## 4 Results and Discussion
The injection of CO2 into CH4 hydrate-filled sediments will lead to the formation of new CO2 hydrate from injected CO2 and free pore water. It is rare to find hydrates with a hydrate saturation exceeding 85% of the pore volume, with 75% or less hydrate filling being more common. The primary mechanism for dissociation of in situ CH4 hydrate will be driven by the latent heat of the CO2 hydrate formation. In a general case, it is safe to assume that CH4 released by the dissociating methane hydrate will migrate away from the CO2 hydrate, whose surface will contain hydrate films separating the fluid CO2 from the released CH4. In the case where we can expect a degree of mixing between the released CH4 and CO2, it will be instructive to examine how this will affect the enthalpies of formation. For ease of implementation, we will use 2D plots of hydrate formation enthalpies evaluated along the hydrate formation pressure limits. These plots should still yield fairly relevant conclusions for hydrate formation enthalpies since any Poynting pressure corrections within the hydrate stability region will be very small. Figure 6 will therefore present the enthalpies for pure CH4 hydrate, pure CO2 hydrate, as well CO2/CH4 mixtures with 98 mol % CO2, 95 mol % CO2, and 90 mol % CO2 plotted as functions of temperature along with the corresponding temperature-pressure stability limits. A similar plot in Figure 7 shows the enthalpies of hydrate formation as functions of pressure along the temperature-pressure stability limits. Since a CO2 molecule is too large to provide any stabilization of small cavities, 25% of small cavities will be empty for these ranges of conditions in the case of a pure CO2 hydrate. As can be seen from Figures 6 and 7 below, the addition of CH4 will therefore result in larger (negative) values for enthalpies of hydrate formation. Figure 6: Enthalpies of hydrate formation in kJ/mol hydrate former as function of temperature. For temperatures below the CO2 phase transition point (roughly 283.4 K), the bottom solid curve corresponds to CO2/CH4 mixture with 10 mol % CH4; the top solid curve is pure CO2; 5 mol % CH4 and 2 mol % CH4 curves fall in between. For temperatures above the CO2 phase transition point, the order of curves is reversed. The dashed curve is the corresponding results for pure CH4 hydrate. The results presented in Figures 6 and 7 will also be relevant when the injected CO\({}_{2}\) comes from a separation plant used for removal of sour gases from hydrocarbons. As an example, millions of tons of CO\({}_{2}\) from the Sleipner field are separated and injected into the Utsira formation. The injection gas contains approximately 5 mol % of CH\({}_{4}\)[42]. In Figures 8 and 9, we plot the enthalpies of hydrate formation as a function of temperature and pressure along the hydrate stability limit curve. The use of pure component values for the enthalpies of hydrate formation will substantially simplify the modeling, with the motivation for including Figures 6-9 being to facilitate a qualitative picture of the sensitivity of hydrate formation enthalpies with respect to certain relevant additives. Other uncertainties taken into account, using pure component  
Figure 8: Enthalpies of hydrate formation in kJ/mol hydrate former as function of temperature. For temperatures below the CO\({}_{2}\) phase transition (around 283.5 K), the bottom solid curve corresponds to pure CO\({}_{2}\) hydrate; the top solid curve is 20 mol % N\({}_{2}\) in CO\({}_{2}\)/N\({}_{2}\) mix; the middle curve is 10 mol % N\({}_{2}\) in CO\({}_{2}\)/N\({}_{2}\) mix. Above the CO\({}_{2}\) phase transition, the order of the curves is reversed. Figure 7: Enthalpies of hydrate formation in kJ/mol hydrate former as function of pressure. The bottom solid curve corresponds to CO\({}_{2}\)/CH\({}_{4}\) mixture with 10 mol % CH\({}_{4}\); the top solid curve is pure CO\({}_{2}\); 5 mol % CH\({}_{4}\) and 2 mol % CH\({}_{4}\) curves fall in between. The dashed curve is the corresponding results for pure CH\({}_{4}\) hydrate.