A Mathematical Model for Water Huff-and-Puff in Tight Oil Reservoirs Considering the Effect of Low-Salinity Water

To address the insufficient consideration of osmotic pressure in existing studies on water huff-and-puff in tight oil reservoirs, this paper introduces both osmotic and capillary pressures into two-phase flow theory and develops a full-cycle model for low-salinity water injection. The effects of salinity, injection rate, and soaking time on pressure, saturation, and salt concentration are analyzed. Results show that lower salinity enhances osmotic drive and improves recovery. Specifically, 1000 mg/L yields the highest recovery (10.94%) and an osmotic gradient of ~3.9 MPa. Capillary pressure acts synergistically with osmotic pressure during the soaking stage, promoting spontaneous imbibition and fluid redistribution. An optimal injection rate exists: excessive rates reduce swept volume, while very low rates prolong the cycle. Quantitatively, 80 m3/d is preferred (recovery 10.94%) over 40 m3/d (11.13%) as it halves the injection time with comparable recovery. There is also an optimal soaking time beyond which incremental gain diminishes. Recovery rises from 9.32% at 20 d to 10.94% at 40 d, with only 0.28% and 0.20% gains afterward; thus, 40 d is optimal. The model captures osmotic-driven behavior and reveals the secondary displacement effect during soaking, supporting parameter optimization.