Study of Microscopic Flow Mechanisms of Water Huff-n-Puff Considering Salinity Effects

Abstract To address the issues of declining performance over multiple cycles of water huff-n-puff and the unclear microscopic oil–water dynamic behaviors, this study employs microfluidic technology and polydimethylsiloxane (PDMS) chips to simulate reservoir structures. We systematically investigate the effects of key parameters─including the number of huff-n-puff cycles, injection rate, soaking period, reservoir geometry, and injection water type─on development performance. The experiments compare oil recovery efficiency in high- and low-permeability zones, analyze the characteristics of oil mobilization in individual cycles and throughout the entire process, and identify the dominant controlling factors. The results show that under a salinity of 5000 mg/L, oil recovery increases with more huff-n-puff cycles, but the rate of improvement slows. Higher injection rates and shorter soaking times lead to reduced recovery. Capillary forces play a less significant role in large-pore and wide-fracture structures. Injection water with high salinity (100,000 mg/L, including NaCl and CaCl2) suppresses oil recovery due to ion-crude oil interactions. In the absence of clay minerals, reservoir geometry and injection water type exhibit a stronger influence on oil recovery. This work provides a basis for optimizing water huff-n-puff operational parameters and elucidating the underlying microscopic mechanisms.