NMR Fluid Analysis Using External Gradients and Multi-Sequence Processing
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Solution Overview
Problem
Existing NMR logging techniques struggle to accurately determine the diffusion properties of fluids in pore matrices due to internal magnetic field gradients and limitations in signal-to-noise ratio, which hinders the effective separation of fluids with similar relaxation times and diffusion coefficients.
Innovation Solution
The method involves obtaining NMR signals at multiple depths, processing them to create bin logs of relaxation and diffusion coefficients, and using similarity measures like Pearson correlation coefficients and mutual entropy to identify hydrocarbon content, while applying external magnetic field gradients to determine diffusion coefficients and estimate water saturation and permeability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If NMR logging is performed without considering internal magnetic field gradients, then the measurement process is simpler, but the diffusion properties of fluids cannot be accurately determined
Solution Approach 1:
The patent applies parameter changes by introducing external magnetic field gradients as a controllable variable to enhance the measurement of diffusion properties. By systematically varying the gradient strength and applying different pulse sequences (CPMG, FID, stimulated echo), the method transforms the measurement process to accurately capture diffusion coefficients while accounting for internal magnetic field gradients. This resolves the contradiction by making the measurement process more complex but yielding significantly improved precision in diffusion property determination.
2Measurement precision
If signal-to-noise ratio is improved to separate fluids with similar relaxation times, then fluid separation accuracy improves, but measurement time or energy consumption increases
Solution Approach 1:
The patent transitions from relying solely on relaxation time differences to a two-dimensional characterization space that includes both relaxation time and diffusion coefficient. By measuring diffusion coefficients through external magnetic field gradients and incorporating them into the analysis, the method creates an additional dimension for fluid differentiation. This allows separation of fluids with similar relaxation times based on their distinct diffusion properties, improving accuracy without requiring excessive measurement time.
Solution Approach 2:
The method employs multiple NMR pulse sequences (CPMG, FID, stimulated echo) with different sensitivity characteristics to diffusion. By changing the measurement parameters and sequence types, the patent optimizes the signal-to-noise ratio for diffusion coefficient determination while managing measurement time. The combination of sequences allows efficient extraction of diffusion information without excessive time investment.
3Measurement precision
If multiple NMR pulse sequences are used to characterize diffusion, then measurement accuracy improves, but device complexity and processing requirements increase
Solution Approach 1:
The patent implements a multi-functional NMR logging tool capable of executing multiple pulse sequences (CPMG, FID, stimulated echo) and performing various measurements (relaxation time, diffusion coefficient, permeability, fluid saturation). This universal tool consolidates what would otherwise require separate specialized tools, managing device complexity while achieving high measurement precision through the coordinated use of multiple sequences.
Solution Approach 2:
The patent combines multiple measurement functions into a single integrated NMR logging system. By merging the capabilities to perform CPMG, FID, and stimulated echo sequences, and to simultaneously determine relaxation times, diffusion coefficients, permeability, and fluid saturations, the patent reduces overall system complexity compared to using separate specialized tools for each measurement type.
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 enables accurate identification of hydrocarbon content and permeability by considering internal magnetic gradients and external field effects, improving the reconstruction of relaxation and diffusion spectra and providing detailed insights into fluid properties within earth formations.
Implementation Method 1
One evolving technique uses nuclear magnetic resonance (NMR) logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. NMR logging tools excite the nuclei of the fluids in the geological formations in the vicinity of the borehole
Implementation Method 2
The purpose of the B0 field is to polarize the magnetic moments of nuclei parallel to the static field
Implementation Method 3
the purpose of the B1 field is to rotate the magnetic moments by an angle controlled by the width tp and the amplitude B1 of the oscillating pulse
Implementation Method 4
After being tipped by 90°, the magnetic moment precesses around the static field at a particular frequency known as the Larmor frequency ω, given by ω=γB0
Implementation Method 5
Inhomogeneities of the B0 field result in dephasing of the magnetic moments and to remedy this, a 180° pulse is included in the sequence to refocus the magnetic moments. This refocusing gives a sequence of n echo signals
Implementation Method 6
The effect of field gradients is given by an equation of the form... where the third term is related to the field gradient G by an equation of the form... where D is the diffusivity of the fluid
Data Source
AI summary
Pulse sequences are applied to a fluid in an earth formation in a static magnetic field and NMR spin echo signals are obtained. The signals are inverted to give T2 distributions at a plurality of depths. Similarities between logs of the T2 bins with resistivity and/or gamma ray logs are used to identify and subtract contributions to the NMR signal from oil. having internal gradients. From the received signals, relaxation and diffusion characteristics of the fluid are determined. The determination takes into account the internal field gradients. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 CFR 1.72(b).