09122-11-PR-NMR_Ratios_Brine_Hydrocarbon_Relaxation_Mature_Organic_Shales-07-27-11.pdf

8/18/2019 09122-11-PR-NMR_Ratios_Brine_Hydrocarbon_Relaxation_Mature_Organic_Shales-07-27-11.pdf

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NMR T1/T2 Ratios

for

Brine

and

Hydrocarbon Relaxation in Mature

Organic Shales

Ayse Ozen

Richard Sigal,

MPGE, University of Oklahoma

July 27, 2011


2/17

NMR in

a Slide

• Magnetize hydrogen nuclei

• Perturb magnetism

• Measure time to relax back to equilibrium – T2 Time for magnetism to loose coherence in plane perpendicular to

primary field direction

– T1 Time to return to initial magnetism state

– Maximum

amplitude

of

relaxation

curve

measures

number

of

hydrogen nuclei.

– Relaxation• Surface T1 surface > T2surface• Bulk T1bulk=T2bulk•

Diffusion

T2 diffusion ( due

to

magnetic

gradients)

• 1/Tsurface =(surface relaxivity)*(surface area)/(Volume)


3/17

NMR Log

Potential

• Stand alone measurement

– Independent of

water

saturation

– Independent of porosity

– Independent of organic content

– Independent of

CH4 pressure (You don’t know it!)

• Deliver

– Moles/cc of free hydrocarbon fluid in reservoir

– Moles/cc of adsorbed hydrocarbon fluid in reservoir

– Fraction of hydrocarbon in organic pores


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NMR inMature Organic Material

• Organic pores – Hydrophobic

–

Nanometer

scale – High entry pressure for water

– Hydrocarbon fluids not displaced by water based mud invasion

– NMR signal contains all CH4 in organic pores

– Fast T2 as gas relaxes at pore surface

– Relaxation mechanism dipole‐dipole interaction not from paramagnetic impurities

–

Surface relaxation

temperature

dependent

• Inorganic pores – Water wet or heterogeneous wettability

– CH4 mostly displaced by mud invasion

– From capillary pressure gas pressure must significantly exceed water pressure.

– Gas could have fast T2

• Mud invasion

– Very low water permeability implies very shallow invasion

– NMR tools can produce signal as function of investigation depth

– Separates out inorganic and organic CH4 signal


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Gas Detection

Ordinary

Reservoirs

• Pore walls not gas wetting

•

Measurement

made

in

invaded

zone• Relatively large pores

• Logging tools permanent magnets have significant gradients

• Implies

for

relaxation

measurement

– High wetting fluid saturation

– Gas in centers of pores

– Diffusion coefficient is approximately bulk value

– T1 bulk

relaxation

implies

long

T1

– 1/T2gas = 1/Tbulk + 1/T2diffusion – Implies short T2


6/17

The Elephants in the Room

Gas Measurement

• The gas signal has poor signal to noise

– Signal has

fast

relaxation

– Maximum signal amplitude small

– Implies very slow logging speeds

• Separation of gas and water signal

– Gas does not relax as bulk fluid

– Small pores

mute

diffusion

effects

– Standard methods for gas identification fail


7/17


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Laboratory Study

T1/T2 Ratios

• 2 MHz system

• No external gradient

• T2 CPMG• T1 Inversion recovery

• Done on ground up samples with carefully controlled volumes – Average volume grain pack 9.53 cc

– Standard deviation .01 cc

• Room conditions

• Primary porosity inter‐ granular porosity

• Carefully controlled saturation fluid volumes – Average fluid volume 2.18 cc

– Standard

deviation

.03

cc

– Brine

– Dead light crude

– Dodecane


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T1/T2

• 1/T 2 = +1/T 2diffusion +1/T 2bulk +1/T 2surface• 1/T 2surface = 2S/V

• 1/T 1 ‐1/T 1bulk =1/T 1surface•

1/T 1surface=

1S/V • Estimate 1/T 2diffusion from T 2 at several TE

values

• Correct

for

bulk

relaxation

• T 1surface/T 2surface = 2/ 1


10/17

T1/T2 ratio for dodecane is 3.84.

T1/T2 ratio for crude is 4.32

T1/T2 ratio for brine is 2.25

Raw Ratios for Typical Sample


11/17

0

1

2

3

4

5

6

7

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

S a m p l e #

T1/T2 ratio

T1/T2 Ratio Organic Shale Samples

Brine

Crude oil

Dodecane

Raw

Data


12/17

0

1

2

3

4

5

6

7

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

S a m p l e #

T1Surface/T2Surface Ratio

Brine

Crude oil

Dodecane

Corrected Data


13/17

Raw T 1/T 2 ratios for brine, dead crude oil, and dodecane for seven

samples from

mature

organic

shale.


14/17

T 1surface/T 2surface ratios for

brine,

dead

crude

oil,

and

dodecane

for

seven samples from mature organic shale.


15/17

The ratios for crude oil to brine and dodecane to brine of the

T1/T2 surface relaxations.


16/17

Conclusions• The T1/T2 ratio provides sensitive way to

identify hydrocarbon

fluids

in

organic

pores

• The difference between the brine and the hydrocarbon ratio is the robust indicator

• Expected other

ratios

– Hydrocarbon liquids in inorganic pores, close to one (slow)

– Gas in

inorganic

pores,

variable.

– Water in organic pores, close to one (slow)


17/17

Selected References

Laboratory NMR Measurements on Methane

Saturated Barnett

Shale

Samples,

Richard

Sigal

and Elijah Odusina,, Petrophysics, February, 2011

• T1/T2 NMR Surface

Relaxation

Ratio

for

Hydrocarbons and Brines in Contact with Mature Organic Shale Source Rocks, Ayse E.

Ozen and Richard

F.

Sigal,

Submitted

to

Petrophysics

09122-11-PR-NMR_Ratios_Brine_Hydrocarbon_Relaxation_Mature_Organic_Shales-07-27-11.pdf

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