Numerical Studies of Gas Production From Class 2 and Class 3 Hydrate Accumulations at the Mallik Site, Mackenzie Delta, Canada
- G.J. Moridis (Lawrence Berkeley Natl. Laboratory, U. of California)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2004
- Document Type
- Journal Paper
- 175 - 183
- 2004. Society of Petroleum Engineers
- 5.3.1 Flow in Porous Media, 4.1.2 Separation and Treating, 5.9.1 Gas Hydrates, 5.5 Reservoir Simulation, 1.1 Well Planning, 7.4 Energy Economics, 4.6 Natural Gas, 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 4.3.1 Hydrates, 4.3.4 Scale, 5.3.2 Multiphase Flow, 5.2 Reservoir Fluid Dynamics, 6.5.2 Water use, produced water discharge and disposal, 1.6 Drilling Operations, 5.4.6 Thermal Methods
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The Mallik site represents an onshore permafrost-associated methane hydrate accumulation in the Mackenzie Delta, Northwest Territories, Canada. This study focuses on gas production at the Mallik site from hydrate deposits that are underlain by either a free-water zone (Class 2) or an impermeable boundary (Class 3). The production analysis was conducted with a numerical simulator that can model the nonisothermal CH4 release, phase behavior, and flow under conditions typical of CH4 -hydrate deposits by solving the coupled equations of mass and heat balance. Accumulations with a CH4 -hydrate saturation of at least 50% were studied. Dissociation was induced mainly by a combination of thermal stimulation and depressurization as hot fluids circulated between injection and production wells. The effects of salinity and of pressure changes at the wells were also accounted for. The production strategy resulted in a zero net water production. The simulation results indicated that the amount of CH4 released from the dissociating hydrate deposits is sensitive to the hydrate saturation, the initial temperature, the specific enthalpy, and the flow rate of the circulating fluids.
Gas hydrates are solid crystalline compounds in which gas molecules are encaged inside the lattices of ice crystals. Vast amounts of hydrocarbons are trapped in hydrate deposits.1 Such deposits exist under favorable thermodynamic conditions that occur in two distinct types of geologic formations in which the necessary low temperatures and high pressures exist: in permafrost and in deep ocean sediments.
Current estimates of the worldwide quantity of hydrocarbon gas hydrates range between 1015 and 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. Although the current energy economics cannot support gas production from hydrate accumulations, their potential clearly demands evaluation.
Gas from hydrates is produced by inducing dissociation. The three main methods of hydrate dissociation are: (1) depressurization, in which the pressure is lowered to a level lower than the equilibrium hydration pressure at the prevailing temperature; (2) thermal stimulation, in which the temperature is raised above the equilibrium hydration temperature at the prevailing pressure; and (3) the use of inhibitors (such as salts and alcohols).1
The Mallik site represents an onshore permafrost-associated methane hydrate accumulation in the Mackenzie Delta, Northwest Territories, Canada. A significant body of literature2 on both the geology and the hydrate accumulations at the site became available following an 1150-m-deep gas hydrate research well that was drilled in 1998.
The Numerical Model.
The numerical studies of gas production in this paper were conducted with the EOSHYDR2 model,3 which is a member of the TOUGH24 family of codes for multicomponent, multiphase fluid and heat flow and transport in the subsurface. By solving the coupled equations of mass and heat balance, EOSHYDR2 models the behavior of methane-bearing binary hydrates that are formed or dissociate in porous media according to the general reaction equation:
where G is the second hydrate-forming gas, n is the hydration number, ? is the mole fraction in the binary hydrate, and the subscripts m and G denote the methane and the second gas, respectively. Obviously, ?m+?G=1. The gas G can be CO2 , H2S, N2, or another gaseous alkane C? ,H2?+2(?=2, 3, 4).
Classification of Hydrate Deposits.
Natural hydrate accumulations are divided into three main classes.5,6 Class 1 accumulations comprise two layers: the hydrate interval and an underlying two-phase fluid zone with free (mobile) gas. In this class, the bottom of the hydrate stability zone (i.e., the location above which the formation of hydrates becomes thermodynamically possible) coincides with the bottom of the hydrate interval. On current evidence, this appears to be the most desirable class for exploitation, necessitating only small changes in pressure and temperature to induce dissociation.6
Class 2 deposits feature two zones: a hydrate-bearing interval, overlying a mobile water zone with no free gas (e.g., an aquifer). Class 3 accumulations are composed of a single zone, the hydrate interval, and are characterized by the absence of an underlying zone of mobile fluids. In Classes 2 and 3, the entire hydrate interval may be well within the hydrate stability zone and can exist under equilibrium or stable conditions.
Objective and Approach.
Class 2 and Class 3 hydrate deposits are more challenging production targets than Class 1 hydrates because of adverse permeability conditions, which limit the effectiveness of depressurization and increase the appeal of thermal stimulation as production strategies.5,7 Previous studies of gas production from Class 2 and Class 3 hydrates involved the production and/or circulation of large amounts of water.5,7 The main objective of this study is to evaluate the potential of Class 2 and Class 3 accumulations when using production strategies that result in zero net water withdrawals. This approach eliminates the environmental and economic problems posed by the disposal of water in the sensitive Arctic environment.
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