Advances in low-field nuclear magnetic resonance (NMR) technologies applied for characterization of pore space inside ro
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REVIEW
Advances in low‑field nuclear magnetic resonance (NMR) technologies applied for characterization of pore space inside rocks: a critical review Jian‑Chun Guo1 · Hang‑Yu Zhou1 · Jie Zeng1,2 · Kun‑Jie Wang3 · Jie Lai1 · Yu‑Xuan Liu1 Received: 12 May 2020 © The Author(s) 2020
Abstract NMR serves as an important technique for probing rock pore space, such as pore structure characterization, fluid identification, and petrophysical property testing, due to the reusability of cores, convenience in sample processing, and time efficiency in laboratory tests. In practice, NMR signal collection is normally achieved through polarized nuclei relaxation which releases crucial relaxation messages for result interpretation. The impetus of this work is to help engineers and researchers with petroleum background obtain new insights into NMR principals and extend existing methodologies for characterization of unconventional formations. This article first gives a brief description of the development history of relaxation theories and models for porous media. Then, the widely used NMR techniques for characterizing petrophysical properties and pore structures are presented. Meanwhile, limitations and deficiencies of them are summarized. Finally, future work on improving these insufficiencies and approaches of enhancement applicability for NMR technologies are discussed. Keywords Low-field NMR · Critical review · Relaxation theory · Unconventional formation List of symbols A Characteristic length (m) An Proportion of the nth eigenstate (–) B0 Initial magnetic flux density (T) 1 c H state change probability (–) ⃖�D⃗ Diffusion coefficient tensor (m2/s) D0 Diffusion coefficient in unrefined environment (m2/s) D(t) Effective coefficient (m2/s) g Magnetic field gradient (A/m2) Handling Editor: Wai Li Edited by Yan-Hua Sun * Jian‑Chun Guo [email protected] * Hang‑Yu Zhou [email protected] 1
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
2
Department of Chemical Engineering, School of Engineering, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia
3
Downhole Operation Branch, Southwest Petroleum Engineering Co., Ltd, Sinopec, Deyang 618014, China
gint Internal magnetic field gradient vector (A/m2) G Diffusion propagation function (–) h Thickness of the surface relaxation region (m) k Wave vector (–) m Probability density of 1H magnetization (A/m4) M Net magnetization vector (A/m) S Pore surface area (m3) T Relaxation times (s) T1 Longitudinal relaxation time (s) T2 Transverse relaxation time (s) TB Bulk relaxation time (s) TS Surface relaxation time (s) Vp Pore volume (m3) 1 H gyromagnetic ratio (42.57 MHz/T) 𝛾 3 𝛿 Dirac function in 3-D space 𝜌 Surface sink strength (m/s) Δ𝜒 Susceptibility difference (H/m) 𝜑n The nth corresponding eigenfunction (–) Abbreviations BE Binomial edited BPPFG Bipolar pulse field gradient BT Brownstein–Tarr CPMG Carr–Purcell–Meiboom–Gill
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