Natural gas hydrates: Comparison between two different applications of thermal stimulation for performing CO2 replacement

Natural gas hydrates represent a valid opportunity for the permanent storage of CO2. In this work thermal stimulation technique has been deepened for replacing CH4 with CO2. Using a lab-scale reactor, eight experimental tests were carried out, characterized by two phases. The first one consists in the methane hydrates formation and is the same for all experiments. The second one consists in the replacement process and has been conducted with two different methods. In the first four tests CO2 was first introduced to reach a solid phase of methane hydrates and a gaseous phase composed of both CH4 and CO2; subsequently the temperature was increased. In the other four tests, the first step consisted in increasing temperature and then introducing CO2. In this case, the temperature required to move the thermodynamic conditions from a region suitable for the formation of both CH4 and CO2 hydrate to a region where the formation process can only involve CO2, was achieved without additional thermal energy; in fact, when the CO2 hydrates began to form, an immediate and massive release of heat took place. The results show that the methane yield and the amount of CO2 that can be stored do not differ between the two types of tests, but the thermal energy required to complete the process is much lower in the tests conducted with the second method.