A new method for the replacement of CH4 with CO2 in natural gas hydrate production

Natural gas hydrate (NGH), as a clean energy, has a great development potential in the future for its immense reserves. The CO 2 replacement method has dual effects in energy exploitation and greenhouse gas (GHG) sequestration, but its CH 4 replacement rate is too low. When an inhibitor is added, CH 4 hydrate and CO 2 hydrate are different in terms of their thermodynamic stability. In this regard, a new method was proposed and experimentally proved for NGH production. This new method combines CO 2 replacement method with thermodynamic hydrate inhibitor technology to accelerate the decomposition of CH 4 hydrate. Three common thermodynamic CH 4 hydrate inhibitors (i.e. methanol, sodium chloride and magnesium chloride) in two categories were compared by means of core displacement experiments. It is shown that, when methanol solution is used as the inhibitor, the decomposition rate of CH 4 hydrate reaches 0.01194 mol/h, which is much higher than that of electrolyte solution (0.00086 mol/h and 0.00141 mol/h respectively). Therefore, methanol solution is used to accelerate hydrate decomposition. The decomposition ratio of CH 4 hydrate was over 92% when methanol solution plug was injected at the early stage and CO 2 was continuously injected at the late stage. Moreover, CO 2 was sequestrated in the form of hydrate. Eventually, the generated CO 2 hydrate accounted for 16–27% of the initial total CH 4 hydrate.