Simulation of Methane Production from Hydrates by Depressurization and Thermal Stimulation Yong Liu, Matteo Strumendo, and Hamid Arastoopour* Department of Chemical and Biological Engineering, Illinois Institute of Technology, 10 W. 33rd Street, Chicago, Illinois 60616 Recently methane hydrates have attracted attention due to their large quantity on the earth and their potential as a new resource of energy. This paper describes a one-dimensional mathematical model and numerical simulation of methane hydrate dissociation in hydrate reserves by both depressurization and thermal stimulation using a one- dimensional radial flow system (axisymmetric reservoir). A moving front that separates the hydrate reserve into two zones is included in this model. A numerical coordinate transformation method was used to solve the moving boundary problem. The partial differential equations were discretized into ordinary differential equations using the method of lines. Our simulations showed that the moving front location and the gas flow rate production are strong functions of the well pressure and reservoir temperature. The impermeable boundary condition at the reservoir results in very low temperature at the moving front and the formation of ice. The formation of ice, which plugs the pore volume for the gas to flow, should be avoided. Compared with a stationary water phase model, our simulations showed that the assumption of a stationary water phase overpredicts the location of the moving front and the dissociation temperature at the moving front and underpredicts the gas flow rate. The thermal stimulation using constant temperature at the well method using a single well was found to have a limited effect on gas production compared to gas production due to depressurization. 1. Introduction Natural gas hydrates are solid molecular compounds composed of water with natural gas; there are huge amounts of methane gas trapped in hydrate reservoirs (Makogon, 1 Ahmadi et al., 2 Sloan and Koh 3 ). Three main methods exist to dissociate the hydrate: depressurization, inhibitor stimulation, and thermal stimulation (Ji et al. 4 ). Mathematical modeling and simulation of the hydrate dissociation process in the hydrate deposits is essential in order to evaluate the rates of natural gas production attainable. Selim and Sloan 5 developed a one-dimensional model consider- ing 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. The water phase is stationary after hydrate dissociation in porous media. Ahmadi et al. 2 developed a numerical method to solve a one- dimensional hydrate dissociation model with both conductive and convective heat transfer and energy balance at the moving front. Ahmadi et al. 6,7 extended the mathematical model into the cylindrical coordinate system. Nazridoust and Ahmadi 8 developed a user-defined function that can be used by the commercial software FLUENT and solved the methane hydrate dissociation problem, and their results compared well with the experimental data by Masuda et al. 9 Besides the mathematical modeling based on the moving boundary model that assumes that the hydrate dissociates only in the moving front, there is another category that is based on the assumption that the hydrate dissociates throughout the entire reservoir. On the basis of the general-purpose multiphase, multicomponent simulator TOUGH2 of fluid flow and heat transport in geologic media, Moridis et al., 10 Alp et al., 11 and Moridis and Sloan 12 simulated the hydrate dissociation using a reaction model. Liu et al. 13 developed a one-dimensional two-phase flow model and numerical solutions coupled with the methane hydrate dis- sociation moving front concept using a Cartesian coordinate system. In our earlier studies, the mathematical model used by Ahmadi et al. 7 was modified to account for the water movement (Liu et al. 13 ). Here, the numerical techniques of Liu et al. 13 are used to simulate the moving boundary problem in the axisym- metric system. The effects of the assumption of a stationary water phase on the moving front speed and the gas flow rate are discussed. In addition the effect of thermal stimulation on the methane gas production, the effect of the effective gas permeability in the undissociated zone, and the effect of the boundary condition at the reservoir are presented. 2. Hydrate Dissociation Model We simulated the case in which a single vertical production well of radius r 0 ) 0.1 m is drilled into a hydrate reservoir. In the reservoir the hydrate coexists with methane gas. The depressurization method and combined depressurization and thermal stimulation methods were used to dissociate the methane hydrate. The hydrate decomposition model may be expressed as (Max et al. 14 ) 164 m 3 (CH 4 ) STP + 0.8 m 3 (H 2 O) water S 1.0 m 3 (hydrate) (1) The hydrate decomposes into methane gas and water when the pressure decreases or the temperature increases. Figure 1 shows that a reservoir in which hydrate coexists with gas. Initially, the gas and the hydrate coexist at a pressure of P e (reservoir pressure) and at a temperature of T e (reservoir temperature). P e is higher than the thermodynamic equilibrium pressure of the hydrate at T e . The hydrate near the well becomes unstable and begins to dissociate into water and gas near the well, as soon as a single vertical production well is drilled into * To whom correspondence should be addressed. E-mail: arastoopour@ iit.edu. Ind. Eng. Chem. Res. 2009, 48, 2451–2464 2451 10.1021/ie8005275 CCC: $40.75  2009 American Chemical Society Published on Web 10/29/2008