Effects of Bottom-Hole Pressure on Energy Recovery from Three-Phase Hydrate-Bearing Sediments with Underlying Free-Gas Reservoir via Depressurization

Methane hydrates are considered as the future clean energy resource. Geological exploration results indicate that the symbiosis of underlying gas is a typical characterization of natural gas hydrate (NGH) reservoirs. Co-production from NGH reservoir and underlying gas reservoir shows significant potential for future commercial production. However, the fluid production, thermal response, and sediment displacement evolution during co-production are still unclear and warrant investigation. In this study, we synthesized three-phase methane hydrate-bearing sediments with hydrate saturations of 12.0 and 26.0% at 15.0 °C and prepared the underlying gas reservoir with gas saturation of 87.7% at 17.5 °C. Fluid production and evolution of temperature and sediment displacement were examined during depressurization from the underlying gas reservoir under four bottom-hole pressures, i.e., 8.0, 6.0, 4.0, and 2.0 MPa. A novel quantification method was developed for estimation of gas and water production from each reservoir. By lowering bottom-hole pressure from 8.0 to 2.0 MPa, gas recovery ratio increased by nearly 30% in both cases. Water production was significantly delayed compared with gas production and only started when water saturation of underlying gas reservoir reached above 40% in all cases. Increasing S_H from 12.0 to 26.0% result in a decrease in the minimum temperature of three-phase methane hydrate-bearing sediments from 7.5 °C to 2.5 °C. Displacement sensor monitors the downward displacement of the three-phase methane hydrate-bearing sediments during depressurization. The volume strain increases from 0.12 to 0.38% when decreasing BHP, while that for low S_H only increases 0.06%. Our findings expand the understanding of fluid production behaviour from three-phase methane hydrate-bearing sediments with underlying gas. It provides guidance in the optimization of producion strategy for future field-scale co-production tests.