Scientists from Skoltech (part of the VEB.RF Group) and
the Keldysh Institute of Applied Mathematics of the Russian
Academy of Sciences have published
in the International Journal of Geotechnical Engineering the
results of laboratory and numerical experiments, which, for
the first time, modeled one-sided heating of frozen sand
containing metastable methane hydrate. The results will help
predict more accurately how gas hydrate-saturated frozen
ground in the Arctic will behave upon heating, so as to
assess the risks of gas emissions, design wells correctly,
and prevent unexpected ground subsidence.
For the
first time, in both a laboratory experiment and a computer
model, researchers were able to simultaneously trace the
dynamics of two processes — ice melting and gas hydrate
dissociation — in frozen sand sample heated from one end.
It was found that pore hydrate dissociation begins earlier
than ice melting: the temperature difference between these
processes can reach 1.1°C or more.
Gas hydrates are
solid compounds of water and gas. In Arctic permafrost, they
can occur in pores together with ice. If a hydrate remains
at a negative temperature where, under equilibrium
conditions, it can no longer be stable, it is called
metastable. Such a hydrate is highly sensitive to heating
and can dissociate, releasing gas — methane. During the
development of Arctic fields, wells pass through permafrost
and heat it, so it is important to know how quickly and in
what order these changes will occur.
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The experiments
used quartz sand samples in the form of cylinders 5 cm in
diameter and 11 to 13 cm long. The sand moisture content was
12%. The pores of the sample were partially filled with
methane hydrate. First, the sand was saturated with methane
at a pressure of 6–8 MPa and a temperature of about
6°C, and then, after hydrate formation, the pressure was
reduced to 0.1 MPa. This converted the hydrate to a
metastable state. The hydrate-bearing sample, with an
initial temperature of 5.4°C, was then heated from below
to +5°C. Temperature was measured at six points. Hydrate
dissociation began at 1.1°C, while ice melting began at
0.01°C. After 4.7 hours, the hydrate dissociation zone
had advanced 6 cm from the heating source. The melting zone
advanced less than 4 cm in 6 hours. Thus, hydrate
dissociation precedes ice melting.
“The main
practical significance of our work is that we have shown how
metastable hydrates in frozen permafrost actually behave
upon heating,” said Evgeny Chuvilin, the
study’s principal investigator and leading research
scientist at the Petroleum Center at Skoltech.
“Previously, many models considered only pressure
reduction as the cause of hydrate dissociation. But in the
Arctic, a well heats the frozen ground, and the process
proceeds somewhat differently. We see that pore hydrate
begins to dissociate at a temperature below zero, even
before the frozen ground begins to melt. This changes our
assessment of how quickly a gas-releasing zone may form
around a well. It means that zones of intrapermafrost
hydrate dissociation form earlier than we thought, and this
must be taken into account when designing production wells
in order to avoid unexpected annular gas influx or ground
subsidence around the well.”
“We built a computer
model that describes the behavior of frozen sand with
metastable hydrate upon heating,” noted Natalia
Alekseeva, first author of the study and a graduate
of Skoltech’s PhD program in Petroleum
Engineering. “The most difficult part was to describe
correctly how, upon heating, the hydrate first dissociates,
releasing water that immediately freezes because the
temperature is still negative, and only then does the ice
melt. We validated the model against laboratory data.
Without accounting for heat losses, the calculation diverged
rather strongly from the experiment near the heater, but
after adding the sink function, the deviations became
minimal. This suggests that the model can be scaled to real
conditions, but this will require refining soil
characteristics — such as particle-size distribution, the
mineral composition of the fine-dispersed component, and
salinity — which will affect the dissociation temperature
of pore hydrate.”
Note:
Skoltech is a private
international university (part of the VEB.RF group) in
Russia, cultivating a new generation of leaders in
technology, science, and business. As a factory of
technologies, it conducts research in breakthrough fields
and promotes technological innovation to solve critical
problems that face Russia and the world. Skoltech focuses on
six priority areas: life sciences, health, and agro;
telecommunications, photonics, and quantum technologies;
artificial intelligence; advanced materials and engineering;
energy efficiency and the energy transition; and advanced
studies. Established in 2011 in collaboration with the
Massachusetts Institute of Technology (MIT), Skoltech was
listed among the world’s top 100 young universities by the
Nature Index in its both editions (2019, 2021). On
Research.com, the Institute ranks as Russian university No.
2 overall and No. 1 for genetics. In the recent SCImago
Institutions Rankings, Skoltech placed first nationwide for
computer science. Website: https://www.skoltech.ru/.
