Understanding Immiscible Natural Gas Huff-N-Puff Seepage Mechanism in Porous Media: A Case Study of CH4 Huff-N-Puff by Laboratory Numerical Simulations in Chang-7 Tight Core

In view of the difficult problems encountered in the development of tight oil reservoirs, using natural gas as an energy supplementary agent and adopting Huff-N-Puff (HNP) production methods have shown great application potential in the development of such reservoirs. In this study, we took the Chang-7 tight oil cores as the research object. Then, we choose natural gas (methane or the associated gas) as the injection agent and used an experimental simulation device to perform natural gas Huff-N-Puff experiment in tight cores. Finally, we analyzed the factors that affect the enhanced oil recovery factor through parallel comparative experiments. The results show that the natural gas HNP development method can effectively increase oil production based on primary depletion production, and its effective production periods were concentrated in the 1st and 2nd rounds. The flooding mechanism of natural gas HNP process focuses on interfacial aggregation and interfacial slippage of crude oil, which are conducive to enhance oil production. Moreover, when the gas agent was CH 4 , it was not necessary to reserve too long soaking time during CH 4 HNP process; when the gas agent was associated gas, a longer soaking time could be appropriately reserved. Additionally, the higher pressure CH 4 injection would aggravate asphaltene precipitation, which will hinder migration of oil and gas in the pore throat of tight cores. Besides, we established the corresponding numerical model via using CMG software. Generally, the CH 4 HNP process with higher gas injection pressure had higher oil recovery. However, the ultimate oil recovery under 30 MPa gas injection pressure was lower than that under 28 MPa gas injection pressure. Therefore, the appropriate gas injection pressure should be selected in accordance with reservoir conditions during CH 4 HNP process, making its benefits overweigh its harms.