Experimental study and spectroscopic analysis of replacement method for $CO_2$ storage and natural gas hydrate production

Abstract

Replacement method emerged as a new technique to achieve both stable $CH_4$ production and nondestructive sequestration of $CO_2$ into deep marine sediments. In this study, we have firstly investigated the effect of soaking on $CH_4$ replacement efficiency as one of the process variables in the replacement method. As a result, sequential soaking steps were beneficial to recover additional $CH_4$ gas and enhanced the $CH_4$ replacement efficiency from 35 % to 60 %. Also, several replenishments of fresh $CO_2/N_2$ gas mixture (short-term soaking frequency) into the vapor phase, represented more effective than solely increasing the soaking time. Second, we have verified the replacement mechanism occurring in sII hydrate with an external $CO_2/N_2$ gas. Notably, we firstly revealed that the $C_3H_8+CH_4$ (sII) hydrate sustained its crystalline structure even after the replacement with a $CO_2/N_2$ gas, and also provided an experimental evidence of the nondestructive replacement by demonstrating that there was no sII hydrate decomposition followed by sI hydrate formation. In addition, the cage-specific replacement pattern of the $C_3H_8+CH_4$ hydrate revealed that $CH_4$ replacement with $N_2$ in the small cages of sII was more significant than $C_3H_8$ replacement with $CO_2$ in the large cages of sII. The total extent of the replacement for the $C_3H_8+CH_4$ hydrate with an external $CO_2/N_2$ gas was found to be approximately 54%. We believe that soaking results will contribute in setting up the process variables of replacement technique for future on/offshore $CH_4$ gas production tests. Also, the structural sustainability of $C_3H_8+CH_4$ hydrate during the replacement will help in proposing the target gas hydrate reservoirs without the concerns of large-scale dissociation of the hydrate-bearing layers, and the cage-specific replacement pattern of $C_3H_8+CH_4$ hydrate with an external $CO_2/N_2$ gas will offer more secure $CO_2$ storage in the large cages of sII hydrate due to the restricted free $CH_4$ attack in the future.