Segmented Refocusing Pulses for NMR Logging SNR

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Solution Overview

Problem

Conventional NMR well logging techniques face limitations due to low signal-to-noise ratio (SNR) in inhomogeneous static magnetic fields, which impede accurate data gathering and analysis of subsurface formations.

Innovation Solution

The implementation of a series of refocusing pulses with specific segment durations and phase shifts, such as RPP pulses, and optimized excitation pulses that align initial magnetization with the refocusing axis, enhancing the SNR and improving data quality in inhomogeneous fields.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Use of energy by moving object

If conventional CPMG sequence is used in inhomogeneous magnetic field, then equipment power consumption is reduced, but signal-to-noise ratio becomes low

Engineering Contradiction:

Improveequipment power consumptionVSAvoidsignal-to-noise ratio

Core Design Contradiction:

Use of energy by moving objectVSMeasurement precision

Solution Approach 1:

The refocusing pulse is divided into multiple segments with different phases. Specifically, the pulse is segmented into portions with 0-degree phase and 180-degree phase shifts, allowing selective refocusing of spins in different frequency regions while managing power consumption and improving signal quality.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different segments of the refocusing pulse apply different phase characteristics to different portions of the spin system. The 0-degree phase segment addresses spins in one frequency region while the 180-degree phase segment addresses spins in another frequency region, optimizing refocusing locally for each spin population.

Inventive Principle:

Principle #3Local quality

2Device complexity

If conventional refocusing pulse is used, then pulse sequence simplicity is maintained, but refocusing effectiveness in inhomogeneous field deteriorates

Engineering Contradiction:

Improvepulse sequence simplicityVSAvoidrefocusing effectiveness

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The pulse sequence employs periodic application of refocusing pulses with alternating phase segments. This periodic structure maintains rhythm and predictability in the sequence while incorporating phase variations that improve refocusing effectiveness across different spin populations.

Inventive Principle:

Principle #19Periodic action

3Speed

If standard excitation pulse is used, then initialization speed is maintained, but magnetization alignment with refocusing axis is insufficient

Engineering Contradiction:

Improveinitialization speedVSAvoidmagnetization alignment

Core Design Contradiction:

SpeedVSMeasurement precision

Solution Approach 1:

The excitation pulse is designed to pre-align the net magnetization vector with the refocusing axis before the refocusing pulse sequence begins. This preliminary alignment ensures that subsequent refocusing operations are optimally positioned to generate strong echoes, improving measurement precision without significantly delaying initialization.

Inventive Principle:

Principle #10Preliminary action

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach nearly doubles the SNR in CPMG sequences and augments the reliable sample slice, allowing for more precise measurements of porosity and pore size distributions, thereby improving the accuracy of petrophysical parameters.

Implementation Method 1

nuclear magnetic resonance (NMR) operation, the spins of nuclei align themselves along an externally applied static magnetic field

Methodology Applied

Scientific EffectNuclear magnetic resonance:

Implementation Method 2

the spins precess around the static field at the Larmor frequency, given by ω0=γ×B0

Methodology Applied

Scientific EffectLarmor precession:

Implementation Method 3

a one-hundred-eighty degree pulse is applied to cause the spins which are dephasing in the transverse plane to refocus. By repeatedly refocusing the spins using one-hundred-eighty-degree pulses, a series of 'spin echoes' appear

Methodology Applied

Scientific EffectSpin echo: Echo

Implementation Method 4

This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. a radio frequency (RF) pulse), which tips the spins away from the static field direction

Methodology Applied

Scientific EffectRF pulse excitation:

Implementation Method 5

In an inhomogeneous field, spins at different locations precess at different rates. Therefore, in addition to the molecular spin-spin relaxation of fluids, spatial inhomogeneities of the applied field also cause dephasing

Methodology Applied

Scientific EffectMagnetic field inhomogeneity: Magnetic Field

Data Source

PatentUS10962674B2Refocusing pulses and excitation pulses for NMR logging

Publication Date: 2021.03.30 SCHLUMBERGER TECH CORP

AI summary

Illustrative embodiments are directed to applying a nuclear magnetic resonance sequence to a substance within an inhomogeneous static magnetic field. Various embodiments include applying a series of refocusing pulses to the substance, each refocusing pulse in the series of refocusing pulses having at least two segments, and a total pulse duration less than or equal to approximately 1.414 times T180. Various embodiments can further include applying an excitation pulse to the substance in the inhomogeneous static magnetic field, where the excitation pulse generates an initial magnetization that is aligned with a refocusing axis produced by a refocusing cycle that is performed after the excitation pulse.