Comparison of kinetic and equilibrium reaction models insimulating the behavior of porous media Page: 1 of 9
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Comparison of kinetic and equilibrium reaction models in simulating gas hydrate behavior in porous media
Michael B. Kowalsky *, George J. Moridis
Earth Sciences Division, Lawrence Berkeley National Laboratory
In this study we compare the use of kinetic and equilibrium reaction models in the simulation of gas (methane)
hydrate behavior in porous media. Our objective is to evaluate through numerical simulation the importance of
employing kinetic versus equilibrium reaction models for predicting the response of hydrate-bearing systems to
external stimuli, such as changes in pressure and temperature. Specifically, we (1) analyze and compare the
responses simulated using both reaction models for natural gas production from hydrates in various settings and for
the case of depressurization in a hydrate-bearing core during extraction; and (2) examine the sensitivity to factors
such as initial hydrate saturation, hydrate reaction surface area, and numerical discretization. We find that for large-
scale systems undergoing thermal stimulation and depressurization, the calculated responses for both reaction
models are remarkably similar, though some differences are observed at early times. However, for modeling short-
term processes, such as the rapid recovery of a hydrate-bearing core, kinetic limitations can be important, and
neglecting them may lead to significant under-prediction of recoverable hydrate. Assuming validity of the most
accurate kinetic reaction model that is currently available, the use of the equilibrium reaction model often appears to
be justified and preferred for simulating the behavior of gas hydrates, given that the computational demands for the
kinetic reaction model far exceed those for the equilibrium reaction model.
Keywords: Gas hydrates; Dissociation; Kinetics; Depressurization; Thermal stimulation
Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) are lodged within the
lattices of ice crystals (called hosts). Under suitable conditions of low temperature and high pressure, a gas G will
react with water to form hydrates according to
G(g)+NHH2O(w)=G NHH2O(h), 61k
where NH is the hydration number and g, w, and h refer to gas, water, and hydrate, respectively. Of particular
interest are methane hydrates (G = CH4), which represent the majority of natural gas hydrates.
The amount of hydrocarbons residing in hydrate deposits is estimated to substantially exceed all known
conventional oil and gas resources [1-3]. Such deposits occur in two distinct geologic settings where the necessary
low temperatures and high pressures exist for their formation and stability: beneath the permafrost and in ocean
Because of the sheer size of the resource and the everincreasing energy demand, hydrocarbon hydrates are attracting
increasing attention as a potential alternative energy resource [4,5]. With hydrates being strong cementing agents,
the geomechanical behavior of hydrate-bearing sediments in response to thermal and mechanical stresses (natural or
anthropogenic) is of particular importance in marine systems because it may lead to deteriorating structural integrity
of the oceanic sediment formations that support structures such as hydrocarbon production platforms [6-8]. There is
also evidence linking the large-scale behavior of gas hydrates to instances of rapid global warming in the geologic
past [9,10]. The scientific and economic implications of all these issues have necessitated the development and
evaluation of models that can accurately predict the behavior of gas hydrates in porous media.
As Makogon  indicated, the three main methods of hydrate dissociation are (1) depressurization, in which the
pressure P is lowered below the equilibrium pressure Pe for hydrate formation at the prevailing temperature T; (2)
thermal stimulation, in which T is raised above the equilibrium temperature Te for hydrate formation at the
prevailing P; and (3) through the use of inhibitors (such as salts and alcohols) which cause a shift in the Pe-Te
equilibrium because of competition with the hydrate for guest and host molecules. Dissociation results in the
production of gas and water, with a corresponding reduction in the saturation of the solid hydrate phase. For the case
of methane hydrates, the endothermic dissociation reaction is:
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Kowalsky, Michael B. & Moridis, George J. Comparison of kinetic and equilibrium reaction models insimulating the behavior of porous media, article, November 29, 2006; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc902720/m1/1/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.