The characteristics and effects of Huff-n-Puff in shale with brine, aqueous surfactant solutions and CO2

In this study, we evaluated the permeability damage and throughput potential of shale cores, focusing particularly on the damage water inflicts on shale permeability and the potential for hydrocarbon extraction using various injection media. Nuclear magnetic resonance (NMR) was utilized to analyze the distribution of crude oil across different pore sizes and to determine the limits of their utilization under varied media injection cycles. The results indicate that the Shengli Oilfield FY shale possesses abundant inorganic pores, primarily composed of intergranular and intragranular pores. Organic matter is dispersed and contains numerous micro-fractures. While formation water has only a minor impact on the permeability of fractured cores, it significantly impairs matrix permeability. Moreover, when the formation water content exceeds 10%, the degree of damage will decrease. In contrast to formation water, distilled water causes serious damage to both matrix and fractured cores. Therefore, anti-swelling measures are essential to ensure effective development in subsequent stages. After five injection cycles, the final recovery rates for brine, surfactant, and CO2 injection are 11.5%, 29.3%, and 45.9%, respectively. CO2 injection demonstrates the best production enhancement, followed by surfactant injection. CO2 is particularly effective in each cycle, primarily due to its expansion and extraction mechanism. Surfactants enhance production mainly by reducing interfacial tension and altering wettability, resulting in significant improvement in the first cycle. Meanwhile, brine injection primarily augments production through improved wettability and capillary pressure, resulting in a moderate production improvement. Based on NMR experiments, the minimum pore size for CO2 injection is estimated to be around 10–15 nm, Surfactant injection necessitates pore sizes with a minimum of 30–40 nm and brine injection requires pore sizes of approximately 60–70 nm. This research provides valuable experimental data for on-site shale oil development and deepens our understanding of the mechanisms involved in hydrocarbon recovery from shale using different injection media.