shortest times corre-sponding to clay-bound and capillary-bound water. Large pores allow long relax-

ation times and contain the most readilyproducible fluids. Therefore the distributionof relaxation times is a measure of the distri-bution of pore sizesa new petrophysicalparameter. Relaxation times and their distri-butions may be interpreted to give other

For most elements the detected signals aresmall. However, hydrogen has a relativelylarge magnetic moment and is abundant inboth water and hydrocarbon in the porespace of rock. By tuning NMR logging toolsto the magnetic resonant frequency ofhydrogen, the signal is maximized and can

be measured.The quantities measured are signal ampli-

tude and decay (see All in a SpinNMRMeasurements, below). NMR signal ampli-tude is proportional to the number of hydro-gen nuclei present and is calibrated to giveporosity, free from radioactive sources andfree from lithology effects. However, the

20 Oilfield Review

All in a Sp in

NMR Mea surem en ts

NMR measureme nts consist of a series of ma nip-

ulations of hydrogen protons in fluid molecules. 1

Protons have a ma gnetic moment and behave like

small bar magnets, so that their orientations may

be controlled by magnetic fiel ds. They also spin,

which makes them act like gyroscopes.

A mea surement sequence starts with proton

alignme nt followed by spin tipping, precession,

and repeated dephasing and refocusing. Trans-

verse relaxation and longitudinal relaxation limit

how long a measurement m ust last. Only after

completion of all these steps which takes a few

seconds can the measurement be repeated. 2

Prece ssing protons. Hydrogen nuc leiprotonsbeh a ve l ike spinning b a r ma gne ts .Once d is turbed from eq ui librium, they prec ess a bout the s ta t ic m a gne t ic f ield (left) in the

sam e w a y tha t a chi lds spinning top prece sses in the Ea r ths gra vitat iona l field (right).

Relaxa t ion cu rves. Wa ter in a test t ube ha s a l ong T 2 rela xa t ion t ime of 3700 msec a t 40C (upper curve).Relaxa t ion t im es of a vu gg y ca rbona tem ight a pp roach th is va lue. However ,wa ter in rock pore spac e ha s shorter rela xa t ion t im es. For exam ple, relax-a t ion t im es for a san dstone typica l ly rang e f rom 10 msec for sm a l l pores to 500 mse c for larg e p ores. The init ia l a m pl itude of the relax a t ion curvegives CMR porosity.

Mag netic f ie ld

Spinning motion

P recession al m otion

Gravi tat ional f ie ld

S pinn ing m otion

W ater in test tub e

T2= 3700 m sec

W ater in p ore space of rock

T2= 10 to 500 m sec

C M R porosity

Tim e, m sec

S

ignalam

plitude

100% porosity

petrophysical parameters such as permeabil-ity, producible porosity and irreducible

water saturation. Other possible applica-tions include capillary pressure curves,hydrocarbon identification and as an aid tofacies analysis.1

Two relaxation times and their distribu-tions can be measured during an NMRexperiment. Laboratory instruments usuallymeasure longitudinal relaxation time, T1and T2 distribution, while borehole instru-ments make the faster measurements oftransverse relaxation time, T2 and T2 distri-bution.2 In the rest of this article T2 willmean transverse relaxation time.

3. Kenyon WE: Nuclear Magnetic Resonance as a Petro-physical Measurement, Nuclear Geophysics6, no. 2(1992): 153-171.

4. Kenyon WE, Howard JJ, Sezinger A, Straley C,Matteson A, Horkowitz K and Ehrlich R: Pore-SizeDistribution and NMR in Microporous Cherty Sand-stones, Transactions of the SPWLA 30th Annual Log-ging Symposium, Denver, Colorado, USA, June 11-14,1989, paper LL.

Howard JJ, Kenyon WE and Straley C: Proton Mag-netic Resonance and Pore Size Variations in ReservoirSandstones, paper SPE 20600, presented at the 65thSPE Annual Technical Conference and Exhibition,New Orleans, Louisiana, USA, September 23-26,1990.

Gallegos DP and Smith DM: A NMR Technique forthe Analysis of Pore Structure: Determination ofContinuous Pore Size Distributions,Journal of Colloidand Interface Science122, no. 1 (1988): 143-153.

Loren JD and Robinson JD: Relations Between PoreSize Fluid and Matrix Properties, and NML Measure-ments,SPE Journal10 (September 1970): 268-278.

5. Freedman R and Morriss CE: Processing of Data Froman NMR Logging Tool, paper SPE 30560, to be pre-sented at the 70th SPE Annual Technical Conferenceand Exhibition, Dallas, Texas, USA, October 22-25,1995.


3/15

T2, m sec

0.1 1.0 10.0 100.0 1000.0

SignalD

istribution

0.03

Signalam

plitude

0

0 100 200 300 400

Tim e, m sec

1

0.00

Signa l am pl itudep roc e ss e d to g iveT2 distribution. TheCMR tool m ea suresdec a ying NMR sig-na l a mp lit ude (top),which i s the sum of a ll the d eca y ing T 2s igna ls ge nera t ed by hydrogen p ro-tons in the m ea sureme n t vo lume . Sepa -

rat ing out ra nge s ofT2 val ues by a m a t h e m a t i c a l inversion process

p rod u c e s th e T2 dis-tribution (bottom).The cu rve repre-sents the d istribu -tion of pore sizes,a n d t h e a r e a u n d e r the c urve is CMR

p oros ity . In te rp re ta -tion of pore size d is-t ribut ion an d loga -r it hmi c mea n T 2 a reused to calculates u c h p a r a m e t e rs a s

p e rm e a b ility a n dfree -fluid porosity.

