Numerical Study on a New Enhanced Gas Recovery Method by Nitrogen Injection from a Methane Hydrate Reservoir

Methane hydrate (MH) is one of the potential resources of natural gas in the near future, because it exists in marine sediments or in permafrost regions worldwide. Some extraction methods of natural gas from the MH reservoir have been proposed, such as depressurization, thermal stimulation and inhibitor injection. These are all based on the in-situ dissociation process of MH that is transformed into methane gas and water. However, there are some technical and economical problems for operation of these methods. Therefore, we have proposed a new enhanced gas recovery method by nitrogen injection. Nitrogen has the effect as an inhibitor as well as methanol and salts to shift an equilibrium condition of hydrate to high temperature and low pressure. In this study, we constructed a numerical model for simulating MH dissociation process in porous media by nitrogen injection on the basis of experimental observations. The gas phase was treated as a two-component system calculation of methane and nitrogen, and equilibrium calculation of methane-nitrogen system was introduced into the numerical model. Through the history-matching of temperature change and gas production behavior in laboratory-scale experiments, we confirmed the validity of the constructed numerical model. Then, using the numerical model for nitrogen injection process, we carried out field-scale simulations. From calculation results, it was found that 1) MH dissociation zone extended depending on pressure gradient to production well, 2) At early-stages of production, water was produced depending on relative permeability characteristics, and 3) Later of the production gas front containing nitrogen and dissociated methane reached to production well and gas production was initiated. Furthermore, we discussed gas production behavior for the gas recovery method that nitrogen injection and depressurization were combined. As a result, we obtained the important knowledge that this combination method had a large advantage for dissociated gas production when well distance was longer.