Pore-Scale Simulations to Enhance Development Strategies in Offshore Weak Water-Drive Reservoirs

Weak water-drive offshore reservoirs with complex pore architecture and strong permeability heterogeneity present major challenges, including rapid depletion of formation energy, low waterflood efficiency, and significant lateral and vertical variability in crude oil properties, all of which contribute to limited recovery. To support more effective field development, alternative strategies and a deeper understanding of pore-scale flow behavior are urgently needed. In this work, CT imaging and digital image processing were used to construct a digital rock model representative of the target reservoir. A pore-scale flow model was then developed, and the Volume of Fluid (VOF) method was applied to simulate and optimize waterflooding schemes aimed at boosting oil recovery. Optimization focused on adjusting injection rates, varying the oil–water viscosity ratio, and implementing a water-alternating-gas (WAG) process. Results show that, for equal injection volumes, higher injection rates cause early water breakthrough through high-permeability pathways, yielding slower gains in recovery. Lowering the oil–water viscosity ratio improves mobility control, suppresses viscous fingering, enlarges sweep volume, and enhances recovery. When CH₄ becomes fully miscible, it dissolves into the crude oil, lowering viscosity and eliminating interfacial tension, thereby providing greater displacement efficiency than partially miscible injection. Following a switch from water to gas injection, residual oil saturation decreases and becomes more uniformly distributed, indicating that the combined action of water and gas significantly improves both sweep efficiency and microscopic displacement.