NMR Applica tions an d Exa m ples

The T2 distribution measured by the Schlum-berger CMR Combinable Magnetic Reso-nance tool, described later, summarizes all

the NMR measurements and has severalpetrophysical applications: T2 distribution mimics pore size distribu-

tion in water-saturated rock the area under the distribution curve

equals CMR porosity permeability is estimated from logarith-mic-mean T2 and CMR porosity

empirically derived cutoffs separate the T2distribution into areas equal to free-fluidporosity and irreducible water porosity.3

Application and interpretation of NMRmeasurement rely on understanding therock and fluid properties that cause relax-ation (see NMR Relaxation Mechanisms,page 26). With this foundation of the mech-

anisms of relaxation, the interpretation of T2distribution becomes straightforward.

T2 DistributionIn porous media, T2

relaxation time is proportional to pore size.4

At any depth in the well the CMR toolprobes a rock sample that has a range ofpore sizes. The observed T2 decay is thesum of T2 signals from hydrogen protons, inmany individual pores, relaxing indepen-dently. The T2 distribution graphically showsthe volume of pore fluid associated witheach value of T2, and therefore the volumeassociated with each pore.

Signal processing techniques are used totransform NMR signals into T2 distributions(right). Processing details are beyond the

scope of this article.5

21Autumn 1995

Proton alignmentHydrogen protons are

aligned by application of a large constant mag-

netic field, B0. Alignment take a few seconds and

the protons will remai n ali gned unless disturbed.

The latest logging tools use elongated permanent

magnets, of about 550 gauss in the measurement

region about 1000 time s larger than the Earths

magnetic field. These are applied to the forma-

tion during the entire measureme nt cycle (right). 3

Spin tippingThe next step is to tip the

aligned protons by transmitting a n oscillating

Proton alignment. The first step in an NMR mea-

surement is to align the spinning protons using

powerful permanent magnets. The protons precess

about an axis parallel to the B0 directionthe net

magnetization being the sum of all the precessing

protons. In logging applications B0 is perpendicular

to the borehole axis.

1. This article uses hydrogen proton when discussing NMR

measurements. However, other texts use proton, nucleus,

moment and spin interchangeably. For the purposes of

NMR theory they are all considered synonyms.

2. For a detailed description of NMR measurements:

Fukushima E and Roeder SBW: Experimental Pulse NMR:

A Nuts and Bolts Approach. Reading, Massachusetts,

USA: Addison-Wesley, 1981.

Farrar TC and Becker ED: Pulse and Fourier Transform

NMR: Introduction to Theory and Methods. New York,

New York USA: Academic Press, 1971.

3. The SI unit of magnetic flux density is the Teslaequal to

1000 gauss.

B 0field

z

y

x

N et m agnetization

along z-axis

Precessing

m agnetic

m om ents


4/15

Pore-size distribution a nd free -fluidindexcarbona te exa m ple. In th is we ll ,t he o il compa ny wa s concerned a bou t wa ter coning dur ing produc t ion. Theinterval be low X405 ft showed ne a rly100% wa ter sa turat ion by c onvent iona l

log interpre ta tion (track 3). Howe ver, theCMR log showe d low T2 distribution va l-ue s over this interval (track 4) indica t ingsma ll pores . La rger pores are indica teda bove X405 ft by highe r T2 distributions.App lying a free-fluid index cutoff of 100m sec to the d ist ribut ions showe d tha t m ost of the w a ter wa s irreduc ible. Thisresult ga ve the oi l com pa ny conf iden ce to a dd the interva l X380 ft to X395 ft to its

p e rfo ra tin g p rog ra m .

22 Oilfield Review

Precession and dephasingWhen protons are

tipped 90 from the B0 direction, they precess in

the plane perpendicular to B0. In this respect

they act like gyroscopes in a gravitation field

(page 20, left).

At first all the protons precess in unison. W hile

doing so they generate a small magnetic field at

the Larmor frequency which is detected by the

antenna and forms the basis of the NMR mea-

surements. However, the magnetic field, B0, is

not perfectly homogeneous, causing the protons

to precess at slightly different frequencies. Grad-

ually, they lose synchronizationthey dephase

causing the antenna signal to decay (next page).

The decaying signal is call ed free i nduction

decay (FID) and the decay time is called

Spin tipping. Aligned protons are tipped 90 by

a magnetic pulse oscillating at the resonance, or

Larmor frequency.

z

x

y

B 0field

B 1field

magnetic field, B1, perpendicular to the direction

of B0 (right). For effective spin tipping:

f= B0

where fis the frequency of B1 cal le d Larmor

frequency and is a constant called the gyro-

magnetic ratio of the nucleus. For example, the

Larmor frequency for hydrogen nuclei in a field of

550 gauss is about 2.3 MHz.

The angle through which spins are tipped is

controlled by the strength of B1 and the length of

time it is switched on. For example, to tip spins

by 90 as in most logging applications a B1

field of 4 gauss is switched on for 16 m icrosec-

onds (sec).

