Numerical analysis of gas production from layered methane hydrate reservoirs by depressurization

Many natural methane hydrate (MH) reservoirs are heterogeneous and are characterized by a layered structure. In this study, we numerically investigate gas production from a multi-layered hydrate reservoir by depressurization through a single vertical and a single horizontal well. This layered MH reservoir is constructed based on field test data at the AT1 site of the Eastern Nankai Trough in Japan, and involves three hydrate-bearing layers (HBLs): Upper HBL-1, middle HBL-2, and lower HBL-3. The simulation results indicate that the horizontal well shows a better gas production performance in comparison to the vertical well. Over a production duration of two years, the average gas production rate by using the horizontal well reached 7.3 x 10(4) ST m(3)/d, which is 5.7 times higher than that by using the vertical well. However, the gas-to-water ratio for both the vertical and horizontal well cases is low in absolute terms. Sensitivity analysis of gas production by the horizontal well indicates that both the very higher and lower levels of permeability in HBL-2 and hydrate saturation in HBL-3 are unfavorable for long-term gas production. In addition, decrease of vertical permeability in HBL-1 and HBL-3 can lead to lower gas production efficiency. (C) 2018 Elsevier Ltd. All rights reserved.