Abstract
This paper offers a summary of the advances in heavy oil and bitumen reservoir
characterization and fluid stream monitoring using low field magnetic resonance
tools. Both laboratory and field advances are presented. Although the bulk of
the work discussed was performed in our laboratory, a selection of other
pertinent technologies is also presented. This overview aims at offering the
reader a quick reference of what has been achieved in the past ten years and it
is hoped that it will be used as a guide for future development in this
area.
Introduction
Low field nuclear magnetic resonance (NMR) relaxometry is a technology that
offers significant benefits in reservoir characterization through the magnetic
resonance logging tools that are offered by oil and gas service
companies(1). These tools can offer measurements of porosity,
permeability, mobile and bound fluids and, potentially saturations, if they are
properly calibrated. This technology has been active in its latest
reincarnation since the middle of the 1980s. It was originally developed with
conventional oil and gas reservoirs in Texas and the North Sea. There are
currently several excellent reviews and two recommended books for those
interested in studying the topic in detail(1-4).
Through low field NMR we measure the amount of and the mobility of
hydrogen-bearing molecules. For reservoir characterization applications, such
molecules translate into gas, water or oil present within a formation. Although
the physics of the process are not the focus of this overview, a brief
introductory summary is included. For details, the reader is directed to the
references above.
The measured parameters in NMR are amplitudes of hydrogenbearing signal and
relaxation times of hydrogen-bearing molecules. The amplitude is directly
proportional to the amount of protons present and it can be correlated to the
volume or mass of fluids within the region of measurement. The relaxation time
is affected by the relative mobility of the hydrogen-bearing molecules. Thus,
as visocsity increases, or the surroundings of the relaxing hydrogen are
restricted, then relaxation occurs faster. There are two relaxation times that
can be measured: longitudinal (T1) and transverse (T2).
The focus of our work deals with transverse relaxation phenomena.
Figure 1 is used as the typical figure to explain different types of relaxation
spectra obtained when exposing different systems in the standard pulse sequence
that is used in logging and laboratory tools alike. The spectrum of bulk water
(i.e. water in a beaker) is a simple narrow peak that shows relaxation at
T2 of ∼2,500 ms. Compared to water, bulk bitumen (viscosity of
∼1,000,000 mPas at room temperature) typically relaxes with a broader peak at
less than 2 ms. Thus, in principle, a beaker that is half-full of water and
half-full of bitumen would show two peaks, as shown in Figure 2.
Figure 1,2,3,4 (available in full paper)
In this case, the oil is somewhat lighter than that of Figure 1 (viscosity of
∼100,000 mPas at room temperature).
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Original manuscript received:
13 January 2009
- Manuscript approved:
4 February 2009
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