Pilot-scale experimental study on natural gas hydrate decomposition with innovation depressurization modes

Natural gas hydrate (NGH), a novel energy source characterized by large reserves, and wide distribution, has attracted worldwide attention in recent years. How to develop NGH economically and efficiently has always been a research hotspot. Although the depressurization method is currently considered the most promising production method, the production efficiency still cannot meet commercialization needs. Therefore, innovation depressurization modes need to be developed to achieve the goal of commercial production of NGH. This study applied a pilot-scale hydrate simulator (PHS) with an effective volume of 117.8 L to conduct three NGH decomposition experiments under different depressurization modes, including the Regular Depressurization (RD), the Cyclic Depressurization (CD), and the Cyclic Depressurization below the Quadruple point (CD-Q). The behavior of the NGH decomposition and the decomposition efficiency were research focus in this study. The experimental results indicate that the lower the final reservoir pressure leads to the higher the total gas production. In comparison to the RD mode, both the CD and CD-Q modes exhibit significant advantages regarding the total gas production volume and the effective average gas production rate. In the initial stage of production, the flow of NGH decomposition products in the wellbore resembles “Slug Flow”, while in the later stage of production, it resembles “Flow with Liquid Entrainment”. The decomposition rates of NGH reached 100% under both CD and CD-Q modes. Particularly, under the CD-Q mode, the decomposition rate of NGH approached nearly 90% at the end of the depressurization stage. The CD mode can be combined with a multi-well network to perform asynchronous cyclic depressurization to achieve high-efficiency continuous gas production. This method provides an innovation strategy for both NGH commercial development and carbon storage.