HydrateResSim (HRS) is an open-source finite-difference reservoir simulation code capable of simulating the behavior of gas hydrate in porous media. The original version of HRS was developed to simulate pure methane hydrates, and the relationship between equilibrium temperature and pressure is given by a simple, 1-D regression expression. In this work, we have modified HydrateResSim to allow for the formation and dissociation of gas hydrates made from gas mixtures. This modification allows one to model the ConocoPhillips Ignik Sikumi #1 field test performed in early 2012 on the Alaska North Slope. The Ignik Sikumi #1 test is the first field-based demonstration of gas production through the injection of a mixture of carbon dioxide and nitrogen gases into a methane hydrate reservoir and thereby sequestering the greenhouse gas CO2 into hydrate form.
The primary change to the HRS software is the added capability of modeling a ternary mixture consisting of CH4 + CO2 + N2 instead of only one hydrate guest molecule (CH4), therefore the new software is called Mix3HydrateResSim. This Mix3HydrateResSim upgrade to the software was accomplished by adding primary variables (for the concentrations of CO2 and N2), governing equations (for the mass balances of CO2 and N2), and phase equilibrium data. The phase equilibrium data in Mix3HydrateResSim is given as an input table  obtained using a statistical mechanical method developed in our research group called the cell potential method. An additional phase state describing a two-phase Gas-Hydrate (GsH) system was added to consider the possibility of converting all available free water to form hydrate with injected gas.
Using Mix3HydrateResSim, a methane hydrate reservoir with coexisting pure-CH4-hydrate and aqueous phases at 7.0 MPa and 5.5°C was modeled after the conditions of the Ignik Sikumi #1 test:
14-day injection of CO2 and N2 followed by
30-day production of CH4 (by depressurization of the well).
During the injection phase, the injection well is modeled as a fixed-condition boundary maintained as a gas phase (23% CO2 + 77% N2) at 9.65 MPa and 5.5 °C. Initially, there is an increase in the saturation of hydrate indicating the formation of secondary hydrate due to the injected gas and the available free water. There is also a slight increase in the temperature due to the exothermic reaction of hydrate formation. As the hydrate becomes saturated with the injected gases it releases CH4. After the initial 14 days of injection, a mixture of the three gases was produced through depressurization. This was modeled by maintaining the well as a fixed-state boundary at the bottom-hole pressure. The amount of CH4 released from the hydrate phase during the injection and production phases and the amount of CO2 and N2 gases sequestered as hydrates have been examined in this study. A model-based history-matching of the gas flow rates from the ConocoPhillips field test will be conducted to validate the code.