ABSTRACT:

Natural gas hydrates have considerable reserves in nature and have been recognized as an energy source in the future. Improving the gas production efficiency from natural gas hydrate-bearing sediments (GHBS) is of great significance to promote its industrial development. Radial jet drilling (RJD) is an effective method for stimulating and developing oil and gas resources. In this study, this method is proposed to exploit oceanic hydrate reservoirs. Based on the geological data of the SH7 site in the South China Sea, a 3D model was developed to simulate the hydrate extraction process with a two-branch radial well. Results indicate that the increase of radial branch length promotes the propagation of pressure drop and enhances the geothermal heat flow, which increases the hydrate decomposition rate in the reservoir. The branch length of radial wells is positively correlated with the gas production rate and the gas recovery ratio. However, the increase of branch length seems incapable of prolonging the duration of gas production. This work provides insights into the potential applications of RJD technology in field trials of hydrate production.

1. Introduction

Natural gas hydrates are solid, non-stoichiometric compounds of small gas molecules and water. They form when the constituents come into contact at low temperature and high pressure (Sloan, 2003). In nature, natural gas hydrates are found mainly in permafrost and shallow sub-seafloor sediments. Global assessments suggest that natural gas hydrates contain approximately 1015-1018 m3 of methane, and their carbon reserves are likely to be more than the total of other fossil energy resources worldwide (Yu et al., 2019; Zhang et al., 2021). Natural gas hydrates' development has essential effects on various issues, including the world's energy landscape, climate change, and natural disaster prevention and control (Li et al., 2016; Cui et al., 2019).

The natural gas hydrate extraction methods mainly include depressurization, thermal stimulation, inhibitor injection, and CO2 displacement (Yuan et al., 2013; Yang et al., 2016; Lee et al., 2017; Tupsakhare et al., 2017). Among these methods, depressurization is relatively simple and efficient. It is regarded as the most promising approach to achieve industrial gas production from GHBS by previous research and field trials (Yang et al., 2016; Terzariol et al., 2017). In 2013, by applying the depressurization method in a vertical well, the world's first offshore methane hydrate production test was implemented in the Nankai Trough, Japan. In 2020, by applying a horizontal well and reservoir stimulation, the hydrates extraction field trial in the South China Sea lasted 30 days. The average gas production rate set a world record of 2.87 × 104 m3/day (Ye et al., 2020). Thus far, the gas production rates in all field trials are much lower than the economic viability level, which is considered at least 5 × 105 m3/day (Sloan, 2003). Therefore, enhancing gas recovery efficiency becomes an urgent issue for the commercial extraction of hydrate resources.

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