Optimization of the pressure drop produced during CO2 replacement in hydrate reservoirs: Balance between gas removal and preservation of structures

Abstract

The replacement of methane molecules with a theoretical equal number of CO2 molecules, is one of the most promising opportunities for the exploitation of natural gas hydrate reservoirs. In this work, the effect of depressurization on CH4 hydrates was analysed in depth. Different pressure drops, from 5 to 15 bar were tested and described in terms of free gas removed (desired event) and hydrates destroyed (undesired event) before the injection of carbon dioxide. A few pronounced pressure drop might be not enough to ensure an effective replacement, while an excessive one might be not sustainable due to the high quantity of hydrates destroyed before the process. In this work, variation of hydrates and free gas was evaluated before and after depressurization; it was then compared and employed to define an optimum range of pressure drop to optimize the process. It was proved that the quantity of hydrates destroyed increases with the pressure drop imposed, while the removal of free gas shows a maximum. When depressurization was close to 10 bar, only 18.57% of hydrates dissociated, while the quantity of free gas removed was three times higher (59.27%). Finally, the CO2/CH4 replacement process was carried out with the optimal depressurization degree previously defined.