Numerical Studies of Gas Production From Methane Hydrates
- G.J. Moridis (Lawrence Berkeley Natl. Laboratory and U. of California, Berkeley)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 2003
- Document Type
- Journal Paper
- 359 - 370
- 2003. Society of Petroleum Engineers
- 5.3.2 Multiphase Flow, 4.1.5 Processing Equipment, 5.4.6 Thermal Methods, 4.3.1 Hydrates, 4.6 Natural Gas, 5.2.1 Phase Behavior and PVT Measurements, 5.9.2 Geothermal Resources, 5.2 Reservoir Fluid Dynamics, 2.2.2 Perforating, 5.3.1 Flow in Porous Media, 5.9.1 Gas Hydrates
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EOSHYDR2 is a new module for the TOUGH2 general-purpose simulator for multicomponent, multiphase fluid and heat flow in the subsurface. By solving the coupled equations of mass and heat balance, EOSHYDR2 can model the nonisothermal gas release, phase behavior, and flow of fluids and heat under conditions typical of common natural hydrate deposits (i.e., in permafrost and in deep ocean sediments) in complex formations, and it can describe binary hydrocarbon systems involving methane.
EOSHYDR2 includes both an equilibrium and a kinetic model of hydrate formation and dissociation. The model accounts for up to four phases (gas phase, liquid phase, ice phase, and hydrate phase) and up to nine components (hydrate, water, native CH4 and CH4 from hydrate dissociation, a second native and dissociated hydrocarbon, salt, water-soluble inhibitors, and a heat pseudocomponent). The mass components are partitioned among the phases. The thermophysical properties of the various mass components can be described at temperatures as low as -110°C. Dissociation, phase changes, and the corresponding thermal effects are fully described, as are the effects of salt and inhibitors. The model can describe all possible hydrate dissociation mechanisms (i.e., depressurization, thermal stimulation, salting-out effects, and inhibitor-induced effects).
Results are presented for four test problems of increasing complexity that explore different mechanisms and strategies for production from typical CH4-hydrate accumulations. The results of the tests indicate that CH4 production from CH4-hydrates could be technically feasible and has significant potential. In particular, thermal stimulation is capable of producing substantial amounts of hydrocarbons, and its effectiveness can be enhanced when coupled with depressurization and the use of inhibitors.
Gas hydrates are solid crystalline compounds in which gas molecules are encaged inside the lattices of ice crystals. These gases are referred to as guests, whereas the ice crystals are called hosts. Of particular interest are hydrates in which the gas is a hydro- carbon. Under suitable conditions of low temperature and high pressure, a hydrocarbon gas M will react with water to form hydrates according to M+NH H2O=M×NH H2O, where NH=the hydration number.
Vast amounts of hydrocarbons are trapped in hydrate deposits.1 Such deposits exist where the thermodynamic conditions allow hydrate formation, and are concentrated in two distinctly different types of geologic formations where the necessary low temperatures and high pressures exist, in permafrost and in deep ocean sediments. The lower depth limit of hydrate deposits is controlled by the geothermal gradient.
Current estimates of the worldwide quantity of hydrocarbon gas hydrates range between 1015 to 1018 m3. Even the most conservative estimates of the total quantity of gas in hydrates may surpass by a factor of two the energy content of the total fuel fossil reserves recoverable by conventional methods.1 The magnitude of this resource could make hydrate reservoirs a substantial future energy resource. While current economic realities do not favor gas production from the hydrate accumulations, their potential clearly demands evaluation.
The majority of naturally occurring hydrocarbon gas hydrates contain CH4 in overwhelming abundance. Simple CH4-hydrates concentrate methane volumetrically by a factor of 164, and require less than 15% of the recovered energy for dissociation. Natural CH4-hydrates crystallize mostly in the I structure, which contains 46 H2O molecules per unit cell. They have an NH ranging from 5.77 to 7.41, with NH=6 being the average hydration number, and NH=5.75 corresponding to complete hydration.
There are three main methods of hydrocarbon recovery from gas hydrates: (a) thermal stimulation,2 in which gas release is effected by heating the hydrate above the dissociation temperature at a given pressure; (b) depressurization,3 in which the gas release is achieved by lowering the pressure below that of the hydrate stability; and (c) "inhibitor" injection,4 in which the hydrocarbon is produced after the injection of substances (e.g., brines, alcohols) that destabilize the hydrate. Combinations of these methods can also be used.
Only two numerical codes have been developed for the simulation of gas production from dissociating hydrates. Drenth and Swinkels5 developed a four-component, three-phase numerical model for the equilibrium dissociation of binary hydrates in marine environments. An improved version of the code with advanced thermodynamics was later developed by the same authors, who provided an in-depth discussion of the challenges facing production from gas hydrates and identified knowledge gaps in numerical simulation of gas production from hydrate dissociation.6
Moridis et al.7 developed EOSHYDR, a TOUGH28,9 module for the simulation of dissociating simple methane hydrates under equilibrium and kinetic conditions in both permafrost and marine accumulations. In this paper, I discuss the development and performance of EOSHYDR2, a new module for the simulation of reservoir fluid flow and behavior in hydrate accumulations. EOSHYDR2 can model any combination of hydrate dissociation mechanism. It describes the nonisothermal formation or dissociation of simple CH4- or composite CH4- and C? H2?+2-hydrates using accurate thermophysical properties and the most recent information on the parameters of equilibrium or kinetic dissociation.
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