Non-equilibrium simulation of CH4 production from gas hydrate reservoirs through the depressurization method

Abstract

Natural gas hydrates (NGHs) in nature are formed from water and hydrate formers from various phases (i.e., aqueous, gas, and adsorbed phases). As a result, owing to Gibbs’ phase rule and the combined first and second laws of thermodynamics, CH4 hydrates cannot reach thermodynamic equilibrium in real reservoir conditions. Thus, there is a competition between hydrate formers, where the most stable hydrates form first. The non-equilibrium nature of hydrates indicates a need for proper kinetic models to describe the various routes that can lead to hydrate formation. Dissociation of hydrates in sediments can also occur as a function of undersaturation of any of the thermodynamic variables. In addition to temperature and pressure being outside the stability region for hydrates, the concentration of water and hydrate formers in co-existing phases can lead to hydrate instability. CH4 hydrate dissociation towards CH4 gas and water has previously been implemented using the RetrasoCodeBright (RCB) hydrate simulator. In the present work, we implement an additional route for hydrate phase transitions that enables hydrate dissociation and reformation towards water and aqueous CH4 and by considering undersaturation or supersaturation with respect to pressure, temperature, and CH4 mole fraction. CH4 hydrate dissociation in contact with water undersaturated with methane is considered, as well. An in-house non-equilibrium thermodynamic package has been written and is inserted into RCB to calculate Gibbs free energies. The driving forces for hydrate phase transitions are calculated from differences in the free energies of hydrates and hydrate formers. Competing phase transitions are handled by Gibbs free energy minimizations. Nucleation theory is used to calculate the impact of heat and mass transport and of non-equilibrium thermodynamics on kinetic rates of hydrate phase transitions. Our modifications are used to simulate CH4 production, covering time spans of 290 days using the depressurization method on a simplified hypothetical model. A complete description of our methodology is presented together with a discussion of our simulation results.