NMR Pulse Sequences for Hydrogen Content in Shale
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Solution Overview
Problem
Current NMR well-logging tools face challenges in accurately measuring hydrogen content in solid samples like shale due to strong dipolar interactions, which result in short transverse relaxation times and low signal-to-noise ratios, especially when using the CPMG sequence, and cannot effectively distinguish between hydrogen in solids and fluids.
Innovation Solution
Implementing an NMR pulse sequence that combines a solid-state pulse sequence followed by a CPMG sequence to isolate the total hydrogen content, and using a 2D line-narrowing pulse sequence in conjunction with CPMG to differentiate between hydrogen species, thereby overcoming dipolar interactions and improving measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional CPMG sequence is used for NMR measurement in solid samples, then measurement speed is improved, but measurement precision deteriorates due to strong dipolar interactions and short transverse relaxation times
Solution Approach 1:
The patent divides the measurement into two distinct phases: first measuring total hydrogen content using a solid-state pulse sequence that is insensitive to dipolar interactions, then measuring fluid hydrogen content using CPMG sequence, and finally calculating solid hydrogen content by subtraction. This segmentation allows each measurement to be optimized for its specific purpose, resolving the contradiction between speed and precision.
Solution Approach 2:
The patent introduces an intermediary measurement approach by using the solid-state pulse sequence as a mediator to obtain total hydrogen content, which then serves as a reference for calculating the solid portion. This intermediary measurement enables the differentiation between solid and fluid hydrogen without directly measuring the difficult solid component alone.
2Productivity
If CPMG sequence is used downhole, then data gathering speed is improved, but signal-to-noise ratio deteriorates due to equipment power limits and downhole conditions
Solution Approach 1:
The patent changes the measurement parameters by switching between two different pulse sequence types (solid-state and CPMG) with different characteristics. The solid-state sequence operates with parameters optimized for precision in difficult conditions, while CPMG provides speed. This parameter change strategy resolves the contradiction between speed and signal quality in downhole environments.
3Device complexity
If conventional NMR measurement is used, then equipment complexity is reduced, but ability to distinguish hydrogen species deteriorates
Solution Approach 1:
The patent employs periodic alternation between two different pulse sequence measurements. The system periodically switches between solid-state sequencing and CPMG sequencing, with each providing complementary information. This periodic action enables full hydrogen species differentiation without requiring permanently complex equipment, as the same hardware performs multiple measurement functions through software-controlled sequence selection.
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 allows for accurate determination of hydrogen content in both solid and fluid phases, enabling better characterization of formation maturity and improving the precision of downhole measurements by enhancing the signal-to-noise ratio and distinguishing between different hydrogen-containing species.
Implementation Method 1
Nuclear magnetic resonance (NMR) technologies can be useful in a wide variety of applications. For example, in the field of oilfield services, NMR logging tools can provide information regarding fluids in a formation as well as porosity of the formation.
Implementation Method 2
in conventional NMR operation, the spins of nuclei align themselves along an externally applied static magnetic field
Implementation Method 3
the spins precess around the static field at the Larmor frequency, given by ω0=γ×B0, where B0 is the strength of the static field and γ is the gyromagnetic ratio
Implementation Method 4
a ninety degree pulse causes the spins to start processing. Then 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
Implementation Method 5
the CPMG sequence is not very well suited for studying solid samples with strong dipolar interactions because the pi (π) pulse rotations that make up the CPMG sequence do not refocus the homonuclear dipole-dipole interaction between nearby hydrogen atoms
Data Source
AI summary
Nuclear magnetic resonance (NMR) methods and apparatus are provided for investigating a sample utilizing NMR pulse sequences having solid state and CPMG pulse sequence portions. Various embodiments of solid state pulse sequences may be utilized including two-dimensional (repetitive) line-narrowing sequences. The hydrogen content of a solid portion of the sample may be determined by using one or more echoes resulting from the solid state sequence portion of the pulse sequence to establish a total organic hydrogen content of the sample, and by using a CPMG echo train to establish a fluid organic hydrogen content, and by subtracting one from the other to obtain the hydrogen content of the sample's solid portion. Additionally, or alternatively, the T2 values obtained from the line-narrowing and CPMG pulse sequences can be compared by plotting to obtain information regarding a characteristic of the sample. The NMR pulse sequence may also include a T1 portion.