Pilot-scale experimental study on gas production from natural gas hydrate using horizontal well cyclic depressurization method

Natural Gas Hydrate (NGH), as a significant and promising energy resource for advancing low-carbon development, has long been the focus of global energy resource research. The research on the extraction methods of submarine natural gas hydrate has attracted much attention. Recently, the most promising production method is depressurization, but the regular depressurization method is difficult to meet the efficiency requirement for commercial production. Therefore, it is necessary to innovate the depressurization method in order to develop a more effective approach for NGH production. The Pilot-scale Hydrate Simulator (PHS), with a volume of 117.8L, was employed in this study to conduct four NGH decomposition experiments with horizontal well under different methods including the Regular Depressurization (RD), the Cyclic Depressurization Above the Quadruple point (CD-AQ), the Cyclic Depressurization at the Quadruple point (CD-Q), and the Cyclic Depressurization Below the Quadruple point (CD-BQ). The experimental results show that the horizontal well exhibits higher production efficiency than the vertical well with the CD-AQ mode. When horizontal well is adopted, compared with the RD, the CD-AQ, the CD-Q, and the CD-BQ all enhance gas production efficiency and hydrate decomposition rate. Among three cyclic methods, lower starting pressure results in greater gas output. In the CD-BQ mode, with an initial pressure of 2.2 MPa, ice formation occurs during the process, which not only hinders gas production but also introduces potential safety risks. Therefore, CD-BQ requires specific geological conditions to be viable. In contrast, CD-Q significantly improves hydrate decomposition efficiency while maintaining process stability during both the depressurization and cyclic depressurization stages. This method optimizes both the well type and the cyclic pressure range, providing a new strategy for the NGH commercial development. Further studies on better optimization methods can be conducted based on this.