Research on the stimulation mechanism of well shut-in imbibition and parameter optimization during fracturing-flooding

Abstract

ABSTRACT In the development of low-permeability tight oil reservoirs, fracturing-flooding technology, with its dual advantages of “fracture creation and energy replenishment, and displacement by imbibition,” effectively addresses challenges such as poor water injection and low oil recovery. However, the mechanism of oil-water migration within the reservoir and the enhanced oil recovery during the shut-in imbibition process remain unclear, and the construction parameters are primarily based on empirical data. In this study, focusing on the Y block in the Bohai Bay Basin, eastern China, we conducted dynamic imbibition experiments and utilized nuclear magnetic resonance (NMR) technology to analyze the key factors influencing imbibition-driven oil displacement and the microscopic oil-water migration mechanisms within the pore space. Numerical simulation was then employed to optimize the construction parameters for fracturing-flooding operations. The results indicate that displacement and imbibition primarily occur in medium-sized pores, while the poor connectivity of micro- and small-sized pores leads to suboptimal effects. The shut-in process provides favorable conditions for imbibition displacement, allowing oil in small and medium pores to be further displaced into fractures. The main factors affecting imbibition efficiency include core permeability (optimal range: 1.5 mD to 4 mD), fracture length (approximately 2/3 of the core length), fracturing-flooding fluid concentration (optimal around 0.2%), and shut-in time (optimal range: 24 to 48 hours). Based on production capacity simulations, the optimal construction parameters for the Y block include an injection rate of 1152 to 1440 m3/min, a total injection volume of 20,000 to 25,000 m3, and a shut-in time of 30 to 45 days. The application of these optimized parameters resulted in an increase in the average daily oil production in the Y block from 4.4 tons to 7.7 tons, demonstrating a significant enhancement in reservoir productivity and oil recovery.