Numerical analysis of gas production potential from a gas-hydrate reservoir at Site NGHP-02-16, the Krishna-Godavari Basin, offshore India-Feasibility of depressurization method for ultra-deepwater environment

Sandy gas-hydrate reservoirs are considered to be promising deposits for gas recovery, but the feasibility of the depressurization method in ultra-deepwater environments remains unclear because high pressure gradient induced by depressurization may increase the water production and reduce the gas-water ratio of produced fluid. In numerical simulations, this study predicts the gas production behavior of a hydrate reservoir (Site NGHP-02-16, in the Krishna-Godavari Basin, offshore India) located in ultra-deepwater (depth 2546.5 m). The reservoir properties such as the hydrate saturation and initial effective (in-situ) permeability were determined from log data and pressure-core analysis. By comparing the hydrate saturations calculated from resistivity log with the estimations from P-wave velocities of pressure-cores, the model parameters a, M, and n of Archie's equation were determined as 1.8, 2.0, and 2.9, respectively. The initial effective permeability of the hydrate-bearing layers (with up to 78% hydrate saturation) was estimated to range from approximately 0.01 to 1.0 millidarcy (mD). At day 180 after the start of depressurization in a single vertical well, the gas production rates ranged from 320 to 9500 Sm-3/d, depending on the bottom-hole pressure. Lowering the bottom-hole pressure increased the gas production rate and gas-water ratio of the produced fluid. The bottom-hole pressure of 10 MPa or less is recommended for the production test condition in this site; however, aquifer below the gas-hydrate-bearing zone and poor hydraulic sealing of the over- and under-burden increased the water production. Hydraulic isolation of the gas-hydrate-bearing layers is essential when applying depressurization in an ultra-deepwater environment.