Control Factors for Gas Recovery in the Second Methane Hydrate Field Test in the Nankai Trough

Japan conducted a methane hydrate field test from the AT1-P3 well in the Nankai Trough in 2017. The system geometry and field data of the layer thickness, pressure, temperature, porosity, and permeability were published. Methane hydrate dissociation in the production test was performed using a depressurization method in a vertical well. The mathematical model was developed to simulate the field test by the full implicit simulator of hydrate (FISH). The simulation domain with the scale of more than 500 m was large enough to meet the boundary constrains of the hydrate deposit. A reduction model was used to match the well-log permeability in the field. The simulation results indicated that excellent history matching of the field data was achieved, and the error of gas and aqueous phases produced were 12.04 and 11.54%, respectively. The low pressure and temperature areas, which almost completely overlapped with each other, were around the production intervals of the wellbore. Sensitivity analyses suggested that an increase of the control factors of (0) and S(G0 )in the hydrate-bearing layers (HBLs) that resulted in a notable improvement of gas production could be obtained by an increase of k(0) and S-G0 in the HBLs, and the fluid produced were mostly from the original gas and aqueous phases in the deposit. The most important reason for the confinement of the hydrate-dissociated area was the limitation of the P gradient around the production well in the short-term production test.