AN EXPERIMENTAL INVESTIGATION INTO THE MECHANISMS OF NATURAL GAS HYDRATE RESERVOIR EXPLOITATION THROUGH NaNO2 -NH4 Cl AUTOGENOUS THERMAL SYSTEM

The efficacy of the autogenic heating system, composed of ammonium chloride, sodium nitrite, and hydrochloric acid, for natural gas hydrate extraction has been demonstrated in the literature, while the mechanisms remained unclear. In this study we carried out a series of experimental analysis to elaborate the mechanisms for natural gas hydrate exploitation. We generated the natural gas hydrate and investigated its reaction with the self-heating system and its by-product. The results show that the mechanisms involve thermal stimulation through exothermic nitrogen gas (N2) replacing methane (CH4), and inhibiting secondary formation of hydrate. Autogenous thermal system releases a large amount of heat during the reaction, generating N2 and sodium chloride (NaCl) solution. At the beginning, the exothermic process becomes the main mechanism, and as the exploitation continues, N2 replacing CH4 starts to dominate. The total recovery rate of natural gas hydrate is 59.24%. Additional tests aiming at the contribution of each process individually show recovery rates of 38.57% and 19.73% for exothermic and replacing, respectively. The reactant, NaCl solution, does not intuitively extract hydrate but prevents the secondary formation of hydrate, which improves the overall extraction efficiency.