Application of horizontal wells to the oceanic methane hydrate production in the Nankai Trough, Japan

Due to the complicated reservoir conditions in the oceanic methane hydrate (MH) reservoir in the Nankai Trough, Japan, the gas production rate for the economical extraction had not been achieved during the current field production tests in 2013 and 2017 using vertical wells. Therefore, this study aimed at the application of the horizontal wells to the oceanic MH production in the Nankai Trough. Since there existed three sub-hydratebearing layers with different physical properties (i.e., initial hydrate saturation, porosity, and intrinsic permeability) in the reservoir, some possible well configurations including single horizontal well patterns and dual horizontal well patterns were designed based on a multilayered geological model simulating the real oceanic MH reservoir in the Nankai Trough. Then, the effectiveness of these two kinds of well designs was verified via long-term simulations of the oceanic MH production by simple depressurization, and the optimal well configuration for each design was recommended. Furthermore, a combined method of depressurization and hot water injection was also tested based on the dual horizontal well pattern, and the sensitivity analyses indicated that a favorable gas production rate of 8.64 x 10(5) m(3)/day could be obtained within the first year, even under a relatively low injection temperature of 40 degrees C and a relatively small injection rate of 2 kg/s/m of well.