In an example taken from a carbonatereservoir, T2 distributions from X340 ft toX405 ft are biased towards the high end ofthe distribution spectrum indicating large

pores (left). Below X405 ft, the bias istowards the low end of the spectrum, indi-cating small pores. This not only provides aqualitative feel for which zones are likely toproduce, but also helps geologists with

facies analysis.Lithology-independent porosityTradi-tional calculations of porosity rely on bore-hole measurements of density and neutronporosity. Both measurements require envi-ronmental corrections and are influenced bylithology and formation fluid. The porosityderived is total porosity, which consists of

Porosity

D olom ite

C alcite

Anhydrite

Q uartz

Bound W ater

Illite

CM R Perm

0.1 m d 1000

Perm

1:240 ft

Perf

X350

X400

X450

C M R Porosity

25 p.u. 0

H ydrocarbon

Free W ater

C ap Bound W ater

W iggle

B ound-Fluid Volum e100-m sec Line

0.1 m sec 1000

750-m sec Vug Line

0.1 m sec 1000

T2 D istribution


5/15

23Autumn 1995

Pulse sequence and refocusing. Each NMR mea-

surement comprises a sequence of transverse mag-

netic pulses transmitted by an antennacalled a

CPMG sequence. Each CPMG sequence starts with a

pulse that tips hydrogen protons 90and is followed

by several hundred pulses that refocus the protons

by tipping them 180 (top). After each pulse the

antenna becomes a receiver that records the signal

amplitude (middle). The fast decay of each

echocalled free induction decayis caused by

variations in the static magnetic field, B0. The decay

of each echo amplitude is caused by molecular inter-

actions and has a characteristic time constant of

T2transverse relaxation time. The circled numbers

correspond to steps numbered in the race analogy(below).

Imagine runners lined up at the start of a race

(bottom). They are started by a 90 pulse (1). After

several laps, the runners are spread around the track

(2, 3). Then the starter fires a second pulse of 180

(4, 5)and the runners turn round and head back to

the starting line. The fastest runners have the far-

thest distance to travel and all of them will arrive at

the same time if they return at the same rate (6a).

With any variation in speed, the runners arrive back

at slightly different times (6b). Like the example of

the runners, the process of spin reversal is repeated

hundreds of times during an NMR measurement.

Each time the echo amplitude is less and the decay

rate gives T2 relaxation time.6b. Som e

spins dephased:echo am plitude

reduced

6a. Spinsreturn at the

sam e rate theyfanned out

90 180

1 2 3 4 600

Start next

C P M Gsequence

Tim e,sec

CPMG Pulse Sequence

160 320 320

Spin echoesFree ind uction

decay

A

m

plitude

Signal

3. Spinsfan out

4. 180pulsereverses spinprecession

2. Spins inhighest B 0precessfastest

1. 90pulsestarts spinprecession

or

3 4 5

2

1 6a 6b

B

1am

plitude

Tim e,sec

5. Spins inhighest B

0precessfastest

Transverse decay. As the protons precess about

the static field, they gradually lose synchronization.

This causes the magnetic field in the transverse

plane to decay. Dephasing is caused by inhomo-

geneities in the static magnetic field and by molecu-

lar interactions.

D ep hasing in

the x-y plan e

B 0field

z

x

y

T2* the asterisk indicates that the decay is not a

property of the formation. For logging tools T2* is

comparable to the span of the tipping pulse a

few tens of microseconds.

Refocusing: spin echoesThe dephasing

caused by the inhomogeneity of B0 is reversible.

Imagine a race started by a starting gun, analo-

gous to a 90 tipping pulse. The runners start off

in unison, but after several la ps they become dis-

persed around the track due to their slightly dif-

ferent speeds. Now the starter gives another sig-

nal by firing a 180 pulse. The runners turn

around and race in the opposite dire ction. The

fastest runners have the greatest distance to

travel back to the start. However, if the conditions

remain the same which is never the case all

the runners arrive back at the same time (above).

Simi larly, the hydrogen protons precessing at

slightly different Larmor frequencies can be

refocused when a 180 pulse is transmitted. The

180 pulse is the same strength as a 90 pulse,

but switched on for twice as long. As the protons

rephase, they generate a signal in the

antenna a spin echo.

Of course, the spin echo quickly decays again.

However, the 18 0 pulses can be applied repeat-

edly typically several hundred times in a single

NMR measurement. The usual procedure is to

apply 180 pulses in an evenly spaced train, as


6/15

Typical decaying spin echo amplitude plot for a rock.

Each dot represents the amplitude of a spin echo. In

this example recorded time is less than 0.3 sec.

1.60

1.20

0.30

0.40

0.00

0.400.1 sec

CPMG Decay for a Rock

Signalam

plitude

dolomite matrix to overlay the CMR porosity.If the lithology is not known or if it is com-plex, CMR porosity gives the best solution.Also, no radioactive sources are used for the

measurement, so there are no environmentalconcerns when logging in bad boreholes.

PermeabilityPerhaps the most importantfeature of NMR logging is the ability torecord a real-time permeability log. The

potential benefits to oil companies are enor-mous. Log permeability measurementsenable production rates to be predicted,allowing optimization of completion andstimulation programs while decreasing thecost of coring and testing.

Permeability is derived from empiricalrelationships between NMR porosity andmean values of T2 relaxation times. These

relationships were developed from brinepermeability measurements and NMR mea-surements made in the laboratory on hun-dreds of different core samples. The follow-ing formula is commonly used:

kNMR = C(NMR)4(T2, log)2

where kNMR is the estimated permeability,NMR is CMR porosity, T2, log is the logarith-

mic mean of the T2 distribution and C is aconstant, typically 4 for sandstones and 0.1for carbonates.

