Comparison of kinetic and equilibrium reaction models insimulating the behavior of porous media Page: 3 of 9
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1.4. Numerical simulator
The numerical studies in this paper were conducted using TOUGH-Fx/HYDRATE , which models the
nonisothermal hydration reaction, phase behavior and flow of fluids and heat under conditions typical of natural
CH4-hydrate deposits in complex formations. It includes both equilibrium and kinetic models of hydrate formation
and dissociation and can handle any combination of the possible hydrate dissociation mechanisms (i.e.,
depressurization, thermal stimulation, and inhibitor-induced effects). TOUGH-Fx/HYDRATE accounts for heat and
up to four mass components (i.e., water, CH4, hydrate, and water-soluble inhibitors such as salts or alcohols) that
are partitioned among four possible phases (gas, liquid, ice or hydrate phases, which may exist individually or in
any of 12 possible combinations).
2. Case A: thermal-stimulation-induced production in hydrate accumulation
The HBL of the Class 3 hydrate accumulation in this case has a thickness of 10 m and involves a cylindrical
domain with maximum radius rmax = 1000 m. The domain was divided into 600 grid blocks in the radial
direction, beginning at the well radius rw = 7.5 cm, and employing a spacing that is Dr = 0.05 m near the well and
that increases logarithmically away from the well. The initial hydrate and aqueous phase saturations (Sh and Sa,
respectively) are spatially uniform, with Sh = Sa = 0.5, and the gas phase saturation Sg = 0. The most relevant
properties of the model are listed in Table 1.
Thermal dissociation is expected to be most useful for cases in which the HBL contains high initial Sh,
corresponding to drastically reduced permeability (rendering depressurization methods impractical). Thermal
stimulation is accomplished by maintaining the well at a constant pressure (equal to the initial HBL pressure) and
an elevated temperature of TW = 45 _C (see Table 1). Heat flows from the well into the HBL mainly by conduction
at a rate that declines over time as the temperature in the vicinity of the well increases.
2.1. Pressure, temperature and phase saturations
Fig. 2 shows the radial distributions of pressure, temperature, and phase saturations after 30 days of heating, as
obtained from simulations performed using the kinetic and equilibrium reaction models.
By this time, the temperature front (Fig. 2a) has propagated into the HBL and induced dissociation over a distance
of 1.3 m, resulting in the evolution of gas (originating exclusively from the hydrate, Fig. 2b) and an increase in
pressure (Fig. 2a). In the region behind the dissociation front (r < 1.3 in), the hydrate has completely dissociated
(Sh = 0), while Sw and Sg have both increased (as water and gas are products of dissociation) from their initial
values (Fig. 2b). We observe a sharp increase in Sh over a short distance immediately ahead of the dissociation front
(r > 1.3 in), mirrored by a corresponding sharp decline in Sa. This is caused by secondary hydrate formation ahead
of the advancing front, caused by (a) outward flow of a faction of the released gas (toward the HBL outer boundaries)
and (b) the increased pressure (Fig. 2a) at the dissociation fiont caused by the gas release. Beyond these saturation
spikes, the phase saturations remain nearly equal to the initial conditions. Note that the pressure rise at the
dissociation front indicates fluid flow in both directions and that the temperature distribution (Fig. 2a) is marked by
a slight discontinuity in the vicinity of the front. The most important observation from reviewing Fig. 2 is that,
although slight deviations in the phase saturations and pressure are observed near the dissociation front (where the
saturation spikes are observed), the profiles obtained from the kinetic and equilibrium reaction models are nearly
2.2. Gas release and production patterns
Fig. 3 shows the gas release and production patterns for the kinetic and equilibrium dissociation models during the
30-day heating period. Specifically, the following quantities are examined: (a) the volumetric rate QR of CH4 release
into the formation (Fig. 3a); (b) the volumetric rate QP of CH4 production at the well (Fig. 3b); and (c) the
cumulative volumes VR and VP of CH4 released in the formation and produced at the well, respectively (Fig. 3c).
The rate of CH4 released to the system during thermal stimulation is shown in Fig. 3a. To allow comparison
between the kinetic and equilibrium release rates QR for the kinetic case is averaged in time using a moving window
of 5 days. For both cases, QR is similar, approximately 50 m3/day. Without performing such averaging for the
kinetic case of QR, the fluctuations are so strong and drastic that a meaningful comparison can not be made with the
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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/m1/3/: accessed December 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.