Page 491 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Air injection 455

Generally, in an LTO process, the temperature increase may not be high,
and the viscosity reduction will not be high. And LTO-generated oxygen-
ated compounds such as carboxylic acids, aldehydes, ketones, alcohols, and
hydroperoxides (Burger and Sahuquet, 1972) will increase oil viscosity.
Thus, the above mechanism 5 may not be feasible. Other mechanisms
(the above mechanisms 1e3) from air injection may not outperform a
nonoxidation gas process like nitrogen or flue gas injection. Such opinions
are supported by Huang et al. (2018) experiments to study the thermal effect
in LTO process (see the above Section 13.6 for details).
To have significant benefits from air injection over nonoxidation gas
injection, combustion must be able to occur. In a shale and tight formation,
even when a combustion process is initiated, sustaining the combustion
process requires the availability of minimum fuel and the injectivity of air
requirement. To ease the low-permeability problem with shale and tight
reservoirs, huff-n-puff air injection was evaluated (Jia and Sheng, 2018).
The simulation results show that about 10% oil can be recovered within
20 years. However, it is assumed that combustion can occur in the model.
Jia and Sheng (2017) discussed the favorable and unfavorable conditions
of air injection in shale reservoirs. Although shale and tight reservoirs have
the disadvantage of low injectivity, and the low-porosity matrix may result
in the difficulty to increase oxidation temperature and in the increase of heat
loss, the fine-grade matrix provides a high surface area that can promote
crude oil oxidation in porous media. A high specific surface area can
make the exothermal temperature range shift to a lower temperature range
(Drici and Vossoughi, 1985). A rich content of clays in shale rocks have a
catalytic effect on crude oil oxidation. Jia et al. (2012b) found that smectite
was ranked first, illite was ranked second, and chlorite and kaolinite were
ranked third in their catalytic ability for crude oil oxidation.
Oils in shale and tight reservoirs are more likely light oils. Freitag (2016)
stated that the aromatics act as the main sources for the oxidation inhibitors
during the LTO reaction process. Hence, the difference on exothermic
behavior for heavy oil and light oil can be explained based on their different
compositions. As the heavy oil contains more aromatic components and the
light oil contains more aliphatic contents, therefore, the light oil tends
to present more intense exothermic activity in the LTO region than the
heavy oil.
The EOR potential of air injection in shale and tight reservoirs cannot be
defined, until EOR mechanisms are well understood and quantified.