Multi-dimensional NMR Spectra for Hydrocarbon Fluid Characterization
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Solution Overview
Problem
Current methods for characterizing hydrocarbon samples, such as crude oils, face challenges in accurately distinguishing between different molecular species due to overlapping signals in NMR spectroscopy, particularly in downhole applications where resolution is limited by viscosity and temperature variations.
Innovation Solution
The use of multi-dimensional NMR spectra obtained by combining NMR spectroscopy with relaxation techniques through specific pulse sequences, such as π/2-τ1-Δ-CPMG, allows for separation of signals based on chemical shift and spin-spin relaxation time, enabling identification of distinct molecules by generating two- or three-dimensional spectra.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional NMR spectroscopy is used for hydrocarbon characterization, then the analysis can be performed with simple equipment, but the signals from different molecular species overlap and cannot be distinguished accurately
Solution Approach 1:
The patent transforms the one-dimensional NMR spectrum into a two-dimensional spectrum by incorporating spin-spin relaxation time (T2) as a second dimension. This is achieved through the π/2-τ1-Δ-CPMG pulse sequence where τ1 provides chemical shift information and the CPMG portion measures T2 relaxation. Molecules with identical chemical shifts but different T2 values are separated in the second dimension, resolving signal overlap problems.
2Ease of operation
If downhole NMR testing is performed in viscous oil environments, then in-situ analysis is achieved, but signal resolution deteriorates due to viscosity and temperature variations
Solution Approach 1:
The invention compensates for the degrading effect of viscosity and temperature on spectral resolution by adding the T2 relaxation dimension. While chemical shift resolution may be compromised in downhole conditions, the CPMG sequence measures T2 relaxation times that remain distinctive for different molecular species even when chemical shifts overlap. This allows accurate characterization despite environmental challenges.
Solution Approach 2:
The patent utilizes the temperature and pressure dependence of T2 relaxation times to differentiate molecular species in downhole conditions. By measuring T2 as a function of these parameters, the method extracts additional information that compensates for the reduced chemical shift resolution inherent in high-temperature, high-pressure environments.
Data Source
AI summary
Methods are disclosed for characterizing samples containing a plurality of molecular species through the use of multi-dimensional spectra obtained by processing of measurements resulting from pulse sequences combining NMR spectroscopy and NMR relaxation techniques.