(728h) Replacement of CH4 in the Hydrate By CO2 and N2 for Methane Recovery and Carbon Dioxide Sequestration: A Modeling Approach

Replacement of CH₄
in the hydrate by CO₂
and N₂
for methane recovery and carbon dioxide sequestration: A Modeling
Approach

Dnyaneshwar R. Bhawangirkar^1,2, Jitendra S. Sangwai^2*

¹Department of Chemical Engineering, Indian Institute of
Technology Bombay, Powai, Mumbai 400076, India

²Gas Hydrate and Flow Assurance Laboratory, Petroleum
Engineering Program, Department of Ocean Engineering, Indian
Institute of Technology Madras, Chennai 600036, India

Keywords: Gas hydrates; Thermodynamic modeling; High pressure phase
equilibria

ABSTRACT

The
replacement mechanism is considered as one of the emerging techniques
for methane gas recovery from the hydrate reservoirs by sequestering
carbon dioxide deep in the oceans mitigating the climate change, and
also by retaining the original form of the hydrate reservoir avoiding
any catastrophic geo-mechanical hazards such as landslides or
earthquakes. When methane (CH₄) hydrate reservoir is
exposed to carbon dioxide (CO₂) gas, the in situ exchange
of methane gas molecules trapped in the hydrate phase is forced to
happen with CO₂ molecules. The CO₂ molecule is
large in size than the CH₄ molecule, resulting in more
affinity towards the larger cages of the hydrate. Hence, CO₂
molecules prefer to replace most of the CH₄ molecules from
the larger cages of the hydrate, leaving behind CH₄
molecules in small cages almost intact. Now, to increase the methane
gas production; it is necessary to introduce new small size gas
molecules which can replace CH₄ gas molecules left behind
in the small cages of the hydrate. Nitrogen (N₂) molecule
is smaller in size than a CH₄ molecule, and it will have
more affinity towards smaller cages of the hydrate. Adding N₂
along with CO₂ will serve the purpose of replacing most of
the CH₄ gas molecules form both small as well as large
cages of the hydrate.

However, it is very important to understand at what ratio the N₂/CO₂
gas mixture needs to be injected into the hydrate reservoir. When CO₂
and N₂ gas mixture is injected into the hydrate reservoir,
it forms a mixed hydrate. Therefore, we have made use of Klauda and
Sandler’s thermodynamic model to understand the distribution of gas
molecules in the hydrate phase. This model utilizes the concept of
equal fugacities of water in the hydrate phase to that in the liquid
phase. The Peng-Robinson-Stryjek-Vera equation of state is used for
evaluating the vapour phase fugacity. Fugacity has found to be
increasing with the increase in temperature. Liquid phase activity
coefficients are determined by using a modified UNIFAC method.

We
have used a total of 109 experimental data points having a different
composition of CH₄+CO₂+N₂ for
calculating the equilibrium pressures from the model, and found that
the percent absolute average deviation is 6.46%. The cage occupancy
which is a function of Langmuir constant and fugacity is being
calculated to know the distribution of gas molecules in the hydrate
phase at equilibrium. We have observed most of the small cages are
occupied by N₂ and CH₄ molecules; displaying
the competitive behavior of N₂ and CH₄
molecules to occupy smaller cages of the hydrate; whereas the
competition between CO₂ and CH₄ molecules is
observed to occupy large cages of the hydrate. We found that the
occupancy of N₂ molecules in smaller cages of the hydrate
is ~20 times larger than in large cages revealing the capability of
N₂ molecules to replace most of the CH₄
molecules from the small cages of the hydrate. We also observed that
the occupancy of CO₂ molecules in larger cages is much
higher than in small cages which will certainly help in replacing
most of the CH₄ gas molecules from the larger cages of the
hydrate. The also observed that with the increase in N₂/CO₂
ratio and at constant CH₄ composition, the total occupancy
of N₂+CO₂ has found to decrease in the hydrate
phase, and CH₄ molecules in the hydrate phase have found
to be increased. This information helped us in deciding the N₂/CO₂
gas ratio to be injected into the hydrate reservoir for better
recovery of methane gas. We conclude that a 1:3 ratio of N₂/CO₂
gas mixture is a good choice to be injected into the hydrate
reservoirs for more methane gas production maintaining the higher CO₂
gas sequestration deep into the ocean reservoir.