Claims
- 1. A method for nuclear magnetic resonance (NMR) measurement of petrophysical properties of an earth formation, comprising:
providing at least one first plurality of NMR pulses having at least one excitation and at least one refocusing pulse followed by a de-phasing delay and further followed by at least one test excitation pulse and at least a test refocusing pulse; and receiving at least one first signal in response to the at least one first plurality of NMR pulses; and combining the received at least one first signal with a second signal to reduce ringing error.
- 2. The method of claim 1, further comprising the step of generating at least one radio frequency pulse covering a relatively broad band of frequencies to saturate nuclear magnetization in the geologic formation, wherein a frequency, F1, of the at least one first plurality of NMR pulses is within the relatively broad band of frequencies.
- 3. The method of claim 1 wherein the at least one first plurality of NMR pulses is phase alternated to generate the second signal.
- 4. The method of claim 1 wherein a second plurality of NMR pulses is generated at a second frequency, F2, which is different from a frequency, F1, of the at least one first plurality of NMR pulses, to generate the second signal.
- 5. The method of claim 4 wherein the de-phasing delay is about (N+1/2)τ, wherein T is an interecho spacing and N is an integer.
- 6. The method of claim 5 wherein N is at least 5.
- 7. The method of claim 1, wherein the at least one first plurality of NMR pulses comprises a CPMG pulse sequence.
- 8. The method of claim 4, wherein the difference between the first and second frequencies is given by
- 9. The method of claim 8, wherein n is zero.
- 10. The method of claim 8, wherein n is ≧1.
- 11. The method of claim 4, wherein the first frequency F1 and the second frequency F2 correspond to non-overlapping resonant volumes in the geologic formation.
- 12. The method of claim 4, wherein pulses at the first frequency F1 are interleaved with pulses at the second frequency F2.
- 13. The method of claim 12, wherein echoes from corresponding interleaved pulses at the first frequency F1 and second frequency F2 are combined to form a composite signal.
- 14. The method of claim 12, wherein echoes from the interleaved pulses are processed to generate a difference signal between a first echo, corresponding to a first recovery time, and a second echo, corresponding to a second recovery time.
- 15. The method of claim 14, wherein the second recovery time is selected to cause substantial loss of signals associated with one or more of a gas phase, a water phase, and an oil phase in the geological formation.
- 16. The method of claim 12, wherein at least two consecutive pulses at the first frequency F1 are followed by at least two consecutive pulses at the second frequency F2.
- 17. The method of claim 14, wherein the first and second echo in the difference signal substantially correspond to a single depth mark.
- 18. The method of claim 4 further comprising providing a fifth plurality of NMR pulses corresponding to a third frequency F3, wherein the first frequency F1 and third frequency F3 correspond to non-overlapping resonant volumes in the geologic formation.
- 19. The method of claim 1, wherein the at least one first plurality of NMR pulses comprises at least two pulses separated by a variable recovery time selected to substantially reduce signals associated with one or more of a gas phase, a water phase, and an oil phase in the geological formation.
- 20. The method of claim 19, wherein the at least one second plurality of NMR pulses comprises at least two pulses separated by the variable recovery time.
- 21. The method of claim 2, wherein the generated at least one broadband saturation pulse and the at least one first plurality of pulses, are according to a pulse sequence defined as:
- 22. The method of claim 21, wherein the pulse sequence is modified as follows:
- 23. The method of claim 22 further comprising the step of repeating the pulse sequence for phase alternation, by replacing the (π)Y and (π)y pulses by (π)−Y and (π)−y, respectively.
- 24. The method of claim 23 further comprising the step of repeating the sequence at frequency F2.
- 25. The method of claim 24 further comprising the step of combining received echoes from the pulse sequences at frequencies F1 and F2.
- 26. The method of claim 25 further comprising the step of estimating an efficiency measure of non-formation signal cancellation in the combined received echoes from the pulse sequences at frequencies F1 and F2 following the (N+1/2)τ delay.