A cored interval of a well was loggedusing the CMR tool. The value of C in theCMR permeability model was calculatedfrom core permeability at several depths.After calibration CMR permeability wasfound to overlay all core permeability pointsover the whole interval (page 26). Over thezone XX41 m to XX49 m the porosity varied

little. However, permeability varied consid-erably from a low of 0.07 md at XX48 m toa high of 10 md at XX43 m. CMR perme-

ability also showed excellent vertical resolu-tion and compared well to that of core val-ues. The value of C used for this well will beapplied to subsequent CMR logs in this for-mation enabling the oil company to reducecoring costs.

Free-fluid indexThe value of free-fluidindex is determined by applying a cutoff tothe T2 relaxation curve. Values above thecutoff indicate large pores potentiallycapable of producing, and values belowindicate small pores containing fluid that istrapped by capillary pressure, incapable ofproducing.

producible fluids, capillary-bound waterand clay-bound water (below).

However, CMR porosity is not influencedby lithology and includes only producible

fluids and capillary-bound water. This isbecause hydrogen in rock matrix and inclay-bound water has sufficiently short T2relaxation times that the signal is lost duringthe dead time of the tool.

An example in a clean carbonate forma-tion compares CMR porosity with thatderived from the density tool to show lithol-ogy independence (next page, top). Thelower half of the interval is predominantlylimestone, and density porosity, assuming alimestone matrix, overlays CMR porosity. AtX935 ft, the reservoir changes to dolomiteand density porosity has to be adjusted to a

24 Oilfield Review

close together as possible. The entire pulse

sequence, a 90 pulse followed by a long series

of 180pulses, is called a CPMG sequence

after its inventors, Carr, Purcell, Meiboom and

Gill.4 The echo spacing is 320 sec for the

Schlumberger CMR tool and 1200 sec for

NUMARs MRIL tool.

Transverse re laxation, T2 The CPMG pulse

sequence compensates for dephasing caused by

B0 field imperfections. However, molecular pro-

cesses also cause dephasing, but this is irre-

versible. These processes are rel ated to petro-

physical properties such as movable fluid poros-

ity, pore size distribution and permeabili ty.

Irreversible dephasing is monitored by mea sur-

ing the decaying ampli tude of the spin echoes in

the CPMG echo train (left). The characteristic echo

amplitude decay time is called the transverse

relaxation time, T2, because dephasing occurs in

the plane transverse to the static field B0. 5

Longitudinal relaxation, T1 After a period of

several times T2, the protons completely lose

coherence, and no further refocusing is possible.

After the CPMG pulse sequence is del ivered the

protons return to their equili brium direction paral-

Free-fluid

C M R

Total

C lay-

bound

w ater

Producible

fluids

C apillary-

bound

w ater

C

onventional

logs

C

M

R

derived

CMR porosity. Hydroge n in rock ma trix a nd in clay-boun dwa ter has shor t rela xa t ion t im es tha t are lost in the de a d t im e of the CMR tool. CMR porosity include s only ca pil la ry-bou nd wa tera nd p roduc ible fluids a nd is not influe nc ed b y li thology. Tota l

p oros ity a s m e a su re d b y c on ve n tio n a l lo g sa lso in c lu d e s c la y-bound wa ter . The dot ted l ine indica tes tha t CMR porosi ty doesnot include m icroporosity associated wi th ha rd sha les .


7/15

Lithology-independent porosity. BelowX935 ft, the lithology is lim eston e w ithsome dolomitization (track 1), whi le a boveis dolomite. Two porosity c urves (track 2)are der ived f rom densi ty measurementsone a ssume s a l ime stone l ithology a nd the

other d olomite. CMR porosity ove rla ys thede nsity limestone porosity in l im estoneregions a nd overlays d olom ite p orosity indolom ite regionsdem onst ra t ing tha t CMR porosity is inde pe nd en t of li thology.

1:240 ft

0 100 30

30

30

0 p.u.

10

10

10

10

X900

X1000

D ensity Porosity, Lim estone

p.u.

p.u.

p.u.

Zero Porosity

C om bined M odel

Illite

B ound W ater

Q uartz

C alcite

D olom ite

O il

W ater

D ensity P orosity, D olom ite

C M R Porosity

%

25Autumn 1995

Longitudinal relaxation, T1. When the CPMG pulse

sequence ends, the protons gradually relax back

towards the static magnetic field. They do so with

characteristic time constant T1, longitudinal relaxation.

x-y plan e

decay

z-axis

buildup

B 0field

z

x

y

4. Carr HY and Purcell EM: Effects of Diffusion on Free Pre-

cession in Nuclear Magnetic Resonance Experiments,

Physical Review94, no. 3 (1954): 630-638.

5. Transverse relaxation is sometimes called spin-spin relax-

ation in early NMR literature. Similarly, longitudinal relax-

ation is sometimes called spin-lattice relaxation.

6. Kleinberg RL, Farooqui SA and Horsfield MA: T1/T2 Ratio

and Frequency Dependence of NMR Relaxation in Porous

Sedimentary Rocks,J ournal of Colloid and Interface

Science158, no. 1 (1993): 195-198.

Kleinberg et al, reference 2 main text.

lel to B0 (left). This process occurs with a different

time constant longitudinal relaxation, T1. The

next spin-tipping measurement i s not started until

the protons have returned to their equilibrium

position in the constant B0 field.

T1 and T2 both arise from m olecular processes.

In ma ny laboratory measurements on water-satu-

rated rocks, it has been found that T1 is frequently

equal to 1.5 T2. 6 However, this ratio varies when

oil and gas are present in rock samples.


