NMR of geophysical drill cores with a mobile Halbach scanner [Elektronische Ressource] / vorgelegt von Elena Talnishnikh

Bernhard Bluemich

Sujets

• Physik

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2007
• Nombre de lectures: 11
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

NMR of Geophysical Drill Cores with a Mobile
Halbach Scanner

Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der Rheinisch-
Westfälischen Technischen Hochschule Aachen zur Erlangung des akademischen
Grades einer Doktorin der Naturwissenschaften genehmigte Dissertation






vorgelegt von

Master of Science
Elena Talnishnikh
aus Sankt Petersburg, Russland





Berichter: Univ.-Prof. Dr. rer. nat. Dr. h. c. Bernhard Blümich
Univ.-Prof. Dr. rer. nat. Christoph Clauser

Tag der mündlichen Prüfung: 21. August 2007
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.
Foreword

Many people were involved in this interdisciplinary project. They worked together from
the beginning until the end. Firstly, they designed and constructed the experimental
equipment and secondly, developed measurement techniques and tested them for
different geophysical applications. Then the author would like to mention personally Dr.
Sophia Anferova, Prof. Vladimir Anferov, and Juliane Arnold for the teamwork.
I would like to underline the collaboration with Prof. Christoph Clauser’s group
from the Applied Geophysics (AG) Institute in RWTH Aachen University. All water-
saturated drill cores and plugs were provided by this institute as well as their
geophysical characteristics measured with independent methods: porosity with a He-gas
pycnometer, and permeability with the gas flow method. Furthermore, magnetic
susceptibility values of all studied cores were recorded with a Multi-Sensor Core
Logger. Therefore, the NMR data were verified with independent measurements. Our
partners from Applied Geophysics tested the developed mobile NMR hardware and
methodologies to estimate rocks characteristics from NMR data. Juliane Arnold (AG)
provided one-dimensional relaxation experiments and calibrated different mobile NMR
instruments to quantify rock porosity, pore size distribution and permeability as
indicated in her thesis.
The author took part in the improvement of the homogeneity of the magnetic
system, the rf circuit with the 24 mm diameter coil, and the gradient system for PFG
experiments. The on-line Halbach core scanner for analysis of large long drill cores was
designed on the basis of an improved magnet. Dedicated software was developed for
on-line measurements of porosity of long large size drill cores and for 2D correlation
experiments. Standard relaxation and diffusion NMR techniques were applied with the
Halbach scanner. All reported experiments were established and the results were
analyzed and are reported in this thesis.
iiiContents
Foreword.........................................................................................................................iii
Contents ........................................................................................................................... v
Abstract..........................................................................................................................vii
Zusammenfassung .........................................................................................................ix
Abbreviations and Symbols ..........................................................................................xi
Introduction..................................................................................................................... 1
1. NMR in porous media ..................................................................................... 5
1.1 Relaxation principles .........................................................................................5
1.2 Basics of NMR in Water-Saturated Rocks.........................................................8
1.3 Samples ............................................................................................................12
2. Halbach core scanner ............................................................................................... 15
2.1. Magnet system.................................................................................................15
2.2. RF system........................................................................................................18
2.3. Gradient coils ..................................................................................................20
2.4. Conclusion.......................................................................................................21
3. Advances in measurements of porosity with the Halbach scanner...................... 23
3.1. Porosity by mobile NMR with exchangeable rf coils .....................................23
3.2. On-line measurements of porosity ..................................................................26
3.3. Conclusions .....................................................................................................32
4. Quantifying diffusion in the presence of internal gradients ................................. 35
4.1. Diffusion in 1D experiments with the CPMG sequence and its influence
on T distributions............................................................................................35 2
4.2. Two-dimensional T – T correlation experiments .........................................43 1 2
4.3. Permeability estimation in the presence of internal gradients from NMR
data...................................................................................................................54
4.4. Accuracy of 1D and 2D experiments ..............................................................59
4.5. Conclusions .....................................................................................................63


v
5. Restricted diffusion in water-saturated core plugs ................................................67
5.1. PFG experiments with the Halbach scanner................................................... 67
5.2. Conclusions..................................................................................................... 73
6. Summary and Outlook..............................................................................................75
Appendix A ....................................................................................................................79
Appendix B.....................................................................................................................89
Appendix C91
References.97
Acknowledgements......................................................................................................105
vi Abstract
Abstract

This thesis is devoted to a mobile NMR with an improved Halbach scanner. This is a
lightweight tube-shaped magnet with sensitive volume larger and a homogeneity of the
magnetic field higher than the previous prototype version. The improved Halbach
scanner is used for analysis of water-saturated drill cores and plugs with diameters up to
60 mm. To provide the analysis, the standard 1D technique with the CPMG sequence as
well as 2D correlation experiments were successfully applied and adapted to study
properties of fluid-saturated sediments. Afterwards the Halbach scanner was calibrated
to fast non-destructive measurements of porosity, relaxation time distributions, and
estimation of permeability. These properties can be calculated directly from the NMR
data using the developed methodology. Any independent measurements of these
properties with other methods are not needed. One of the main results of this work is the
development of a new NMR on-line core scanner for measurements of porosity in long
cylindrical and semi cylindrical drill cores. Also dedicated software was written to
operate the NMR on-line core scanner.
The physical background of this work is the study of the diffusion influence on
transverse relaxation. The diffusion effect in the presence of internal gradients in porous
media was probed by 1D and 2D experiments. The transverse relaxation time
distributions obtained from 1D and from 2D experiments are comparable but different
in fine details.
Two new methodologies were developed based on the results of this study. First is
the methodology quantifying the influence of diffusion in the internal gradients of
water-saturated sediments on transverse relaxation from 2D correlation experiments.
The second one is the correction of the permeability estimation from the NMR data
taking in account the influence of the diffusion. Furthermore, PFG NMR technique was
used to study restricted diffusion in the same kind of samples. Preliminary results are
reported.


viiZusammenfassung
Zusammenfassung

Diese Arbeit widmet sich der mobilen NMR mit einem verbesserten Halbach-Scanner.
Dieser ist ein leichter röhrenförmiger Magnet, der im Vergleich zum früheren
Prototypen ein größeres sensitives Volumen und eine höhere Homogenität des
Magnetfeldes aufweist. Der verbesserte Halbach-Scanner wird für Untersuchungen von
wassergesättigten Bohrkernen und Scheiben mit einem Durchmesser bis zu 60 mm
verwendet. Um die Eigenschaften von fluid-gesättigten Sedimentgesteinen zu
untersuchen, wurden sowohl die standardmäßige 1D