Review on Carbon Dioxide Replacement of Natural Gas Hydrate: Research Progress and Perspectives

Natural gas hydrate (NGH) is a new energy with great potential that has the advantages of large reserves and high energy density and has become the key exploitation target of many countries in the world. Compared with traditional exploitation methods, such as the heat injection method and the depressurization method, the CO2 replacement method can meet the demand of NGH exploitation and achieve the geological storage of CO2 at the same time, which has more advantages in environmental protection. This paper summarizes the feasibility study of this method, including thermodynamics, kinetics, and mechanical stability of reservoirs, and discusses the replacement mechanism of different types of hydrate; then it summarizes the research progress of improving the replacement efficiency and puts forward corresponding suggestions for the existing problems. The relevant conclusions are as follows: (1) The replacement reaction is feasible in terms of thermodynamics and kinetics, but the mechanical stability of the reservoir is uncertain during actual exploitation. (2) The contact position between CO2 molecules and CH4 hydrates is the main replacement position. The fugacity difference between different guest molecules dominates the replacement reaction and is affected by the mass transfer capacity. (3) At present, there is no consensus on whether the CH4 hydrate will dissociate during the replacement reaction. (4) sII and sH hydrates will undergo structural transformation during the replacement reaction and eventually become sI mixed hydrates. (5) Different phase CO2 and different guest molecules have different replacement efficiencies for CH4.