8/15

Many experiments have been made onrock samples to verify this assumption.6T2distributions were measured on water-satu-rated cores before and after they had been

centrifuged in air to expel the produciblewater. The samples were centrifuged under100 psi to simulate reservoir capillary pres-sure. Before centrifuging, the relaxation dis-tribution corresponds to all pore sizes. It

seems logical to assume that during cen-trifuging the large pore spaces empty first.Not surprisingly, the long relaxation timesdisappeared from the T2 measurement (nextpage, top).

Observations of many sandstone samplesshowed that a cutoff time of 33 msec for T2distributions would distinguish betweenfree-fluid porosity and capillary-boundwater. For carbonates, relaxation times tendto be three times longer and a cutoff of 100msec is used.7 However, both these valueswill vary if reservoir capillary pressure dif-fers from the 100 psi used on the cen-

trifuged samples. If this is the case, theexperiments may be repeated to find cutofftimes appropriate to the reservoir.

In a fine-grained sandstone reservoirexample, interpretation of conventional logdata showed 70 to 80% water saturation

26 Oilfield Review

1. Kleinberg RL, Kenyon WE and Mitra PP: On the Mecha-

nism of NMR Relaxation of Fluids in Rocks,J ournal of

Magnetic Resonance108A, no. 2 (1994): 206-214.2. Kleinberg RL and Horsfield MA: Transverse Relaxation

Processes in Porous Sedimentary Rock,J ournal of Mag-

netic Resonance88, no. 1 (1990): 9-19.

Sezginer A, Kleinberg RL, Fukuhara M and Latour LL:

Very Rapid Simultaneous Measurement of Nuclear Mag-

netic Resonance Spin-Lattice Relaxation Time and Spin-

Spin Relaxation Time,J ournal of Magnetic Resonance

92, no. 3 (1991): 504-527.

NMR Rela xa t ion Mech a nism s

There are three NMR relaxation mechanisms that

influence T1 or T2 relaxation tim es: grain surface

relaxation, relaxation by molecular diffusion in

magnetic fiel d gradients and relaxation by bulk

fluid processes. 1

Grain surface relaxationMolecules in fluids

are in constant motionBrownian motion and

diffuse about a pore space, hitting the grain sur-

face several times during one NMR measurement.

When this happens, two interactions may occur.

First, hydrogen protons can transfer nuclear spin

energy to the grain surface allowi ng realignment

with the static magnetic field, B0 this con-

tributes to longitudinal relaxation, T1. Second,

protons may be i rreversibly dephased contribut-

ing to transverse relaxation, T2. Researchers have

shown that in most rocks, grain-surface relaxation

is the m ost important influence on T1 and T2. The

abili ty of grain surfaces to relax protons is called

surface rel axivity, . 2

Compa rison of log a nd core da ta. CMR porosi ty shows a good m a tch wi th core poros-i ty me a sureme nts . Compu ted CMR perm ea bi li ty ha s been f ine- tuned to m a tch core

p e rm e a b ility e n a b lin g CM R log s to re p la c e c on ve n tio n a l c or in g on su b se q ue n t w e lls .

C aliper

125 375m m

B it Size

125 375m m

G a m m a R ay

0 150g API

SP

-120 30m V 1:120 m

C ore P erm eability

.01 m d 100

C M R Perm eability

.01 m d 100

Logarithm ic M ean T21 m sec 10000

C ore P orosity

0.2 m 3/m 3 0

C M R Porosity

0.2 m 3/m 3 0

C M R Free-Fluid P orosity

0.2 m 3/m 3 0

XX40

XX50

6. Straley C, Morriss CE, Kenyon WE and Howard JJ:NMR in Partially Saturated Rocks: Laboratory Insightson Free Fluid Index and Comparison with BoreholeLogs,Transactions of the SPWLA 32nd Annual Log-ging Symposium, Midland, Texas, USA, June 16-19,1991, paper CC.

7. Akbar M, Petricola M, Watfa M, Badri M, Charara M,Boyd A, Cassell B, Nurmi R, Delhomme JP, Grace M,Kenyon B and Roestenberg J: Classic InterpretationProblems: Evaluating Carbonates, Oilfield Review7,no. 1 (January 1995): 38-57.

Chang D, Vinegar H, Morriss C and Straley C: Effec-tive Porosity, Producible Fluid and Permeability inCarbonates from NMR Logging, Transactions of theSPWLA 35th Annual Logging Symposium, Tulsa,Oklahoma, USA, June 19-22, 1994, paper A.

Surfaces are not equally effective in relaxing

hydrogen protons. For example, sandstones are

about three tim es more efficient in re laxing pore

water than carbonates. Also rocks with a high

content of iron or other magnetic mi nerals have

larger than usual values of and, hence, shorter

NMR relaxation times.

Pore size al so plays an important role in sur-

face relaxation. The speed of relaxation depends

on how frequently protons can collide with the

surface and this depends on the surface-to-vol-

ume ratio (S/V) (next page, bottom). Collisions


9/15

27Autumn 1995

n Grain surface relaxation. Precessing protons move about pore space colliding with other protons and with grain surfaces (left). Every

time a proton collides with a grain surface there is a possibility of a relaxation interaction occurring. Grain surface relaxation is the most

important process affecting T1 and T2 relaxation times. Experimenters have shown that when the probability of colliding with a grain sur-

face is highsmall pores (center)relaxation is rapid and when the probability of colliding with a grain surface is lowlarge pores

(right)relaxation is slower.

