Methods of investigating formation samples using NMR data

Patent metadata

Field	Value
Publication number	US-9939506-B2
Application number	US-201314406503-A
Country	US
Kind code	B2
Filing date	May 24, 2013
Priority date	Jun 8, 2012
Publication date	Apr 10, 2018
Grant date	Apr 10, 2018

Abstract

A methods are provided for investigating a sample containing hydrocarbons by subjecting the sample to a nuclear magnetic resonance (NMR) sequence using NMR equipment, using the NMR equipment to detect signals from the sample in response to the NMR sequence, analyzing the signals to extract a distribution of relaxation times (or diffusions), and computing a value for a parameter of the sample as a function of at least one of the relaxation times (or diffusions), wherein the computing utilizes a correction factor that modifies the value for the parameter as a function of relaxation time for at least short relaxation times (or as a function of diffusion for at least large diffusion coefficients).

First claim

What is claimed is:

1. A method of investigating a geological formation traversed by a borehole, comprising:
a) obtaining a nuclear magnetic resonance (NMR) tool;
b) subjecting a sample of known porosity to a nuclear magnetic resonance (NMR) sequence with a pulse echo spacing using the NMR tool;
c) determining a normalized bias B(T 2 ) of said NMR tool as a function of T 2 relaxation times;
d) locating the NMR tool in the borehole and detecting signals in response to said NMR sequence at at least one depth in the borehole;
e) analyzing a decay of the detected signals with a computer processor, in order to extract a distribution of the T 2 relaxation times; and
f) computing and providing, with the computer processor, a value for porosity of the formation at said depth as a function of at least one of said T 2 relaxation times, wherein said computing utilizes a correction factor that modifies the value of the parameter as a function of the T 2 relaxation time for at least the T 2 relaxation times occurring on the order of said pulse echo spacing, wherein said correction factor is a function of said normalized bias.

2. A method according to claim 1 , wherein:
said pulse echo spacing is approximately 0.2 ms, and
said relaxation times on the order of said pulse echo spacing are 1 ms and less.

3. A method according to claim 1 , wherein:
said correction factor is c f (T 2 )=1/(1+B(T 2 )).

4. A method according to claim 2 , wherein:

B
⁡

(

T
2

)

≈

ϕ
^

⁡

(

T
2

)

-

ϕ
T

ϕ
T

where ϕ T and {circumflex over (ϕ)}(T 2 ) are respectively a true and an estimated porosity of a calibration sample obtained from a porosity sensitivity curve for said NMR tool.

5. A method according to claim 1 , wherein:
said correction factor is

c
f

⁡

(

T
2

)

=

1

1
+

B
⁡

(

T
2

)

⁢

R
⁡

(

T
2

)

β
⁡

(

R
⁡

(

T
2

)

)

+

R
⁡

(

T
2

)

where
⁢

⁢

R
⁡

(

T
2

)

=

ϕ
^

⁡

(

T
2

)

σ
ϕ

⁡

(

T
2

)

and corresponds to a signal to noise ratio for a given T 2 , β is a scalar, < > is an average computed over T 2 ,

B
⁡

(

T
2

)

≈

ϕ
^

⁡

(

T
2

)

-

ϕ
T

ϕ
T

is the relative bias obtained from an NMR sensitivity curve of said NMR tool where ϕ T is a true porosity of a calibration sample and ϕ(T 2 ) is an estimated porosity of the calibration sample, and σ ϕ is a standard deviation of the estimated porosity.

6. A method according to claim 5 , wherein:
said correction factor tends to a value of 1 with respect to a particular T 2 relaxation time when said signal to noise ratio is small for that particular T 2 relaxation time.

7. A method acco

Assignees

• Schlumberger Technology Corp

Inventors

• Venkataramanan Lalitha
• Gruber Fred K
• Habashy Tarek M
• Akkurt Ridvan
• Vissapragada Badarinadh
• Lewis Richard E
• Rylander Erik

Classifications

• G01R33/448

  Relaxometry, i.e. quantification of relaxation times or spin density (G01R33/50 takes precedence) · CPC title

• G01V3/38

  Processing data, e.g. for analysis, for interpretation, for correction · CPC title

• G01R33/54 Primary

  Signal processing systems, e.g. using pulse sequences {; Generation or control of pulse sequences; Operator console} · CPC title

• G01V3/32

  operating with electron or nuclear magnetic resonance · CPC title

• G01N24/081

  Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity · CPC title