Enhanced Methane Production Efficiency with In Situ Intermittent Heating Assisted CO2 Replacement of Hydrates

Natural gas hydrates attract worldwide attention for extensive reserve. Current exploitation methods of thermal stimulation, depressurization, and inhibitor injection consume much energy, inducing formation instability or marine biological problems. In this work, a novel in situ intermittent heating-assisted CO2 replacement (IIAR) method was proposed to enhance methane production efficiency of hydrates. Experiments investigated recovery performance with an electrical rod simulating the in situ heating, tested at 275.15-279.15 K and 4.00-13.06 MPa. The results indicated that the presence of N-2 and liquid CO2 was conducive to CH4 hydrate exploitation. Rising temperature and decreasing pressure assisted IIAR dissociate partial hydrates and promote further CH4-CO2/N-2 replacement. The maximum CH4 recovery percentage was 52.42% obtained at 279.15 K and 8.01 MPa in 48 h. The exploitation time could be shortened by 40-60% using IIAR compared with pure CO2 and CO2/N-2 replacement. The improved recovery performance indicated that IIAR was more competitive than pure CO2 and CO2/N-2 replacement and CO2 replacement combined with thermal stimulation. Further reduction of exploitation time and improvement of methane production efficiency demonstrated that intensive heating modes of IIAR increased exploitation economy and reduced methane consumption.