Thermodynamic implications of adding N2 to CO2 for production of CH4 from hydrates

Abstract

The huge resources of natural gas trapped in hydrate form are widely distributed worldwide in permafrost and offshore sediments. Pressure reduction and thermal stimulation have been dominating the research into production methods over the latest decades. More recently, a novel approach has emerged based on conversion of in situ methane hydrate to carbon dioxide-dominated hydrate through injection of carbon dioxide. This work applied the free energy analysis to determine whether addition of nitrogen into the injection mixture would result in a win-win situation of simultaneous methane production and safe long term storage of CO2. Our evaluation of data from two permafrost and two offshore fields indicates that injection of carbon dioxide at concentrations exceeding 50 mol % and pressures ranging between 9 and 25 MPa will result in formation of new carbon dioxide-dominated hydrate for all of these fields. While only reservoir simulations implementing reliable thermodynamic models can verify whether given injection will result in substantial storage of carbon dioxide in the form of hydrate, thermodynamic models developed in this work have their own significance. Pressure and temperature dependencies of hydrate stability have frequently been reported in studies of hydrates in sediments as the only criteria. Extending these criteria to include the concentration dependency will make it possible to implement an efficient free energy minimization scheme able to probe local phase distributions. Since one of four hydrate reservoirs used in our thermodynamic analysis is located in Alaska, we have also investigated the upper limit of water that can be tolerated during transport under extreme conditions prevailing the winter seasons in this region. It was found that hydrate formation triggered by water adsorbing on rusty surfaces will dominate the tolerance limit, which will correspond to practically zero water concentration.