Summary
Canada contains vast reserves of heavy oil and bitumen. Viscosity
determination is key to the successful recovery of this oil, and low-field
nuclear magnetic resonance (NMR) shows great potential as a tool for estimating
this property. An NMR viscosity correlation previously had been developed that
is valid for order-of-magnitude estimates over a wide range of viscosities and
temperatures. This correlation was built phenomenologically, using experiments
relating NMR spectra to viscosity. The present work details a more thorough
investigation into oil viscosity and NMR, thus providing a theoretical
justification for the proposed correlation. A novel tuning procedure is also
presented, whereby the correlation is fitted using the Arrhenius relationship
to improve the NMR viscosity estimates for single oils at multiple
temperatures. Tuning allows for NMR to be potentially used in observation wells
to monitor thermal enhanced oil recovery (EOR) projects or online to monitor
the viscosity of produced-fluid streams as they cool.
Introduction
With approximately 400 million m3 of oil in place, the Canadian deposits of
heavy oil and bitumen are some of the most vast oil resources in the world.1
Heavy oil and bitumen are characterized by high densities and viscosities,
which is a major obstacle to their recovery. The waning of conventional-oil
reserves in Canada, coupled with increasing worldwide demand for oil, has
forced the industry focus to shift rapidly to the exploitation of these
heavy-oil and bitumen reserves.
The most important physical property of heavy oil that affects its recovery
is its viscosity.1 This parameter dictates both the economics and the technical
chance of success for any chosen recovery scheme. As a result, oil viscosity is
often directly related to recoverable reserves estimates.2 Unfortunately,
laboratory measurements of oil viscosity become progressively more difficult to
obtain as viscosity increases.3 The oil that has been removed from the core
also may have been physically altered during sampling and transport. Thus, the
viscosity at reservoir conditions may be different from the value obtained
later from the laboratory.2 In light of the shortcomings of conventional
viscosity measurements, low-field NMR is considered as an alternative for
estimating heavy-oil and bitumen viscosity.
The main appeal of NMR as a tool for assessing reservoir-fluid viscosities
and phase volumes is that the measured signal comes only from hydrogen, which
is present in both oil and water found in hydrocarbon reservoirs.4,5 Most of
the low-field NMR applications in the petroleum industry have been in
conventional oil, contained in sandstone reservoirs.6 To use low-field NMR
technology in heavy-oil and bitumen formations like the ones present in
Alberta, new methods of interpretation are required. The eventual goal for
using NMR to estimate viscosity is to make these predictions in the field
through logs. Currently, research toward this goal is conducted in the
laboratory.
In previous work,7–9 an oil-viscosity correlation was presented that is
capable of providing viscosity predictions for samples with viscosities less
than 1 mPa×s to more than 3 000 000 mPa×s. This is a wider range than any other
viscosity correlation presented in the literature.10–15 The correlation is only
order-of-magnitude accurate but still could be valuable for applications on a
logging tool, where the goal would be to determine viscosity variations with
depth or areal location in a reservoir. The theoretical justification behind
the NMR correlation is given in this work, along with a procedure for tuning
the correlation to improve the viscosity predictions for individual oils as a
function of temperature. Low-field NMR experiments are simple to perform and
nondestructive. The same test also can be run by different technicians to yield
the same results, which is a concern for conventional viscosity tests.3 In this
manner, a properly calibrated NMR model for viscosity can be a very accurate
and useful tool for predicting heavy-oil and bitumen viscosity at different
temperatures.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
14 January 2003
- Revised manuscript received:
19 February 2004
- Manuscript approved:
30 November 2004
- Version of record:
15 February 2005
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