海洋可燃冰采掘利用系统设计及储层尺度的数值模拟

Oceanic methane hydrate utilization system design and reservoir scale numerical modeling

近年来,海洋可燃冰的开发利用已经成为国际能源研究领域的重要新兴课题.本文介绍了一种利用海洋可燃冰系统的低碳排放的开采,并进行了经济性论证和开采策略的数值分析工作.以日本南部海域海槽和中国神狐海域的开采情景为例,着重针对近年中日两国的开采活动及其开采策略进行了分析.研究表明:单一的降压开采和注热开采会遇到复杂的井口效应和能效问题,策略性组合设计能够获得较好的开采效率.研究确证了地层热管理和注热水、地层热流对可燃冰储层分解过程的影响.总结了当前海洋可燃冰开采方式的利弊,提出了低碳排放的开采方法,以期为未来进一步的海洋可燃冰试采工作和规模化利用提供有益的参考.

In recent years, the exploration and utilization of methane hydrate from permafrost and off-coast regions has become a very hot topic. Aiming at its potential as a vast energy resources in future use, the current study is focused on the low emission extraction and utilization system for oceanic methane hydrate and its economic and technological feasibility. In this study, recent real extraction tests in both China and Japan in 2017 are firstly discussed and compared, where the challenges in production period and production rate are explained. In those tests, depressurization method is used to dissociate the methane hydrate for gas production form the unconsolidated reservoir layers below the seabed. Based on the conditions of Japan, a systematic utilization system is proposed for oceanic methane hydratewhich is consisted of extraction system, power generation system, Carbon Capture and Sequestration（CCS） system and hot fluid injection system. Vertical well systems are adopted in this design and the strategic warming up process is used before depressurization extraction, so as to obtain optimal production rate of methane gas. The inclusion of CCS makes it possible to utilize methane hydrate with very low carbon emission. It is estimated that the electricity price obtained by this system can be comparable with the current civil electric price in Japan if the production rate is around 1.0×10~5 m^3/d. Also in this study, the real extraction scenarios and possible strategies of Nankai Trough in Japan and Shenhu Area in China are re-visited and discussed by numerical models. It is found that the only depressurization or thermal stimulation method have the problems of wellbore deformation, pressure problem and energy efficiency problems, while combined strategic modes show the possibility of long-term continuous production. For the basic dissociation parameter analysis, the conditions near the equilibrium curve show higher production rate, which is due to higher initial reservoir temperature. Such results indicate the importance of heat supply inside the reservoir layer as the methane hydrate dissociation is endothermic. It is shown that with the increase of reservoir pressure, such temperature effects become more critical as the optimal initial temperature increases according to equilibrium curve. This results support the system design with strategic warming-up process in a sister well during methane hydrate extraction. In addition, the effects of reservoir layer thermal management, hot reservoir fluid flow effects on the dissociation behaviors are analyzed in this study. For vertical well analysis, the production rates under different permeability parameters show the same trend with real production, which in turn help confirm the in-lab permeability tests. Horizontal well simulation show the importance of overburden and under burden. If the confining layers are less permeable, the production rate becomes higher due to the limited intrusive water flow from the confining layers. In sum, this study summarized the advantages and disadvantages of oceanic methane hydrate extraction/utilization methods and put forward a low-carbon emission system. It is hoped that this study will contribute to the future development of both production prediction and real extraction tests.