- 27. A method for determination of petrophysical properties of a volume of earth formation in a borehole using an NMR tool, the method comprising the steps of:
providing a static magnetic field in said volume of earth; providing oscillating magnetic fields in said volume of earth formation, to detect echoes thereto as an induced signal, according to pulse sequence comprising a first set of CPMG pulses associated with a pulse echo spacing τ1 and a second set of CPMG pulses associated with a pulse echo spacing τ2, shorter than τ1; and processing the induced signal with additional induced signals to determine petrophysical properties of the volume of earth formation.
- 28. The method of claim 27 wherein the additional induced signals correspond to at least one saturation recovery time different than a saturation recovery time of the pulse sequence.
- 29. The method of claim 27 wherein data, the data comprising the induced signal and the additional induced signals, is acquired in at least two orthogonal channels.
- 30. A method for making nuclear magnetic resonance (NMR) measurements of a geologic formation using an NMR logging tool, comprising the steps of:
providing a static magnetic field in the a volume of the formation; applying oscillating magnetic fields according to at least one modified CPMG pulse sequence characterized by at least one first echo spacing TD and a second echo spacing TE; wherein the at least one first echo spacing TD is selected to correspond to diffusion characteristics of fluids in the formation and cause corresponding amplitude loss in induced NMR echo signals, and TE is relatively short, such that diffusion in the corresponding induced NMR echo signals is substantially negligible; measuring the induced NMR echo signals; determining the amount of amplitude loss resulting from at least one TD interval; and computing diffusion properties of fluids in the formation based on the determined amplitude loss.
- 31. The method of claim 30, wherein the modified CPMG sequence is repeated with at least one first echo spacing TD≧TE.
- 32. The method of claim 30, wherein the at least one TD is selected to cause loss of signals associated with one or more of a gas phase, an oil phase, and a water phase to derive petrophysical properties of the geological formation.
- 33. The method of claim 30 further comprising the step of estimating diffusivity values for gas, water and oil phases in the formation, and selecting values for the TD and TE echo spacings based on the estimated diffusivity values.
- 34. The method of claim 33, wherein the selection of values for the TD and TE echo spacings is based on forward-modeling of signal components to achieve maximum contrast between fluid phases.
- 35. A system for increasing the resolution of NMR log data obtained using a multi-frequency NMR tool having N operating frequencies, comprising:
means for providing a NMR pulse echo signal comprising components corresponding to at least two operating frequencies of the tool; means for separating the provided pulse echo signal into two or more data-flow paths, each data flow path corresponding to an operating frequency of the tool; means for processing the signal in each data flow path separately to remove coherent noise components; means for combining output signals from the separately processed data flow paths to remove random noise components; and means for estimating residual coherent noise.
- 36. The system of claim 35 further comprising means responsive to a residual coherent noise estimate for initiating additional noise reduction by stacking at least one more signal.
REFERENCE TO RELATED APPLICATIONS AND PATENTS
[0001] This application is a continuation-in-part of and claims the benefit of the U.S. patent application Ser. No. 09/874,028, filed on Jun. 25, 2001, which is a continuation-in-part of the U.S. patent application Ser. No. 08/822,567, filed on Mar. 19, 1997 and issued as U.S. Pat. No. 6,242,912, which is a continuation of U.S. patent application Ser. No. 08/542,340 filed on Oct. 12, 1995, now abandoned, all of which applications and issued patents are incorporated herein by reference in their entirety for all purposes. This application also incorporates by reference in its entirety for all purposes the U.S. patent application Ser. No. 09/736,754, filed on Dec. 14, 2000.
Continuations (1)
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Number |
Date |
Country |
Parent |
08542340 |
Oct 1995 |
US |
Child |
08822567 |
Mar 1997 |
US |
Continuation in Parts (2)
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Number |
Date |
Country |
Parent |
09874028 |
Jun 2001 |
US |
Child |
10352763 |
Jan 2003 |
US |
Parent |
08822567 |
Mar 1997 |
US |
Child |
09874028 |
Jun 2001 |
US |