Experimental analysis on the possibility of coupling chemical inhibitors injection and CO2 replacement strategies in natural gas hydrates reservoirs

Natural gas hydrates reservoirs may be considered one of the most important energy sources worldwide. During the last decades, several estimates of methane amount contained into hydrates deposits were made: even if results presented significant differences between each other, in all cases the value estimated falls in the range 1015 – 1018 m3, making them the most abundant natural gas reserve in the world. Moreover, in literature is widely affirmed how the energy which can be produced from natural gas hydrates is more than two times greater than energy producible from all conventional energy sources put together. Research interest is justified for the possibility of improving a completely carbon neutral energy source. Lots of strategies were performed for deposit exploitation; from them, the CO2/CH4 exchange process is gaining more and more importance. Carbon dioxide is able to form hydrates with lower pressure and/or higher temperature values than methane. Thus, injecting CO2 inside the reservoir layers and, at the same time, slightly modifying thermodynamic conditions (as mentioned previously), the replacement of methane with carbon dioxide into already existing water cages may be improved. Considering that the exchange process’ ratio is equal to 1:1 (in case of maximum theoretical efficiency) and taking into account that the combustion of one methane molecule leads to the production of only one carbon dioxide molecule, the process impact on the atmosphere is very contained, until be considered, in cases where efficiency is particularly elevated, carbon neutral. Several techniques were thought for intervening in hydrates reservoirs; among them, the most explored and performed solutions are depressurization, thermal stimulation, chemical inhibitors injection and, obviously, CO2 replacement. All methods present critical issues and disadvantages, which make the ratio between spent energy and produced energy too elevated for making the hydrates reservoirs exploitation an industrial scale process. However, better results were reached by coupling different techniques. In literature the use of depressurization together with thermal stimulation was widely explored and documented; nowadays it is considered the most effective solution for intervening in hydrates deposits. Within this PRIN project, another combination was proposed and is now object of experimental investigation: the contemporary use of chemical inhibitors injection and CO2 replacement strategies. In particular, sodium chloride case was taken into account. If its inhibitor effect is well known and documented in literature, very few information are available on its possible selective behaviour in function of the gaseous species involved into hydrates. To prove this, several CO2 and CH4 hydrates formation tests were carried out in presence of demineralised water and salt, adopting different concentration: 0, 32 and 37 g/l. For all salinity degree, equilibrium curves of both species hydrates were firstly tracked and then compared among them, with the aim of observing their mutual distance variations. Considering temperature values in the range 2.0 – 3.8 °C, for tests realized in pure demineralised water, the pressure gap existing between methane equilibrium curve (the higher one) and carbon dioxide (the lower) is comprised between 9.55 bar and 12.21 bar. Considering the same temperature interval, equilibrium curves of tests carried out with a salt concentration of 32 g/l showed a gap in the range 16.96 – 20.16 bar. Finally, in 37 g/l test the pressure difference was 15.40 – 19.67 bar. Results clearly proved how sodium chloride negative impact is greater for methane hydrates than carbon dioxide ones, making the pressure gap between their equilibrium curves greater. A wider thermodynamic region between the two species equilibrium means being able to promote the CO2 replacement strategy with higher efficiency values, because of the possibility of moving deposit pressure – temperature conditions at higher distances from methane hydrate stability zone while remaining inside the CO2 hydrates stability area.