Simulation and experimental measurements of internal magnetic field gradients and NMR transverse relaxation times (T-2) in sandstone rocks

Paul R. J. Connolly, Weichao Yan, Daniel Zhang, Mohamed Mahmoud, Michael Verrall, Maxim Lebedev, Stefan Iglauer, Peter J. Metaxas, Eric F. May, Michael L. Johns

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3 Citations (Scopus)

Abstract

NMR T-2 measurements are widely used to determine various petrophysical properties of rock cores. Internal magnetic field gradients, which occur in rock cores during NMR measurements due to magnetic susceptibility differences between the rock matrix and the pore fluid, can however distort these T-2 measurements. Here we implement a FEM simulation of these internal magnetic field gradients on 3D digital mu CT images for five different sandstone rocks, coupled with a random walk simulation of the T-2 NMR signal relaxation process. The FEM simulations required the magnetic susceptibility of each sandstone, this was directly measured using a SQUID magnetometer over a range of magnetic field strengths. The resultant probability distributions of internal magnetic field gradients were then compared against equivalent experimental measurements; they were generally in reasonable agreement, however the simulations failed to capture the larger magnetic field gradients that were observed experimentally. By consideration of various potential reasons for this, we identify the assumption of a single mean magnetic susceptibility as being the primary source of the variation between simulated and measured results. Simulations of 2 MHz T-2 relaxation process are shown however to be in good agreement with experimental measurements across the five sandstones studied.

Original languageEnglish
Pages (from-to)985-997
Number of pages13
JournalJournal of Petroleum Science and Engineering
Volume175
DOIs
Publication statusPublished - Apr 2019

Cite this

@article{0189be11ff5e49f9a5a49d42069626c3,
title = "Simulation and experimental measurements of internal magnetic field gradients and NMR transverse relaxation times (T-2) in sandstone rocks",
abstract = "NMR T-2 measurements are widely used to determine various petrophysical properties of rock cores. Internal magnetic field gradients, which occur in rock cores during NMR measurements due to magnetic susceptibility differences between the rock matrix and the pore fluid, can however distort these T-2 measurements. Here we implement a FEM simulation of these internal magnetic field gradients on 3D digital mu CT images for five different sandstone rocks, coupled with a random walk simulation of the T-2 NMR signal relaxation process. The FEM simulations required the magnetic susceptibility of each sandstone, this was directly measured using a SQUID magnetometer over a range of magnetic field strengths. The resultant probability distributions of internal magnetic field gradients were then compared against equivalent experimental measurements; they were generally in reasonable agreement, however the simulations failed to capture the larger magnetic field gradients that were observed experimentally. By consideration of various potential reasons for this, we identify the assumption of a single mean magnetic susceptibility as being the primary source of the variation between simulated and measured results. Simulations of 2 MHz T-2 relaxation process are shown however to be in good agreement with experimental measurements across the five sandstones studied.",
keywords = "NMR, T-2, Internal magnetic field gradients, Simulation, POROUS-MEDIA, SURFACE RELAXIVITY, PORE-SIZE, DIFFUSION, SUSCEPTIBILITY, DISTRIBUTIONS, ENHANCEMENT, EQUATIONS, DECAY, WATER",
author = "Connolly, {Paul R. J.} and Weichao Yan and Daniel Zhang and Mohamed Mahmoud and Michael Verrall and Maxim Lebedev and Stefan Iglauer and Metaxas, {Peter J.} and May, {Eric F.} and Johns, {Michael L.}",
year = "2019",
month = "4",
doi = "10.1016/j.petrol.2019.01.036",
language = "English",
volume = "175",
pages = "985--997",
journal = "Journal of Petroleum Science & Engineering",
issn = "0920-4105",
publisher = "Pergamon",

}

TY - JOUR

T1 - Simulation and experimental measurements of internal magnetic field gradients and NMR transverse relaxation times (T-2) in sandstone rocks

AU - Connolly, Paul R. J.

AU - Yan, Weichao

AU - Zhang, Daniel

AU - Mahmoud, Mohamed

AU - Verrall, Michael

AU - Lebedev, Maxim

AU - Iglauer, Stefan

AU - Metaxas, Peter J.

AU - May, Eric F.

AU - Johns, Michael L.

PY - 2019/4

Y1 - 2019/4

N2 - NMR T-2 measurements are widely used to determine various petrophysical properties of rock cores. Internal magnetic field gradients, which occur in rock cores during NMR measurements due to magnetic susceptibility differences between the rock matrix and the pore fluid, can however distort these T-2 measurements. Here we implement a FEM simulation of these internal magnetic field gradients on 3D digital mu CT images for five different sandstone rocks, coupled with a random walk simulation of the T-2 NMR signal relaxation process. The FEM simulations required the magnetic susceptibility of each sandstone, this was directly measured using a SQUID magnetometer over a range of magnetic field strengths. The resultant probability distributions of internal magnetic field gradients were then compared against equivalent experimental measurements; they were generally in reasonable agreement, however the simulations failed to capture the larger magnetic field gradients that were observed experimentally. By consideration of various potential reasons for this, we identify the assumption of a single mean magnetic susceptibility as being the primary source of the variation between simulated and measured results. Simulations of 2 MHz T-2 relaxation process are shown however to be in good agreement with experimental measurements across the five sandstones studied.

AB - NMR T-2 measurements are widely used to determine various petrophysical properties of rock cores. Internal magnetic field gradients, which occur in rock cores during NMR measurements due to magnetic susceptibility differences between the rock matrix and the pore fluid, can however distort these T-2 measurements. Here we implement a FEM simulation of these internal magnetic field gradients on 3D digital mu CT images for five different sandstone rocks, coupled with a random walk simulation of the T-2 NMR signal relaxation process. The FEM simulations required the magnetic susceptibility of each sandstone, this was directly measured using a SQUID magnetometer over a range of magnetic field strengths. The resultant probability distributions of internal magnetic field gradients were then compared against equivalent experimental measurements; they were generally in reasonable agreement, however the simulations failed to capture the larger magnetic field gradients that were observed experimentally. By consideration of various potential reasons for this, we identify the assumption of a single mean magnetic susceptibility as being the primary source of the variation between simulated and measured results. Simulations of 2 MHz T-2 relaxation process are shown however to be in good agreement with experimental measurements across the five sandstones studied.

KW - NMR

KW - T-2

KW - Internal magnetic field gradients

KW - Simulation

KW - POROUS-MEDIA

KW - SURFACE RELAXIVITY

KW - PORE-SIZE

KW - DIFFUSION

KW - SUSCEPTIBILITY

KW - DISTRIBUTIONS

KW - ENHANCEMENT

KW - EQUATIONS

KW - DECAY

KW - WATER

U2 - 10.1016/j.petrol.2019.01.036

DO - 10.1016/j.petrol.2019.01.036

M3 - Article

VL - 175

SP - 985

EP - 997

JO - Journal of Petroleum Science & Engineering

JF - Journal of Petroleum Science & Engineering

SN - 0920-4105

ER -