Numerical study of depressurization and hot water injection for gas hydrate production in China's first offshore test site

Abstract

In 2017, China successfully conducted its first offshore gas hydrate production test by using vertical well in the Shenhu area, achieving a total gas production of 3.09 × 105 m3. However, there is still a long way to commercial production. In this study, to evaluate and improve the gas production potential of China's first hydrate production site, depressurization, and the combination of depressurization and hot water injection, were applied to natural gas hydrate (NGH) reservoir in China's first offshore hydrate production site to analyze 10 years' hydrate production features. The results indicated that the decomposition front of hydrate when using depressurization in two horizontal production wells had obvious non-uniform characteristics, and there were rapid decomposition regions and hydrate reformation regions. Among them, the hydrate reformation region may have adverse effects on hydrate decomposition. The combination of depressurization and hot water injection could improve total gas production and alleviate hydrate reformation. However, when the temperature of hot water injected increased from 35 °C to 80 °C, the energy efficiency decreased from 10.22 to 1.53, as a lot of energy was consumed to heat the sediments in the reservoir. When the hot water injection time was reduced from 3650 days to 1825 days, the energy efficiency could be effectively increased from 10.22 to 21.78 without significant reduction of the gas production. This work contributed to the present understanding of production behavior of depressurization and the combination of depressurization and hot water injection, and helped to evaluate the influence of hot water injected on gas production from NGH reservoir.