Correction of NMR artifacts due to axial motion and spin-lattice relaxation

• US 7,358,725 B2
• Filed: 05/16/2006
• Issued: 04/15/2008
• Est. Priority Date: 08/16/2004
• Status: Expired due to Fees

• Current Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Original Assignee: Baker Hughes Incorporated (Baker Hughes Company)
• Inventors: Blanz, Martin
• Primary Examiner(s): Shrivastav; Brij B.
• Application Number:
  US11/435,265
• Publication Number:
  US 20060273787A1
• Time in Patent Office: 700 Days
• Field of Search: 324300-303, 175/40, 175/45, 175/50, 166/66, 166/100, 166/11, 702/10, 702/9, 702/8, 702/7, 167/81
• US Class Current: 324/303
• CPC Class Codes: G01N 24/081 Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity
• CPC Class Codes: G01R 33/5617 using RF refocusing, e.g. RARE
• CPC Class Codes: G01R 33/56308 Characterization of motion or flow; Dynamic imaging
• CPC Class Codes: G01V 3/32 operating with electron or nuclear magnetic resonance

Abstract

NMR spin echo signals are corrected for axial motion of the borehole logging tool. The correction is applied to an averaged echo train using an average velocity determined from surface measurements over the duration of averaging of the echo trains. An additional correction may be applied to correct for excess polarization or incomplete polarization of nuclear spins due to an insufficient wait time between pulse sequences.

Claims

1. A method of processing Nuclear Magnetic Resonance (NMR) signals from an earth formation, the method comprising:

(a) conveying a NMR logging tool into a borehole in said earth formation and moving it axially therein with a velocity;

(b) polarizing nuclear spins in said earth formation;

(c) pulsing an antenna on said NMR logging tool with a plurality of pulse sequences, each of the plurality of pulse sequences producing an associated echo train;

(d) averaging the plurality of echo trains to give an averaged echo train;

(e) correcting the averaged echo train using a function of an average of the velocity and producing a corrected echo train.

2. The method of claim 1 wherein said logging tool is conveyed into said borehole on one of (i) a wireline, and, (ii) a slickline, (iii) a drillstring, and, (iv) coiled tubing.

3. The method of claim 1 wherein said correcting further comprises scaling the averaged echo train by a normalizing function related the average velocity and a reference velocity.

4. The method of claim 2 wherein said correcting further comprises using an equation of the form

$A_{\mathrm{cor}}$

( t ) = A ⁡

( t ) f c ⁡

( t ·

v axial v ref )

wherein A(t) is one of (i) an in-phase component of the averaged echo train, (ii) a quadrature component of the averaged echo train, and, (iii) an amplitude of the averaged echo train, t is a time, A_cor(t) is a corrected signal, v_axial is the average velocity, and v_ref is the reference velocity, and f_c is said normalizing function.

5. The method of claim 4 wherein said function f_c comprises a polynomial function and a perturbing term.

6. The method of claim 5 wherein said perturbing term comprises an exponential term.

7. The method of claim 1 wherein an excitation pulse of the plurality of pulse sequences has a tip angle that is substantially equal to 90°

.

8. The method of claim 1 wherein said at least one pulse sequence comprises a plurality of pulse sequences with a wait time TW between an ending time of a pulse sequence and a starting time of a subsequent pulse sequence.

9. The method of claim 8 further comprising using a saturation sequence after an ending time of a pulse sequence of said plurality of pulse sequences.

10. The method of claim 9 wherein said correcting further comprises scaling the averaged echo train by a normalizing function related to the average velocity, a longitudinal relaxation time of the earth formation, a reference velocity and a wait time.

11. The method of claim 9 wherein said correcting further comprises scaling the averaged echo train by a normalizing function related to the average velocity, a longitudinal relaxation time of the earth formation, a reference velocity and a wait time.

12. The method of claim 1 wherein said logging tool is conveyed in said borehole on a bottom hole assembly (BHA), the method further comprising measuring the velocity at a surface location.

13. The method of claim 1 wherein said nuclear spins are characterized by a transverse relaxation time (T₂) distribution and a longitudinal relaxation time (T₁) distribution, the method further comprising estimating a porosity of the earth formation using the average velocity, an estimate of the T₂ distribution and an estimate of the T₁ distribution.

14. An apparatus for Nuclear Magnetic Resonance (NMR) logging of an earth formation, the apparatus comprising:

(a) a NMR logging tool conveyed into a borehole in said earth formation by a conveyance device and moved axially therein with a velocity of motion;

(b) a magnet on said NMR logging tool which polarizes nuclear spins in the earth formation;

(c) an antenna on said logging tool which is pulsed with a plurality of pulse sequences, each of the plurality of pulse sequences producing an associated echo train;

(d) a receiver which receives said spin echo signals; and

(e) a processor which;

(A) averages the plurality of echo trains to produce an averaged echo train; and

(B) corrects the averaged echo train using a function of an average of the velocity, and produces a corrected echo train.

15. The apparatus of claim 14 wherein said conveyance device is selected from the group consisting of (i) a wireline, and, (ii) a slickline, (iii) a drillstring, and, (iv) coiled tubing.

16. The apparatus of claim 14 wherein said processor corrects said spin echo signals using a normalizing function related to the average of the velocity and a reference velocity.

17. The apparatus of claim 15 wherein said processor corrects said spin echo signals using an equation of the form

$A_{\mathrm{cor}}$

( t ) = A ⁡

( t ) f c ⁡

( t ·

v axial v ref )

wherein A(t) is one of (i) an in-phase component of the averaged echo train, (ii) a quadrature component of the averaged echo train, and, (iii) an amplitude of the averaged echo train, t is a time, A_cor(t) is a corrected signal, v_axial is the average of the velocity, and v_ref is the reference velocity, and f_c is the normalizing function.

18. The apparatus of claim 14 wherein said nuclear spins are characterized by a transverse relaxation time (T₂) distribution and a longitudinal relaxation time (T₁) distribution, and wherein said processor further estimates a porosity of said earth formation, based in part on the average of the velocity, an estimate of the T₂ distribution and an estimate of the T₁ distribution.

19. A computer readable medium for use with an apparatus for Nuclear Magnetic Resonance (NMR) logging of an earth formation, the apparatus comprising:

(a) a NMR logging tool conveyed into a borehole in said earth formation by a conveyance device and moved axially therein with a velocity of motion;

(b) a magnet on said NMR logging tool which polarizes nuclear spins in the earth formation;

(c) an antenna on said logging tool which is pulsed with a plurality of pulse sequences, each of the plurality of pulse sequences producing an associated echo train; and

(d) a receiver which receives said spin echo signals;

the medium including instructions that enable a processor to

(e) average the plurality of echo trains to produce an averaged echo train; and

(f) correct the averaged echo train using a function of an average of the velocity, and produces a corrected echo train.

20. The medium of claim 19 further comprising at least one of (i) a ROM, (ii) an EPROM, (iii) an EAROM, (iv) a flash memory, and (v) an optical disks.