GENERAL RESEARCH Numerical Simulation of Methane Production from a Methane Hydrate Formation Yong Liu, Matteo Strumendo, and Hamid Arastoopour* Department of Chemical and Biological Engineering, Illinois Institute of Technology, 10 West 33rd Street, Chicago, Illinois 60616 This paper describes a one-dimensional model for hydrate dissociation in porous media by the depressurization method. A moving boundary, which separates the total simulation zone into two zones, is used. The governing equations consider the convective-conductive heat transfer and mass transfer in the gas and hydrate zones together with the energy balance at the moving front. These equations were transformed into a new coordinate system using a coordinate transformation method. The numerical method of lines was used to discretize the governing equations after coordinate transformation. Distributions of temperature and pressure for different well pressure and reservoir temperature are presented. The speed of the moving front and the gas production rate were shown to be strong functions of the well pressure and the absolute permeability of the porous media. Our simulations also showed that the assumption of stationary water phase, underpredicts gas production and overpredicts the speed of the moving front. 1. Introduction Natural gas hydrates are solid molecular compounds com- posed of water and a large amount of methane gas is trapped in the reservoirs. 1-3 Methane is a less carbon-intensive fuel than other hydrocarbons, such as oil or coal. The combustion of methane yields 44% less CO 2 than coal, per unit energy release, and 29% less CO 2 than oil. 2 Natural gas hydrates occur in two zones: in permafrost and under the sea floor. Because the hydrate density is smaller than the density of seawater, the hydrate is cemented with the sediment of the sea floor to be stable. 2 There are three main methods to dissociate the hydrate: depressurization, 4-9 inhibitor stimulation, and thermal stimula- tion. 10-13 Mathematical modeling and simulation of the hydrate dissociation process and of the transport phenomena associated with the flow of natural gas in the hydrate deposits is essential in order to evaluate the rate of natural gas production attainable. Consequently, it is also necessary to determine the importance of the hydrate reserves in the panel of the future energy supplies. Selim and Sloan 14 developed a one-dimensional model considering the convective and conductive heat transfer in the gas hydrate reserve. They obtained an analytical solution under the assumption that a moving front separates the reserve into two zones: the dissociated gas zone and the hydrate zone. They also assumed that no gas phase exists in the hydrate zone and the water phase is stationary after hydrate dissociation in porous media. Makogon 15 proposed a model with an analytical solution considering the adiabatic and throttling effects of methane gas, together with the gas convective energy, with the assumption that both gas phase and hydrate phase exist in the hydrate zone. Ji et al. 7 calculated the undetermined values in Makogon’s analytical solution and applied it to given sets of reservoirs with different operational parameters. Both Makogon and Ji’s models neglected the energy balance at the moving front. Ahmadi et al. 4 solved a one-dimensional model numerically with both conductive and convective heat transfers and an energy balance at the moving front with the assumption of stationary water phases in the dissociated zone. They also considered only the convective heat transfer in the dissociated gas zone in the energy balance equation. More recently, Nazridoust and Ahmadi 12 simulated the hydrate dissociation using commercial software FLUENT. Our model followed the approach of Ahmadi. 4 We modified the governing equations to account for the water movement. The energy balance equations include heat conduction in sand that composed the porous media; and the convective heat transfer due to the movement of the gas and water after hydrate dissociation. A coordinate transformation method was applied, to avoid the calculation of the moving front explicitly. 2. Hydrate Dissociation Model Depending on the physical property of the hydrate reservoir, the gas hydrate may exist by itself or coexist with the pressurized gas. We simulated the case that a well that is drilled into a hydrate reservoir coexists with methane gas. The depressuriza- tion method was used to dissociate the methane hydrate. The depressurization method is the least expensive method of hydrate dissociation and most feasible when the gas hydrate-bearing sands have a large free gas. 2 The hydrate decomposition model may be expressed as 2 The hydrate decomposes into methane gas and water when the pressure decreases or the temperature rises. Figure 1 shows a schematic of a reservoir of hydrate coexisting with gas. Initially, the gas and hydrate coexist at pressure of P e (reservoir pressure) and at temperature of T e (reservoir temperature). P e is higher than the thermodynamic 164 m 3 (CH 4 ) STP + 0.8 m 3 (H 2 O) water T 1.0 m 3 (Hydrate) (1) 2817 Ind. Eng. Chem. Res. 2008, 47, 2817-2828 10.1021/ie071398b CCC: $40.75 © 2008 American Chemical Society Published on Web 03/19/2008