NMR Direct-Echo Pulse Sequence for Porosity Overcall
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Solution Overview
Problem
Nuclear magnetic resonance (NMR) measurements in formations with short relaxation time T2 and T1 greater than T2 lead to porosity overcall, particularly in shale intervals, due to unaccounted second-order stimulated echo effects.
Innovation Solution
A direct-echo pulse sequence is employed, which includes a series of pulses to separate direct and stimulated NMR echoes, allowing for accurate estimation of porosity by processing the amplitudes of clean direct echoes, thereby reducing errors from second-order stimulated echo effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional NMR pulse sequences are used in formations with short T2 and T1>T2, then measurement speed is maintained, but porosity overcall occurs due to unaccounted second-order stimulated echo effects
Solution Approach 1:
The pulse sequence is segmented into specific pulse groups (e.g., 90-degree pulses followed by 180-degree pulses) with carefully controlled timing intervals. This segmentation allows direct echoes to be separated from stimulated echoes in the echo train, enabling accurate identification and measurement of true porosity signals while excluding stimulated echo contributions that cause overcall.
Solution Approach 2:
The pulse sequence employs specific parameter settings including precise pulse spacing intervals (e.g., tau and delta tau), pulse flip angles (90 degrees and 180 degrees), and echo train timing parameters. These parameter changes are optimized to maximize direct echo separation while minimizing stimulated echo contamination, directly addressing the porosity measurement accuracy issue in formations with T1>T2.
2Measurement precision
If conventional pulse sequences are used, then equipment operation remains simple, but porosity overcall reaches 20 percent in shale intervals
Solution Approach 1:
The pulse sequence is pre-configured with specific timing parameters and pulse patterns that automatically separate direct echoes from stimulated echoes. This preliminary action is built into the sequence design, so operators simply need to select the appropriate pre-programmed sequence rather than manually adjusting multiple parameters, maintaining ease of operation while achieving accurate porosity measurements in shale formations.
3Measurement precision
If standard echo trains are acquired, then data acquisition speed is maintained, but stimulated echoes contaminate direct echo signals
Solution Approach 1:
The pulse sequence uses periodic pulse patterns with specific repetition intervals and echo spacing. This periodic structure creates a predictable echo train where direct echoes and stimulated echoes occur at distinct, regularly spaced intervals. By analyzing the periodic pattern, the system can identify and extract pure direct echo signals without requiring extended measurement times, thus maintaining data acquisition speed while improving signal separation.
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
The direct-echo pulse sequence effectively reduces porosity overcall by providing accurate microporosity measurements, even in formations with fast relaxation times, such as shale and heavy oil, by fully or partially separating non-direct echoes from direct echoes.
Implementation Method 1
a magnet assembly disposed in the carrier and configured to generate a static magnetic field in the formation
Implementation Method 2
at least one transmitting assembly disposed in the carrier and configured to generate an oscillating magnetic field in a volume of interest within the formation
Implementation Method 3
nuclear magnetic resonance (NMR), can be used to estimate formation characteristics such as mineralogy-independent porosity and permeability of rocks
Data Source
AI summary
A nuclear magnetic resonance (NMR) apparatus includes a carrier configured to be deployed in a borehole, a magnet assembly configured to generate a static magnetic field in an earth formation, and at least one transmitting assembly configured to generate an oscillating magnetic field in a volume of interest within the formation. The apparatus also includes a pulse generator configured to apply a direct-echo pulse sequence to the at least one transmitting assembly, the direct-echo pulse sequence having a plurality of successive pulses including a first pulse and a second pulse configured to generate a first direct NMR echo, and a third pulse, the third pulse selected to at least partially separate a stimulated NMR echo from a second direct NMR echo occurring after the third pulse. The apparatus further includes at least one receiving assembly configured to detect the first and second direct echoes of an NMR echo train.