Search

Gas hydrate reservoir properties provide critical information on the controlling mechanisms of gas hydrate formation and accumulation in natural depositional environments. They also are essential to understanding and accurately predicting gas production characteristics. However, the properties of fine-grained reservoirs that host hydrates are poorly studied. Guangzhou Marine Geological Survey acquired a comprehensive set of logging-while-drilling (LWD) logs, in situ tests, and lab measurements from 2015 to 2018 in the W11-17 fine-grained, gas hydrate reservoir of the Shenhu on the South China Sea. Porosity, hydrate saturation, and gas saturation were derived from neutron porosity, nuclear magnetic resonance (NMR), and electrical resistivity logs of SH-W17 and SHSC-4J1 wells and compared with lab measurements of pressurized cores. Units A to C contain three hydrate-concentrated intervals, with a maximum thickness of 32 m and an average hydrate saturation of 0.32 ±0.05. The hydrate saturation was estimated using resistivity and NMR models. Units D and E were inferred to be approximately 20 m thick, free-gas layers containing gas hydrates. Compared to hydrate saturation, the estimations of gas saturation from resistivity and NMR models are less reliable with a range of 0.05–0.4 due to the coexistence of gas hydrates and free gas. Permeabilities estimated from the NMR log agree well with those from neuron porosity in non-hydrate bearing intervals but are slightly higher in hydrate-bearing intervals. By incorporating the lab measurements and in situ tests into the NMR pore-size analysis, the permeability of water-bearing sediments in the hydrate intervals in the SHSC-4J1 well can be constrained to the range of 0.002–0.1 md, with 0.015 md being our best estimate. The NMR pore-size geometries indicate gas hydrates appear to preferentially fill bigger pores within fine-grained reservoirs, which exhibit a similar behavior to coarse-grained reservoirs. Our resistivity and relative permeability modeling indicate that the growth of gas hydrates within pore spaces is characterized by pore-filling and cementation behaviors. • We report reservoir properties from the first hydrate production site offshore China. • Stable hydrate intervals are 32 m thick with a hydrate saturation of 0.32 ±0.05. • Unstable layers are 20 m thick and gas saturation is estimated to be 0.05–0.4. • Permeability is estimated to be in a range of 0.002–0.1 md. [ABSTRACT FROM AUTHOR]

• The gas production from hydrate dissociation accounting for about 85%. • Analysis of the radius of the hydrate dissociation area in this production test. • Five-year prediction of long-term influence radius of pressure drop. • Exploration of favorable areas for the formation of secondary hydrates. • Analysis of the response of pressure and temperature of hydrate reservoirs. The first offshore natural gas hydrate production test of China in 2017 has proved the feasibility of hydrate exploitation from clayey-silt reservoirs, which possesses the highest reservoirs than other types of hydrate resources. However, owing to the absence of monitoring wells in this production test, the hydrate dissociation behavior cannot be analyzed through pressure and temperature changes of hydrate reservoirs. This paper focuses on the simulation study on the detailed response of the temperature and pore pressure of hydrate reservoirs of Well SHSC-4 during the gas production by depressurization. Meanwhile, it highlights the analysis of favorable areas for the formation of secondary hydrates and the influence of the secondary hydrates on pressure and temperature field of hydrate reservoirs. The simulation results indicate that in the first 60 days, the hydrate reservoirs feature a dissociation radius of about 5 m, and the gas production from hydrate dissociation accounts for about 85%. After 1 year, 2 years and 5 years of hydrate exploitation, the influence radius of low-pressure area (<10 MPa) is 15 m, 16 m and 17 m, respectively, suggesting that the hydrate reservoirs have higher gas production efficiency in the first year. Furthermore, the temperature and pressure of hydrate reservoirs are not favorable to the formation of secondary hydrates in the first 60 days. In long-term production, secondary hydrates are mainly formed at hydrate dissociation front. This can increase the pore pressure and further decrease the effective stress in the local areas of hydrate reservoirs, thus affecting mechanical stability of the local hydrate reservoirs. [ABSTRACT FROM AUTHOR]