(118c) Injection of Carbon Dioxide and Nitrogen Into Methane Hydrate Reservoirs: Binary Hydrateressim Simulations

Natural gas hydrates, non-stoichiometric crystalline inclusion compounds are likely to contain more carbon than in all other fossil fuel reserves combined. Different techniques that are currently being proposed for production of CH₄ from the gas hydrates include depressurization, thermal stimulation and injection of inhibitors. These methods involve dissociation of gas hydrate and the release of a significant amount of water that may cause geomechanical stresses on the reservoir that could lead to subsidence. A newly proposed method is to replace CH₄ in the hydrate by the injection of pure CO₂ or CO₂+N₂ mixtures, which serves dual purpose of long-term storage of a greenhouse gas (CO₂) and the production of natural gas, while maintaining the hydrate structure in situ.

Field-scale experiments and the equipment required for production tests on hydrate deposits are very expensive. Therefore, reservoir simulations can be used to predict the production potential of gas hydrates. HydrateResSim (HRS) is the only open-source code available for public through National Energy Technology Laboratories (NETL), written in FORTRAN.  The current version of HRS can only simulate pure methane hydrates and the relationship between equilibrium temperature and pressure is given by a simple, 1-D regression expression parameterized by Moridis (2003). In this work the code has been modified to simulate mixed hydrates including CH₄-CO₂ and CH₄-N₂ in order to understand simultaneous CH₄ production rate as well as CO₂ sequestration rate in naturally-occurring hydrate reservoirs. The original code allows distribution of heat and up to 3 components (H₂O, CH₄, and inhibitors) whereas the modified code, can allow distribution of heat and up to 4 components (H₂O, CH₄, CO₂/N₂ and inhibitors) between four possible phases (gas, aqueous, ice, and hydrate). A major component to the new code is the implementation of two-component equilibrium surface. The equilibrium surface obtained using a cell potential method is incorporated in to the code in tabular form and a bi-linear interpolation is used to interpolate data at given conditions. The phase equilibrium is given as input to the code in two data files, T_eq=f(P,y_CH₄) & P_eq=g(T,y_CH₄) where T is temperature (°C), P is pressure (MPa) and y_CH₄is CH₄ composition in gas phase. Simulations have been performed to analyze the behavior of the hydrate reservoir by injecting pure CO₂ and N₂. 

A methane hydrate reservoir is considered with coexisting hydrate and aqueous phases and at the boundary CO₂ gas is injected and the reservoir behavior is studied. It is found that CO₂ hydrate was formed due to the available free water phase which caused dramatic reduction in permeability of the reservoir to the injection of fluid. Therefore it is necessary to remove the free water within the vicinity of the injection well in a reservoir so as to avoid the excessive hydrate formation and blockage close to the well. In order to remove the free water before injection of CO₂ gas the reservoir can be flooded with N₂ gas. The reservoir behavior with injection of N₂ is also studied and it is observed that there is an ice formation near the injection well which can block the well. The solution could be injection of N₂+CO₂ mixture and to study the exchange process in the reservoir by injection of mixed gas. Therefore HydrateResSim has been modified to allow for the simulation of a ternary gas system, CH₄-N₂-CO₂.