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Considerations for the acquisition and inversion of NMR T2 data in shales.

Considerations for the acquisition and inversion of NMR T2 data in shales.

Authors :
Testamanti, M. Nadia
Rezaee, Reza
Source :
Journal of Petroleum Science & Engineering. Mar2019, Vol. 174, p177-188. 12p.
Publication Year :
2019

Abstract

Abstract Low-field Nuclear Magnetic Resonance (NMR) is a non-invasive method widely used in the petroleum industry for the evaluation of reservoirs. Pore structure and fluid properties can be evaluated from transverse relaxation (T 2) distributions, obtained by inverting the raw NMR signal measured at subsurface conditions or in the laboratory. This paper aims to cast some light into the best practices for the T 2 data acquisition and inversion in shales, with a focus on the suitability of different inversion methods. For this purpose, the sensitivity to various signal acquisition parameters was evaluated from T 2 experiments using a real shale core plug. Then, four of the most common inversion methods were tested on synthetic T 2 decays, simulating components often associated with shales, and their performance was evaluated. These inversion algorithms were finally applied to real T 2 data from laboratory NMR measurements in brine-saturated shale samples. Methods using a unique regularization parameter were found to produce solutions with a good balance between the level of misfit and bias, but could not resolve adjacent fast T 2 components. In contrast, methods applying variable regularization – based on the noise level of the data – returned T 2 distributions with better accuracy at short times, in exchange of larger bias in the overall solution. When it comes to reproducing individual T 2 components characteristic of shales, the Butler-Reeds-Dawson (BRD) algorithm was found to have the best performance. In addition, our findings suggest that threshold T 2 cut-offs may be derived analytically, upon visual inspection of the T 2 distributions obtained by two different NMR inversion methods. Highlights • Short echo spacings (TE) can improve signal quality but also enhance early experimental noise. • Inversion methods using fixed smoothing cannot resolve adjacent T 2 components correctly. • Inversion methods based on noise level produce better fits but may bias solutions. • The BRD algorithm can differentiate the contribution from adjacent peaks at short T 2 times. • Threshold T 2 cut-offs are determined analytically by using two different NMR inversion methods. [ABSTRACT FROM AUTHOR]

Details

Language :
English
ISSN :
09204105
Volume :
174
Database :
Academic Search Index
Journal :
Journal of Petroleum Science & Engineering
Publication Type :
Academic Journal
Accession number :
134296725
Full Text :
https://doi.org/10.1016/j.petrol.2018.10.109