In situ combustion
(ISC) is an important thermal recovery technique.
Significant open ISC questions include the effect of coke formation
on the pore structure and permeability. In the study, an experimental
apparatus was constructed to not only physically simulate coke formation
similar to the crude oil ISC process but also to in situ measure postdeposition
permeability. Effects on coke deposition with the Xinjiang crude oil
were studied, including reaction atmosphere, temperature, and time.
The results indicate that the critical coking temperature differs
significantly by at least 200 °C between low-temperature oxidation
(LTO) runs with air flow and coking runs with nitrogen flow for the
Xinjiang crude oil. The coke generation promoted by LTO and the coke
consumption via high-temperature oxidation (HTO) result in a maximum
coke production with temperature in the LTO runs. In addition, the
study found that many resins and the small amount of asphaltenes in
the Xinjiang crude oil prolonged the induction coking period in the
coking runs. This understanding of the coke deposition process led
to the production of core samples with different amounts of coke deposition
for selected reaction conditions. The pore structure of the coked
grain clusters was viewed with a scanning electron microscopy (SEM)
and mercury porosimeters. The results showed the complicated pore
structure and increasing number of micropores with increasing coke
deposition, which not only reduced the permeability rapidly so that
it deviated from the Kozeny–Carman relationship at the Darcy
scale but also further promoted the Klinkenberg effect. In addition,
the global permeability damage would be further underestimated regardless
of the coke concentration heterogeneity in the core samples. The permeability
change was then correlated with coke deposition for numerical simulations
of ISC or ToeHeel Air Injection (THAI) processes in sandstone
reservoirs.
Non-isothermal reactive transport in complicated porous media is diverse in nature and industrial applications. There are challenges in the modelling of multiple physicochemical processes in multiscale pore structures with various length scales ranging from nanometres to micrometres. This study focuses on coke combustion during in situ crude oil combustion techniques. A micro-continuum model was developed to perform an image-based simulation of coke combustion through a multiscale porous medium. The simulation coupled weakly compressible gas flow, species transport, conjugate heat transfer, heterogeneous coke oxidation kinetics and structural evolution. The unresolved nanoporous coke region was treated as a continuum, for which the random pore model, permeability model and species diffusivity model were integrated as sub-grid models to account for the sub-resolution reactive surface area, Darcy flow and Knudsen diffusion, respectively. A Pe–Da diagram was provided to present five characteristic combustion regimes covering the ignition temperature and air flux in realistic field operations and laboratory measurements. The present model proved to achieve more accurate predictions of the feasible ignition temperature than previous models. Compared with the air flux of
$\phi \sim O\textrm{(1) s}{\textrm{m}^\textrm{3}}(\textrm{air})\;{({\textrm{m}^\textrm{2}}\ \textrm{h})^{ - 1}}$
in the field, the increasing air flux in the laboratory transformed the combustion regime from diffusion-limited to convection-limited, which led to an overpredicted burning temperature. Reactive fingering combustion was analysed to understand the potential risks in some experimental measurements. The findings provide a better understanding of coke combustion and can help engineers design sustainable combustion methods. The developed image-based model allows other types of multiscale and nonlinear reactive transport to be simulated.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.