Experimental investigation of the behavior of methane gas hydrates during depressurization-assisted CO2 replacement

CO2 replacement with depressurization assistance is a crossover concept that has been proposed in recent years for natural gas hydrate development. It combines the merits of two traditional methods to improve production and reduce risks. To verify the feasibility of this new method and evaluate the corresponding efficiency, experimental apparatus has been designed to simulate the horizontal interface between a CO2 injection well and a CH4 production well based on the independent extraction mechanism and well structure design. To prepare natural gas hydrate-bearing samples under conditions that conform to actual field conditions, silica sand is packed into a core-holding tube with methane gas and distilled water at appropriate pressures and temperatures. Then a horizontal extraction process with an injection well and a production well are simulated, and the parameters influencing the pressure system, such as inlet pressure (pressure of injection well), outlet pressure (pressure of production well), and confining pressure (pressure of the overburden layer), are controlled as variables and analyzed. The ratio of generated methane and injected CO2 is used to evaluate the corresponding utilization efficiency (UE) in this work. Results indicate that all these pressure parameters have certain effects on CO2 replacement behavior. Generally, the fluid flow driven by pressure differences, and the special phase equilibria properties are the essence of their influence. The feasibility of enhancing CO2 utilization efficiency with depressurization assistance is verified based on experimental data. In consideration of time and cost, the relative pressure parameters should be optimized comprehensively to maximize commercial efficiency before field application.