Analyzing the process of gas production for natural gas hydrate using depressurization

• Hydrate dissociation behavior was analyzed in porous media by depressurization.
• The gas production process can be divided into three main stages.
• Methane hydrate first dissociates simultaneously throughout the hydrate zone, and then from the outside.
• The sensible heat of the reservoir and ambient heat transfer play a dominant role in hydrate dissociation.

Natural gas hydrate is a vast energy resource with global distribution in permafrost regions and in the oceans; its sheer volume demands that it be evaluated as a potential energy source. Understanding the mechanisms of natural gas extraction from hydrate-bearing sediments is critical for the utilization of hydrate accumulations. In this work, methane hydrate dissociation was performed in three kinds of porous media at production pressures of 2.2 MPa, 2.6 MPa, and 3.0 MPa. Results show that the methane gas production process can be divided into three main stages: free gas liberation, hydrate dissociation sustained by the sensible heat of the reservoir, and hydrate dissociation driven by ambient heat transfer. In the process of gas production, hydrate dissociation occurs simultaneously throughout the hydrate zone along the phase equilibrium curve, and then spreads radially from the outside as a result of ambient heat transfer. Hydrate reformation and ice generation always occur in the reservoir interior due to insufficient heat transfer. The use of porous media with increased thermal conductivity accelerates the gas production rate; however, it has little influence on the final percentage of gas production. Furthermore, the Stefan (Ste) number and dissociation rate constant were employed to evaluate the impact of the sensible heat of the reservoir and ambient heat transfer. Results indicate that the sensible heat of the reservoir and ambient heat transfer play a dominant role in hydrate dissociation, and that both are dependent on production pressures.

Schematic diagram illustrating the process of gas production in hydrate-bearing sediment induced by depressurization. When depressurization occurs, the reservoir pressure and temperature change along the trajectory of A–B–C–D. Character of gas production process is outlined.Figure optionsDownload as PowerPoint slide