Hybrid saturation recovery-inversion recovery pulse sequence for improved NMR logging of boreholes
A nuclear magnetic resonance (NMR) logging method includes providing a hybrid pulse sequence having a saturation pulse, an inversion pulse, and a detection sequence. The method also includes measuring echo signals in response to the hybrid pulse sequence. The method also includes deriving a spin-lattice time constant (T 1 ) distribution from the measured echo signals. A NMR system includes a hybrid pulse sequence module to provide a hybrid pulse sequence with a saturation pulse, an inversion pulse, and a detection sequence. The NMR system also includes a control module to select a time interval between the saturation pulse and the inversion pulse.
1. A nuclear magnetic resonance (NMR) method that comprises:
repeatedly applying, to a formation sample, a hybrid pulse sequence having a saturation pulse, an inversion pulse, and a detection sequence;
selecting a time interval between the saturation pulse and the inversion pulse during the repeated application of the hybrid pulse sequence to increase sensitivity to fast-relaxing components in the formation sample, wherein the time interval is determined based, at least in part, on a T 1 value associated with the fast-relaxing components in the formation sample;
measuring echo signals in response to the repeated application of the hybrid pulse sequence; and
deriving a T 1 distribution from the measured echo signals.
2. The NMR method of claim 1 , further comprising displaying a representation of the T 1 distribution.
3. The NMR method of claim 1 , wherein the hybrid pulse sequence comprises multiple saturation pulses.
4. The NMR method of claim 1 , wherein the detection sequence comprises a free-induction decay (FID) pulse.
5. The NMR method of claim 1 , wherein the detection sequence comprises a spin-echo sequence, a Carr-Purcell sequence, a Carr-Purcell-Meiboom-Gill (CPMG) sequence, or a sequence with different phase cycling schemes.
6. The NMR method of claim 1 , further comprising adjusting a time interval between the saturation pulse and the inversion pulse from a first value to a second value that is different than the first value during the repeated application of the hybrid pulse sequence to increase sensitivity to said fast-relaxing components.
7. The NMR method of claim 1 , wherein said repeated application of the hybrid pulse sequence is performed by a downhole NMR logging tool or logging-while-drilling (LWD) NMR tool.
8. The NMR method claim 1 , wherein said repeated application of the hybrid pulse sequence is performed by a laboratory NMR tool.
9. The NMR method of claim 1 , wherein the T 1 value of the fast-relaxing components comprises a value below 10 ms.
10. The method of claim 1 , further comprising:
performing one or more simulations, wherein results of the simulation are utilized in determining the T 1 value associated with the fast-relaxing components.
11. A nuclear magnetic resonance (NMR) system, comprising:
a hybrid pulse sequence module to apply a hybrid pulse sequence with a saturation pulse, an inversion pulse, and a detection sequence; and
a control module to select a time interval between the saturation pulse and the inversion pulse during repeated application of the hybrid pulse sequence to a formation sample to increase sensitivity to fast-relaxing components in the formation sample, wherein the time interval is determined based, at least in part, on a T 1 value associated with the fast-relaxing components in the formation sample.
12. The NMR system of claim 11 , wherein the hybrid pulse sequence module and the control module correspond to software modules stored in computer-readable memory and executable by a processor.
13. The NMR system of claim 11 , wherein the hybrid pulse sequence module and the control module correspond to hardware modules.
14. The NMR system of claim 11 , further comprising:
a static magnetic field source;
a pulsed magnetic field source; and
a measurement storage unit to store spin-lattice time constant (T 1 ) distribution measurements based on the repeated application of the hybrid pulse sequence.
15. The NMR system of claim 11 , wherein the NMR system is part of a downhole logging tool or logging-while-drilling (LWD) tool.
16. The NMR system of claim 11 , wherein the NMR system is part of a laboratory tool.
17. The NMR system of claim 11 , wherein the T 1 value of the fast-relaxing components comprises a value below 10 ms.
18. The NMR system of claim 11 , wherein the T 1 value associated with the fast-relaxing components is determined according to one or more simulations.
19. A nuclear magnetic resonance (NMR) data acquisition method that comprises:
exposing a measurement region of a formation sample to a static field;
repeatedly applying a pulse sequence to the measurement region, the pulse sequence including, in order: a saturation pulse, an inversion pulse, a 90° pulse, and optionally one or more 180° pulses to induce echo signals, wherein a time interval between the inversion pulse and the 90° pulse varies for repeated applications of the pulse sequence to increase sensitivity to fast-relaxing components in the formation sample, wherein values for the time interval are spaced on a logarithmic scale;
collecting measurements of a free-induction decay signal caused by the 90° pulse or measurements of the echo signals;
deriving a measurement of a T 1 distribution from said measurements; and
displaying to a user a representation of the T 1 distribution.
20. The method of claim 19 , wherein values for the time interval are equally spaced on the logarithmic scale.
21. The method of claim 19 , wherein, the values for the time interval are more closely spaced for lower values than for higher values.
22. The method of claim 19 , wherein the fast-relaxing components have a T 1 value below 10 ms.