Sm all pore

Am

pl

itude

Tim e, m sec

Large pore

Am

plitude

Tim e, m sec

n Free-fluid p orosity. Free -fluid p orosity isdeterm ined by a pp lying a c utoff to the T2distr ibut ion cu rve. The a rea und ernea th the c urve a bove the c utoff gives free -fluid

p oros ity (top). This zone rela tes to thelarge p roducible pores of a rock sam ple.The va lue for the c utoff wa s determ inedin the lab oratory from a large nu mb er of wa ter-satura ted c ore sam ples. Fi rs t , T2distr ibut ions we re m ea sured. Then thecores were c ent r ifuge d u nde r 100 psi pres-sure to sim ulate d raining down to typica l reservoi r cap i llary pressures . The a moun t of fluid d ra ined eq ua ls the free -fluid

p oros ity , w h ic h is c on ve rte d in to a nequ i va l en t a r ea on t he T 2 distributioncurve. The a rea sha ded from the r ightdeterm ines cutoff values. A comp a risonof T2 distr ibut ions ta ken before a nd a fter ce ntrifug ing show s the va lidity of thist echn i que (top). Free -fluid porosity m ea -sured in sa ndstone forma t ions in twowe lls using the c utoff va lue of 33 mse cobtained a bove shows good correla t ionwhen plotted versus centrifuge porosity(bottom).

R ock grain

R ock grain

R ock grain

5 10 15 20

centrifuge, p.u.

Free-fluid

porosity

at33m

sec,p.u.

20

15

10

5

W ell A

W ell B

0

Free-fluid cutoff

33 m sec

T2original

T2spun sam ple

Tim e, m sec

Pore diam eter, m icrons

Signal

distribution

1 10 100 1000

0.01 0.1 1 10

Free-fluid


10/15


11/15


12/15

30 Oilfield Review

History of Nuc lea r Ma g ne tic Reson a nc e Log g ing

As far back as 1946 , nuclear magnetic resonance

(NM R) signals from hydrogen atom nucleiprotons were fi rst observed independently by

Purcell and Bloch, a nd have since been used

extensively to characterize ma terials. 1 Magnetic

resonance i maging instruments are commonly

used as diagnostic tools in medicine today.

Oil industry interest followed in the 19 50s

with several patents for NMR l ogging tools filed

by companies such as California Research

Corporation, Schlumberger We ll Surveying

Corporation, Texaco Incorporated and Socony

M obil Oil Company.

The first NMR log was run in 1960. Developed

by Brown and Gamson of Chevron Research Com-

pany, the tool used the Earths magnetic fiel d for

proton alignment the principle underlying NMR

logging tools for the next 30 years (below).2

Schlumberger ran two versions of this tool in the1960 s and early 197 0s under license from Chevron

and later developed a third-generation tool

NM L-C commerciali zed at the end of the 1970s. 3

During this time, continuing research into NMR

interpretation produced some outstanding contri-

butions. Seevers developed a rela tionship

between relaxation time and permeability of sand-

stones in 1 965, and Timur developed the concept

of free-fluid i ndex and new methods to measure

permeability using NMR principles in 1968. 4 The

relationship between pore size, fluid and matrix

properties was presented by Loren and Robinson

of Shell Oil Company in 1969. 5

The 1970s a nd 1980s saw continuation of this

work by many oil companies in parallel with labo-

ratory NMR techniques developed to characterize

core samples. Schlumberger also has a long tra-dition of research into NM R techniques and pro-

duced important work on relaxation me chanisms

and pore-size distributions in the late 19 80s. 6

Much of this work continues today.

In the late 1980s the first experimental pulsed-

NMR logging tools were developed. A major dis-

advantage of early logging tools was having to

dope the mud system with magnetite to eliminate

the NMR signal from the borehole. To make the

technique more widely acceptable meant a radi-

cal design change to profit from a dvances in

pulsed-NMR technologym ore commonly used

in the la boratory for core measurements.

A patent for a pulse-echo NMR logging tool

was filed by Jackson in 1978. This was followed

Principle of early NMR tools. The Schlumberger NML Nuclear Magnetism Logging tool measured transverse relaxation, T2. Protons in the

formation are aligned to the Earths magnetic fieldconsidered to be homogeneous on these scales. A horizontally-mounted coil tips the

protons 90 (Borehole). They then start to precess about the Earths field and gradually relax back towards it (Magnetization). The same coil

measures the decaying horizontal magnetization as protons relax. The envelope of the decaying signal gives T2 (NML Signal). T2 amplitude

was extrapolated back to the start of the measurement to give NML porosity, assumed equal to free-fluid index. One big drawback with this

type of tool was that the borehole signal had to be eliminated by doping the mud system with magnetitenot very popular with drillers.

B orehole Fields M agnetization N M L Signal

Energize

R eceive

Tim

e

Free-fluidind ex

y

xToolEarth

z

Earth

N et

R otatingfram e ofreference

fL


13/15

B polarization

B earth

S

N

NS

NS

Perm anent m agnet

Static m agnetic field lines

R adio frequency coil

R ad io freq uency field lines

Sensitive zones

NS

N

S

S

N

1. Bloch F, Hansen W and Packard ME: The Nuclear Induc-

tion Experiment, Physical Review 70 (1946): 474-485.

Bloembergen RM, Purcell EM and Pound RV: Relaxation

Effects in Nuclear Magnetic Resonance Absorption,

Physical Review73 (1948): 679.

J ackson J and Mathews M: Nuclear Magnetic Resonance

Bibliography,The Log Analyst34, no. 4 (May-J une

1993): 35-69.

2. Brown RJ S and Gamson BW: Nuclear Magnetism

Logging,J ournal of Petroleum Technology12

(August 1960): 199-207.

