An Experimental Study on Improved Production Performance by Depressurization Combined with CO2-Enriched Air Injection

Efficiently and safely producing natural gas stored as gas hydrates remains an urgent and, as yet, unsolved challenge. The CO2 replacement method is attractive for combining methane production and carbon dioxide storage but with low efficiency attributed to a barrier formed at interface between vapor and solid phase. Although there has been previous work in this area, development is restricted by the obscure underlying mechanism of improving recovery efficiency for long-term. In this study, a series of experiments were conducted aiming to obtain a new concept for improved recovery by the combination of depressurization and gas replacement method, in which air and CO2-enriched air were injected into artificial multilayer hydrate sediment at pressure ranges from 8.5 to 18.7 MPa. The recovery efficiency was investigated using a method that combines three-stage depressurization assisted with CO2-enriched air injection. Initial production pressure was found have a positive effect on recovery of methane via injection of air, while an opposite influence was observed for injection of CO2-enriched air. Compared with injecting air, injecting zCO-enriched air promotes the performance of gas hydrate production up to 74.4% recovery ratio. A novel multilayer hydrate cap nechanism is therefore proposed to describe the improved efficiency during the replacement depressurization process for the first me. The multilayer hydrate cap and its composition are largely dependent on initial conditions of injected gas, thereby causing limited recovery efficiency. The results obtained from this study are beneficial for future optimization of operation conditions to maximize efficiency and develop planning for natural gas hydrate resources.