Nano-Emptiness Space of Oil-Bearing Rocks-Juniper Publishers
Juniper Publishers
Introduction
The electron microscopic and X-ray tomography
studies of oil-bearing rocks showed a great variety and wide volume
limits of an emptiness space. It includes macro- and with the
corresponding micro-fragments of space closely connected each other.
Micro-emptiness space predominates in volume in thin-grained terrigenous
rocks, especially in argillites. But even in carbonate rocks it plays
very important role as system of canals for oil migration.
Micro-emptiness space is composed by micro-cracks,
micro-pores, and micro-cavities, as well as intermediate and
transitional morphological types. The nature of the micro-emptiness
space (structure, volume, filling in organic or mineral matter, etc.)
determines many properties of reservoir rocks and their oil saturation.
Its study is carried out by many methods, and it is very useful for
sorting out the geophysical information used in the correlation of
oil-bearing strata sections.
This short communication devoted to the problems of
emptiness space study under a high-resolution scanning electron
microscope. The objects of the author’s investigation were some oil
deposits of the Perm Region, Russia. The methods of high-resolution
electron microscopy were applied to study the nano-sized fragments of
rock emptiness space. The polished sections of terrigenous and carbonate
rocks were studied under JSM 7500F (Jeol) microscope with cool
emission. The regime for standard scanning electron image for analysis
was acceleration voltage of 15 kV, emission current of 10 μA, work
distance of 8 mm.
The use of high-resolution electron microscopy
methods has shown that the micro-emptiness space of reservoir rocks is
practically inextricably linked with the nano-emptiness one. The main
structural fragments of the nano-emptiness space are nano-cracks and
nano-pores, which directly contacted with the corresponding
micro-fragments. The wide spread of nano-cracks is due to the fact that
virtually every micro-crack
before the wedging along the strike and into the depth turns into
nano-tracks, the extent of which can significantly exceed that of the
mother micro-crack. Besides, many micro-cracks narrow up to nano-sized
width in definite their parts (Figure 1).
Accordingly, the study of micro-pores under an
electron microscope with high resolution shows that they gradually
narrow to the depth, and several deeper canals of nano-scale width
usually remain at the bottom (Figure 2).
Besides the common joint systems of micro- and
nano-defects, there are the separate analogous nano-systems. Presumably,
they fix the places of weak influences of external factors: tectonic
and catagenesis processes, interstitial solution, etc.
At present, the share of the nano-emptiness space in
the total volume of reservoir rock can be estimated only very
approximately, and the oil industry does not particularly interest this
task in connection with the impossibility of oil migration through the
nano-canals. However, in some cases nano-space can play a significant
role. As an example we may discuss on thin-layered argillites.
The nano-emptiness space in argillites is the important
component of their structure, and its transformation may
remarkably influence on oil migration. Note that a significant
part of the basic mineral matter of argillites (montmorillonite,
hydromica, and kaolinite) in particle size refers to the nanoscale
range if we take into account the thickness of flakes. They
can largely fill the nano-cracks, and gradually expand them
due to the swelling of montmorillonite packets.
In some argillaceous strata the extensive systems of
crossing and interlayer micro-cracks have been forming
during the stage of catagenesis. It is assumed that many of
them pass into nano-cracks in separate segments and when
they are wedged out. This factor may be disregarded. However,
the abundance of micro- and nano-cracks leads to a noticeable
change in the petro-physical properties of the reservoir rocks,
which must be taken into account in the correlation of the
strata. In addition, such rock behaves differently under the
drilling process.
The processes in the nano-emptiness space should be
examined when assessing the effect of drilling fluids on oil
reservoirs. In particular, the precipitation of salt nano-crystals
constantly occurs from these solutions. They arbitrarily fill the
micro- and especially the nano-emptiness space. In the future,
salt nano-crystals begin to grow and, due to the crystallization
force, they make significant changes in the structure of the
reservoir rocks (such as expansion and elongation of cracks
and pores). These transformations lead to the appearance of
the secondary emptiness space of another nature.
The processes that take place in the reservoir are also
directly related to the structure of the nano-emptiness space
when solving the problems of increasing of oil recovery from
thin-grained rocks. Many of them require special consideration
from theoretical positions and by setting appropriate
experiments.
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