Numerical Simulation Study on Unstable Water Injection in Shale Oil Reservoirs Considering Stress Sensitivity and Threshold Pressure Gradient

Unstable water injection is widely used in low-permeability shale oil reservoirs, but its performance under the combined effects of stress sensitivity and threshold pressure gradient remains uncertain. This study aims to determine how these two mechanisms jointly affect unstable water injection performance and to optimize the switching timing and half-cycle period. Core-scale stress sensitivity and flow experiments were conducted, and the results were incorporated into reservoir simulation using the ROCKTAB and Threshold Pressure keywords in Eclipse, enabling a coupled numerical modeling approach. The simulation results indicate that both stress sensitivity and threshold pressure gradient significantly affect ultimate recovery, with stress sensitivity exerting a considerably greater influence. Among the half-cycle scenarios evaluated, a 15-day half cycle yields the best development performance, and longer half cycles are associated with reduced stimulation effects. Extended shut-in periods cause greater formation pressure fluctuations, which may induce additional stress-sensitive damage and impair well productivity. These findings provide practical guidance for designing unstable water injection strategies in low-permeability shale reservoirs. Among the discrete cases simulated, the optimal timing for switching from continuous to unstable water injection is when the water cut reaches 25%, although this value is case-specific; conversion at water cut beyond this threshold leads to progressively diminished oil response.