Towards Online Natural Gas Composition Analysis by Low-Field NMR Spectroscopy

Abstract

The most common NMR methods in the oil and gas industry are NMR relaxometry and diffusometry. Relaxation times T1 and T2 as well as diffusivity D are determined for characterizing fluids and their surrounding pores in logging applications or laboratory analyses. NMR spectroscopy is rarely used for petrophysical investigations but primarily in medical, food, and material applications. In a joint operator-academia-service company project, we explore opportunities for gas component quantification by low-field NMR spectroscopy with the prospect of rig-site, production-plant, and sea-floor deployment. Using an affordable benchtop device in a production-like environment, we gained a fundamental understanding for a commercially viable application of low-field NMR spectroscopy and translated this knowledge into a practical workflow for online natural gas composition analysis. We measured typical natural gas components (all isomers from methane to n-hexane) with a portable desktop NMR spectrometer working at a proton resonance frequency of 60 MHz A hardware setup was manufactured for mimicking a gas production environment up to 200 bar. A database was populated by NMR signatures of pure gas components, derived from measurements on pure components and binary mixtures. Additional efforts were dedicated to understanding and quantifying systematic effects on the hydrocarbon NMR spectra connected to sample composition and pressure. Pre- and post-processing data procedures were developed and applied for substantially increasing robustness of the method and further improving the gas composition analyses results. Using an indirect hard modeling (IHM) analysis, the constituting pure components in binary, ternary, and more complex gas mixtures were identified and quantified. IHM automatically accounts for small variations and uncertainties in the NMR spectra. The results from the NMR spectrum analysis are in very good agreement with vendor certificates of gas composition obtained from gravimetrics and gas chromatography.