Abstract
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. • Site NGHP-02-16 was modeled by combining pressure-core analysis and log data. • Empirical parameters of Archie's equation were determined. • A bottom-hole pressure of 10 MPa or less is a possible field-test scenario at this site.
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