Dual-Step TE NMR Pulse Sequence for Gas Identification
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Solution Overview
Problem
Existing NMR logging methods face challenges in accurately identifying gas quantities in geological formations due to uncertainties in magnetic field gradients, which affect spin dephasing and relaxation time measurements, leading to errors in fluid identification and porosity analysis.
Innovation Solution
A dual-step TE pulse sequence is employed, using a short TE for the initial subset of pulses to minimize the internal gradient effect and a longer TE for the latter subset to distinguish gas from liquids based on diffusion techniques, while averaging echo trains to estimate gas saturation and T2 distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a long TE is used to enhance diffusion contrast for gas identification, then gas identification capability is improved, but the internal gradient effect increases causing measurement uncertainty
Solution Approach 1:
The pulse sequence is divided into two distinct segments: a first set of refocusing pulses with short TE to minimize internal gradient effects and establish a baseline, and a second set with long TE to maximize diffusion contrast for gas identification. This segmentation allows the harmful internal gradient effect to be isolated and subtracted from the measurement, enabling accurate gas identification while maintaining measurement reliability.
Solution Approach 2:
The first set of refocusing pulses with short TE serves as a preliminary measurement that captures the internal gradient effect before it interferes with the gas identification measurement. By acquiring this baseline data first, the method prepares for subsequent subtraction to eliminate the internal gradient effect from the long TE measurement, thereby preventing measurement uncertainty.
2Reliability
If a short TE is used to minimize internal gradient effect, then measurement reliability is improved, but diffusion contrast for gas identification is reduced
Solution Approach 1:
The measurement process is segmented into two functional parts: short TE pulses for reliable baseline acquisition and long TE pulses for gas-specific diffusion contrast. This segmentation enables each part to optimize for its specific purpose while the combination achieves both reliability and gas identification capability.
Solution Approach 2:
The internal gradient effect is extracted as a separate measurable component using short TE pulses. By isolating this effect in the first measurement set, it can be removed from the second measurement set, allowing the long TE measurement to purely reflect diffusion contrast without the confounding internal gradient effect.
3Measurement precision
If multiple pulse sequences with different TEs are used to separate internal gradient effect from diffusion contrast, then measurement accuracy is improved, but device complexity increases
Solution Approach 1:
The method employs periodic alternation between short TE and long TE pulse sequences in a systematic manner. This periodic structure, while involving multiple TEs, follows a regular pattern that simplifies data processing and interpretation compared to more complex multi-parameter sequences, thereby achieving good measurement accuracy with manageable device complexity.
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 method reduces uncertainty in NMR log data analysis by effectively distinguishing gas from liquids and improving the accuracy of gas saturation and porosity estimates, even in low-field gradient conditions.
Implementation Method 1
NMR operates based on the fact that the nuclei of many elements have angular momentum (spin) and a magnetic moment. The nuclei have a characteristic Larmor resonant frequency related to the magnitude of the magnetic field in their locality.
Implementation Method 2
NMR tools generate a uniform or near uniform static magnetic field in a region of interest surrounding the wellbore. Over time the nuclear spins align themselves along an externally applied magnetic field.
Implementation Method 3
This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field, which tips the spins with resonant frequency within the bandwidth of the oscillating magnetic field away from the static field direction.
Implementation Method 4
After tipping, the spins precess around the static field at a particular frequency known as the Larmor frequency ω0, given by ω0=γB0
Implementation Method 5
At the same time, the spins return to the equilibrium direction (i.e., aligned with the static field) according to an exponential decay time known as the spin-lattice relaxation time, or longitudinal relaxation time, T1.
Implementation Method 6
Existing NMR logging methods face challenges in accurately identifying gas quantities in geological formations due to uncertainties in magnetic field gradients, which affect spin dephasing and relaxation time measurements
Data Source
AI summary
A first NMR echo train is acquired with a single TE. A second NMR echo train is acquired with a first portion having the same TE and the second portion having a longer TE. When measurements are made with a low magnetic field gradient, processing the two echo trains can be used to determine gas in the formation.