Summary
Recovery of oil from oil shales and the natural primary migration of
hydrocarbons are closely related processes that have received renewed interest
in recent years because of the ever tightening supply of conventional
hydrocarbons and the growing production of hydrocarbons from low-permeability
tight rocks. Quantitative models for conversion of kerogen into oil and gas and
the timing of hydrocarbon generation have been well documented. However, lack
of consensus about the kinetics of hydrocarbon formation in source rocks,
expulsion timing, and how the resulting hydrocarbons escape from or are
retained in the source rocks motivates further investigation. In particular,
many mechanisms have been proposed for the transport of hydrocarbons from the
rocks in which they are generated into adjacent rocks with higher
permeabilities and smaller capillary entry pressures, and a better
understanding of this complex process (primary migration) is needed. To
characterize these processes, it is imperative to use the latest technological
advances. In this study, it is shown how insights into hydrocarbon migration in
source rocks can be obtained by using sequential high-resolution synchrotron
X-ray tomography. Three-dimensional images of several immature "shale" samples
were constructed at resolutions close to 5 μm. This is sufficient to resolve
the source-rock structure down to the grain level, but very-fine-grained silt
particles, clay particles, and colloids cannot be resolved. Samples used in
this investigation came from the R-8 unit in the upper part of the Green River
shale, which is organic rich, varved, lacustrine marl formed in Eocene Lake
Uinta, USA. One Green River shale sample was heated in situ up to 400°C as
X-ray-tomography images were recorded. The other samples were scanned before
and after heating at 400°C. During the heating phase, the organic matter was
decomposed, and gas was released. Gas expulsion from the low-permeability
shales was coupled with formation of microcracks. The main technical difficulty
was numerical extraction of microcracks that have apertures in the 5- to 30-μm
range (with 5 μm being the resolution limit) from a large 3D volume of X-ray
attenuation data. The main goal of the work presented here is to develop a
methodology to process these 3D data and image the cracks. This methodology is
based on several levels of spatial filtering and automatic recognition of
connected domains. Supportive petrographic and thermogravimetric data were an
important complement to this study. An investigation of the strain field using
2D image correlation analyses was also performed. As one application of the 4D
(space + time) microtomography and the developed workflow, we show that fluid
generation was accompanied by crack formation. Under different conditions, in
the subsurface, this might provide paths for primary migration.
Key words in this work include 4D microtomography, 3D image processing,
shale, strain field analysis, kerogen, petroleum generation, primary migration,
petrography, and thermogravimetry.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
31 March 2011
- Revised manuscript received:
6 May 2012
- Manuscript approved:
16 May 2012
- Published online:
27 November 2012
- Version of record:
5 April 2013
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