Recovery of methane from a gas hydrate reservoir using depressurization and N2 injection

Natural gas hydrate deposits are predominantly found in deep sea sediments and permafrost regions of earth. They store a large amount of methane in a chemically bonded form with water. In the present work, the methane recovery technique based on depressurization is augmented by the injection of gaseous nitrogen. The continuous and periodic modes of injection are compared. An underground Class 3 hydrate reservoir studied has production and injection wells at its two ends. The reservoir initially contains free CH4 gas, methane hydrate, and water. Constant pressure nitrogen gas is introduced at the injection well, and depressurization is carried out at the other. The reservoir is numerically simulated to obtain estimates of methane recovered and nitrogen injected over a period of time. The mathematical model includes transient, non-isothermal, multiphase multicomponent transport through a porous region. Mass, momentum, and energy balances form the system of governing equations. The production well pressure, injection well pressure, medium porosity, and heat transfer coefficient with the bounding rock are varied. Consequently, methane recovery rate and the reservoir response to N2 injection are estimated. Results of N2 injection-assisted depressurization technique show definite improvement in the methane gas production over that due to pure depressurization alone. Methane recovery increases with the pressure of injection till the flow is blocked by the formation of secondary gas hydrates. In place of continuous injection, cyclic injection of nitrogen is shown alleviate this difficulty without impeding gas recovery. Jointly, the quantity of nitrogen consumed is lowered. © 2021 by Begell House, Inc.