Comparison of kinetic and equilibrium reaction models insimulating the behavior of porous media Page: 2 of 9
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CH4 NHH2O(h)=CH4(g)+NHH2O(w), 62b
where the hydration number NH is approximately 6. Depending on the thermodynamic state, the water produced in
the reaction of Eq. (2) can exist as liquid (the common product of dissociation in geologic systems) or ice. Two
approaches are possible for predicting hydrate dissociation. The first considers the reaction of Eq. (2) to occur at
chemical equilibrium, while the second treats it as a kinetic reaction. The equilibrium relationship between Pe and
Te is described by Fig. 1 . In the equilibrium model, the system is composed of heat and two mass components
(CH4 and H20) that are distributed among four possible phases: the gas phase (composed of CH4 and H20 vapor),
the aqueous phase (composed of H20 and dissolved CH4), the solid ice phase (involving exclusively H20), and the
solid hydrate phase. Thus, the system always exists at equilibrium, with the occurrence of the various phases and
phase transitions determined by the availability and relative distribution of heat and of the two components.
In the kinetic model, the system is composed of heat and three mass components: CH4, H20, and CH4 zE NHH20.
As opposed to the equilibrium model, the hydrate is not treated as a thermodynamic state of CH4 and H20 but as a
third distinct compound. In this case the solid hydrate phase is considered to be composed exclusively of the CH4 zE
NHH20 component. Phase changes and transitions are determined by a kinetic rate of dissociation or formation,
which acts as a source/sink term and is given by the equation of Kim et al. :
dmH dt 1 KO exp E RT F AA6fe f D; 63b
where f and fe are the values of fugacity (Pa) for the pressure at temperature T ( C) in the gas phase and at
equilibrium, respectively; E is the hydration activation energy (J mol 1); KO is the hydration reaction constant (kg
m 2 Pa 1 s 1); A is the surface area (m2) for the reaction; FA is the area adjustment factor [dimensionless],
which accounts for deviations from the assumption of grain sphericity used in calculating A ; and R is the
universal gas constant (J mol 1 C 1). Values of KO and the E which are used in this study have been determined
from laboratory data in pure hydrate systems [12,13] and in hydrate-bearing media .
It is difficult to know a priori which reaction model, equilibrium or kinetic, is most appropriate for the description of
problems of hydrate dissociation in porous media. While the kinetic model may more accurately model hydrate
dissociation, the use of the equilibrium model may be justified in some cases due to its computational efficiency (as
it involves one less equation per grid block than the kinetic one) and because predictions made using both models
are in many cases remarkably similar . Prior to this study, we worked with the assumption that, in general,
thermalstimulation-induced production is accurately described by an equilibrium model, while a kinetic model may
be more appropriate for depressurization-induced dissociation.
The objective of this study is to investigate through numerical simulation the conditions under which the use of each
of the two models (equilibrium and kinetic) is appropriate, and to evaluate differences in predictions from the two
models. Specifically, we aim (1) to investigate whether the rate of CH4-hydrate dissociation in a variety of realistic
situations is limited by kinetics; (2) to compare model predictions obtained by using the kinetic and equilibrium
models of dissociation for a wide range of production scenarios and geological settings; and (3) to investigate the
relative sensitivity of the two dissociation models to a number of parameters, including numerical discretization,
initial hydrate saturation and the area adjustment factor FA (Eq. (3)).
1.3. Test cases
We investigate four test cases (A-D). The first two cases involve production from a Class 3 hydrate accumulation
, which is characterized by a hydrate-bearing layer (HBL) underlain and overlain by impermeable layers. In Case
A dissociation is induced by thermal stimulation, in which the temperature of the HBL is increased above Te at the
prevailing pressure (Fig. 1), while in Case B dissociation is induced by depressurization, in which the pressure of
the HBL is reduced below the Pe at the prevailing temperature (Fig. 1). In Case C we examine production at a
constant rate from a Class 1 hydrate accumulation. This type of accumulation is characterized by a HBL overlain by
an impermeable layer and underlain by a two-phase zone of water and mobile gas, and it has been identified as a
particularly promising target for gas production [15,16]. In Case D, we simulate the response of a hydrate-bearing
core as it is extracted from depth (in situ conditions) and transported to the surface.
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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/m1/2/: accessed October 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.