3. Herrick RC, Couturie SH and Best DL: An ImprovedNuclear Magnetism Logging System and its Application to

Formation Evaluation, paper SPE 8361, presented at the

54th SPE Annual Technical Conference and Exhibition,

Las Vegas, Nevada, USA, September 23-26, 1979.

Chandler RN, Kenyon WE and Morriss CE: Reliable

Nuclear Magnetism Logging: With Examples in Effective

Porosity and Residual Oil Saturation, Transactions of the

SPWLA 28th Annual Logging Symposium, London, Eng-

land, J une 29-J uly 2, 1987, paper C.

31Autumn 1995

in the late 1980 s and 1990s by designs from

NUM AR and Schlumberger ( left).7

The two tools currently availa ble are the

Schlumberger second generation, pulse-echo

NMR tool the CMR tool and NUMARs MRIL

Magnetic Resonance Imager Log. 8 Both use per-

manent ma gnets, instead of the Earths magnetic

field, to align protons, a nd a system providing

controllable radio-frequency (rf) magnetic pulses

allowing T2 measurements. The use of perma-

nent magnets means that the position of the mea -

surement volume can be controlled by tool

design elim inating borehole doping.

Borehole NMR tool designs. Earth-field tool proposed by Brown and Gamson in

1960 (top left)was the principle behind three generations of Schlumberger NML

tools. Inside-out NMR uses permanent magnets and pulsed NMR technology (top

right). This configuration was first proposed by Jackson in 1984. The MRIL Mag-

netic Resonance Imager Log built by NUMAR Corporation in 1990 (bottom left)was

the first commercial pulsed NMR tool. A side-looking inside-out configuration

invented by Schlumberger is the basis for the CMR tool commercialized this year

(bottom right). The top two figures are side views of the borehole and the bottom

two figures are cross sections.

4. Seevers DO: A Nuclear Magnetic Method for Determining

the Permeability of Sandstones, Transactions of the

SPWLA 7th Annual Logging Symposium, Tulsa, Okla-

homa, USA, May 8-11, 1966.

Timur A: Effective Porosity and Permeability of Sand-

stones Investigated Through Nuclear Magnetic Reso-

nance Principles, Transactions of the SPWLA 9th Annual

Logging Symposium, New Orleans, Louisiana, USA,

J une 23-26, 1968, paper K.

5. Loren J D and Robinson J D: Relations Between Pore

Size Fluid and Matrix Properties, and NML Measure-

ments, paper SPE 2529, presented at the 44th SPEAnnual Fall Meeting, Denver, Colorado, USA, September

28-October 1, 1969.

Loren J D: Permeability Estimates from NML Measure-

ments,J ournal of Petroleum Technology24 (August

1972): 923-928.

6. Kenyon WE, Day PI, Straley C and Willemsen J F: Com-

pact and Consistent Representation of Rock NMR Data

for Permeability Estimation, paper SPE 15643, presented

at the 61st SPE Annual Technical Conference and Exhibi-

tion, New Orleans, Louisiana, USA, October 5-8, 1986.

Kenyon WE, Day PI, Straley C and Willemsen J F: A

Three-Part Study of NMR Longitudinal Relaxation Proper-

ties of Water-Saturated Sandstones,SPE Formation

Evaluation3, no. 3 (1988): 622-636.

Kenyon et al, reference 4 main text.

7. J ackson JA and Cooper RK: Magnetic Resonance Appa-

ratus, US Patent No. 4,350,955, August 1980.

J ackson J A: Nuclear Magnetic Resonance Well Log-

ging, The Log Analyst25 (September-October 1984):

16-30.

Kleinberg RL, Griffin DD, Fukuhara M, Sezginer A and

Chew WC: Borehole Measurement of NMR Characteris-tics of Earth Formations, US Patent No. 5,055,787,

October 1991.

8. Kleinberg et al, reference 12 main text.

Miller MN, Paltiel Z, Gillen ME, Granot J and Bouton J C:

Spin Echo Magnetic Resonance Logging: Porosity and

Free Fluid Index Determination, paper SPE 20561, pre-

sented at the 65th SPE Annual Technical Conference and

Exhibition, New Orleans, Louisiana, USA, September 23-

26, 1990.


14/15

T2 distribution measurements were alsomade on crude oil samples having viscosi-ties of 2.7 cp to 4300 cp (below). Highlyviscous oils have less mobile hydrogen pro-

tons and tend to relax quickly. The CMR logshowed the T2 oil peak and correctly pre-dicted oil viscosity. It also showed that theupper 150 ft of the diatomite formationundergoes a transition to heavier oil.

Capillary pressure curves, used byreservoir engineers to estimate the percent-age of connate water, may also be predictedfrom T2 distributions. Typically thesecurvesplots of mercury volume versuspressureare produced by injecting mer-cury into core samples. Under low pressurethe mercury fills the largest pores and, aspressure increases, progressively smallerpores are filled. The derivative of the capil-lary pressure curve approximates the T2 dis-

tribution. Some differences in shape areexpected as mercury injection measurespore throat sizes, whereas NMR measure-

ments respond to the size of pore bodies.11

Other applications and techniques arelikely to follow with more complex opera-

tions that might involve comparing logs rununder different borehole conditions. Forexample, fluid may be injected into the for-mation that is designed to kill the waterNMR signal so that residual oil saturation

may be measured. This type of technique,called log-inject-log, has been used with

other borehole logs to monitor injectivity orto monitor acid treatments.

