Multiphysical Responses of a Gas Hydrate Reservoir to Extended-Duration Gas Production on the Alaska North Slope

Abstract The field test of extended-duration gas production from a gas hydrate-bearing reservoir via depressurization─formally titled the JOGMEC–DOE–USGS Collaborative Gas Hydrate R&D Project in Alaska─was conducted in the Prudhoe Bay Unit on the Alaska North Slope during 2023–2024. The project was jointly implemented by the Japan Organization for Metals and Energy Security (JOGMEC), the National Energy Technology Laboratory (NETL) under the U.S. Department of Energy (DOE), and the U.S. Geological Survey (USGS). The primary objective of the project was to investigate reservoir responses during extended-duration gas production and to provide technical insights for future industrialization and commercialization. The operational period lasted 315 days (approximately 10 months), from the start of electric submersible pump (ESP) operation on September 19, 2023, to the end of jet pump operation on July 30, 2024. Excluding shutdown periods associated with surface facility and downhole equipment issues, troubleshooting, and pump replacement, the net gas production duration was 216 days. Although frequent gas-locking events of the ESP, caused by higher-than-expected gas-to-water production ratios, hindered stable pump operation, sustained gas production was maintained for about 2 months at a flowing bottomhole pressure (FBHP) of approximately 850 psi (≈5.9 MPa). During this period, the gas and water production rates were approximately 60 Mscf/day (1,700 m3/day) and 17 bbl/day (2.7 m3/day), respectively. In the final stage of production operation with the ESP, pressure communication across the sand control device decreased significantly. Even after replacing the ESP with a jet pump to substantially reduce the FBHP, a sufficient reduction in reservoir pressure could not be achieved. Temporal and spatial variations in reservoir pressure, temperature, and strain associated with gas hydrate dissociation were successfully monitored using multiple gauges and sensor cables installed in three observation wells at different distances from the production test well. This paper presents an overview of the key data obtained from the production test and provides analysis results of the observed hydraulic, thermal, and mechanical responses of the reservoir. The most notable reservoir responses observed during the production test included (1) during approximately 2 months of continuous production at a constant FBHP, increases in reservoir permeability associated with gas hydrate dissociation and expansion of the dissociation zone were confirmed; however, no clear increase or decrease in the gas production rate was observed; and (2) the gas-water ratio (GWR) of the produced fluids was higher than that reported in previous gas production tests from gas hydrate-bearing reservoirs. These findings, derived from the acquired data, provide guidance for identifying evaluation points and for shaping the direction of future studies focused on understanding phenomena within the reservoir through geological, reservoir, and geomechanical modeling and simulation.