Title: Key factors influencing the low-field NMR characterisation of gas- and oil-bearing shales: a case study of the shales from the southern Sichuan Basin and Dongying sag, China

Authors: Chen Chen; Shuangfang Lu; Pengfei Zhang; Junqian Li; Wenbiao Li; Siyuan Wang

Addresses: School of Geosciences, China University of Petroleum(East China), Qingdao, 266580, China ' School of Geosciences, China University of Petroleum(East China), Qingdao, 266580, China ' School of Geosciences, China University of Petroleum(East China), Qingdao, 266580, China ' School of Geosciences, China University of Petroleum(East China), Qingdao, 266580, China ' School of Geosciences, China University of Petroleum(East China), Qingdao, 266580, China ' School of Geosciences, China University of Petroleum(East China), Qingdao, 266580, China

Abstract: The response characteristics of transverse relaxation time (T₂) of key experimental parameters [waiting time (TW), number of scans (NS) and echo number (NECH)], sample size and probe fluid, are analysed systematically based on NMR experiments of oil- and gas-bearing shales from the Shahejie Formation in Dongying sag and the Wufeng and Longmaxi Formations in the southern Sichuan Basin. The results indicate that: 1) the optimised NMR parameters for TW, NS and NECH are 1,500 ms, 64 and 6,000, respectively, which will ensure higher accuracy and signal-to-noise ratio (SNR); 2) the total signal amplitude of the T₂ spectrum increases with decreasing grain size. The T₂ spectra of shale which grain size bigger than 10 mesh can characterize the shale PSD; 3) water porosity clearly decreases with increasing brittle minerals but decreasing clay content than n-dodecane-saturated porosity, which indicates the hydration of clay and brittle minerals together controls the reconstruction of PSD. [Received: May 30, 2018; Accepted: October 24, 2018]

Keywords: shale; low-field nuclear magnetic resonance; NMR; waiting time; TW; number of scans; NS; grain size; probe liquid; echo number; NECH; signal-to-noise ratio; SNR.

DOI: 10.1504/IJOGCT.2020.108049

International Journal of Oil, Gas and Coal Technology, 2020 Vol.24 No.4, pp.466 - 482

Received: 30 May 2018
Accepted: 24 Oct 2018
Published online: 02 Jul 2020 *

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