n CMR tool. The CMR tool (left) i s only 14 ft [4.3 m] long a nd i s combina ble wi th m a nyother Schlum berge r logging tools. The sensor is skid-mounted to cut through m ud ca kea nd h a ve good c ontac t wi th the form a t ion over most hole s izes . Conta ct is provided bya n e cce nt ral izing a rm or by p owered ca lipers of o ther logging tools. Two p owerful per-ma nen t m a gnet s p rov ide a s ta t ic m a gnet ic f ie l d (right). By de sign the tool m ea sure-m en t volum e is a re gion of a bou t 0.5 in. to 1.25 in. [1.3 cm to 3.2 cm ] into the forma tiona nd st retches the length of the a ntenna a bout 6 in . [15 cm ], providing the tool with exc ellen t vert ica l resolution. The a rea in front of the a nten na doe s not contribu te to thesigna l, wh ich a llows the tool to ope rate in holes with a ce rtain a m ount of rugosity, simi-la r to densi ty tools . The a ntenna a cts as b oth t ra nsm i tter an d rece ivertra nsm i tt ing theCPMG ma gne t ic pu lse sequ enc e a nd rece iving the pulse echoes from the forma t ion.

32 Oilfield Review

B orehole w all

Antenna

W ear plate

Perm anent m agnet

Perm anent m agnet

B ow spring

eccentralizer

Electronic

cartridge

C M R skid

Sensitive zone

14

ft

3.9 m sec865 cp

414 m sec3.6 cp

T2

0.1 1.0 10.0 100.0 1000.0 10000.0

n T2 distributions for two oils of differe ntviscosit ies. Whe n b ulk fluid relax a tiondom inates , as in the ca se of flu id sa m ples,viscosity influe nc es rela xa tion t im e.Hydrogen protons in highly viscous fluidsa re less mobi le a nd ten d to relax q uickly .

Function of a Pulsed Ma gn etic

Resona nce Tool

The CMR tool i s the latest generationSchlumberger NMR tool (see History ofNuclear Magnetic Resonance Logging,page 30). The measurement takes placeentirely within the formation, eliminatingthe need to dope mud systems with mag-netite to kill the borehole signala bigdrawback with the old earth-field tools. Ituses pulsed-NMR technology, which elimi-

nates the effects of nonuniform static mag-netic fields and also increases signalstrength. This technology, along with the

sidewall design, makes the tool only 14 ft[4.3 m] long and readily combinable withother borehole logging tools (above).12

The skid-type sensor package, mountedon the side of the tool, contains two perma-nent magnets and a transmitter-receiverantenna. A bowspring eccentralizing arm or

powered caliper armif run in combinationwith other logging toolsforces the skidagainst the borehole wall, effectively remov-ing any upper limit to borehole size.

An important advantage of the sidewalldesign is that the effect of conductive mud,which shorts out the antenna on mandrel-type tools, is greatly reduced. What littleeffect remains is fully corrected by an inter-

nal calibration signal. Another advantage isthat calibration of NMR porosity is simpli-fied and consists of placing a bottle of wateragainst the skid to simulate 100% porosity.T2 properties of mud filtrate samplesrequired for interpretation correctionsmayalso be measured at the wellsite in a similarfashion. Finally, the design enables high-res-


15/15

33Autumn 1995

11. Morriss et al, reference 10.

12. Kleinberg RL, Sezginer A, Griffin DD and FukuharaM: Novel NMR Apparatus for Investigating anExternal Sample,Journal of Magnetic Resonance97, no. 3 (1992): 466-485.

Sezginer A, Griffin DD, Kleinberg RL, Fukuhara Mand Dudley DG: RF Sensor of a Novel NMR Appa-ratus,Journal of Electromagnetic Waves and Appli-cations7, no. 1 (1993): 13-30.

Morriss CE, MacInnis J, Freedman R, Smaardyk J,Straley C, Kenyon WE, Vinegar H and Tutunjian PN:Field Test of an Experimental Pulsed Nuclear Mag-netism Tool, Transactions of the SPWLA 34thAnnual Logging Symposium, Calgary, Alberta,Canada, June 13-16, 1993, paper GGG.

olution logginga 6-in. [15-cm] long mea-surement aperture is provided by a focusedmagnetic field and antenna.Two permanent magnets generate the

focused magnetic field, which is about1000 times stronger than the Earths mag-netic field. The magnets are arranged sothat the field converges to form a zone ofconstant strength about one inch inside the

formation. NMR measurements take placein this region.By design, the area between the skid and

the measurement volume does notcontribute to the NMR signal. Coupled withskid geometry, this provides sufficientimmunity to the effects of mudcake andhole rugosity. The rugose hole effect is simi-lar to that of other skid-type tools such asthe Litho-Density tool.The measurement sequence starts with a

wait time of about 1.3 sec to allow for com-plete polarization of the hydrogen protonsin the formation along the length of the skid

(for measurement principles, see All in aSpinNMR Measurements, page 20).Then the antenna typically transmits a trainof 600 magnetic pulses into the formationat 320-msec intervals. Each pulse inducesan NMR signalspin echofrom thealigned hydrogen protons. The antenna alsoacts as a receiver and records each spinecho amplitude. T2 distribution is derivedfrom the decaying spin echo curve, some-times called the relaxation curve.

Logs for the Future?

The prospects have always looked bright for

nuclear magnetic resonance logging.Research has shown that interpretation ofNMR relaxation times provides a wealth ofpetrophysical data. The latest generationlogging toolsusing pulsed NMR tech-niquesare building on that research andare providing powerful wellsite answersthat shed new insight into the basic ques-tion, What will the well produce? AM

nuclear magnetic resonance imaging-technology for the 21st century

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