Abstract: Natural gas hydrate resources have great potential. The key to development and utilization of hydrate resources is an efficient mining method. Huff-and-puff hot water injection has been widely used in the petroleum industry. To better understand how huff-and-puff hot water injection is used to extract hydrate reservoirs, the mining performance of hydrate reservoirs during huff-and-puff hot water injection in a horizontal well are explored by numerical simulations, based on the actual hydrate reservoir parameters. The mining performance of huff-and-puff hot water injection is also compared with the simple depressurization method and simple heat injection methods. The results show that when extracting the hydrate resources using the huff-and-puff hot water injection method, the gas production rate changes periodically. With increasing number of cycles, the production time for one cycle becomes longer and the overall extraction rate of decomposition gas shows an increasing trend. These findings indicate a better extraction performance. However, the fraction of the decomposition gas in the output is relatively small, and proper approaches are required to enhance the production ratio. The decomposition of hydrates expands outwards gradually from the horizontal wellbore, but the most effective decomposition occurs in the region close to the well. The huff-and-puff hot water injection method can realize both depressurization and heat injection functions when used for extracting hydrate resources; it therefore exhibits a better extraction performance compared with only using either the depressurization or heat injection methods. In general, the huff-and-puff hot water injection method allows for a greater gas production rate, higher gas decomposition rate, and high decomposition level of hydrates.

Key words: hydrate reservoir, huff-puff hot water injection, mining performance, horizontal well, numerical simulation