Mechanism and Parameter Optimization of Energy Replenishment and Permeability Enhancement by Waterflooding in Tight Reservoirs

Tight reservoirs in the Yanchang Oilfield face challenges such as low formation energy and rapid production decline under depletion-driven development, necessitating effective formation energy replenishment to sustain enhanced oil production. Waterflooding-based energy replenishment and permeability enhancement is an innovative strategy designed to improve tight reservoir performance. Specifically, the rapid injection of high-rate water coupled with the cyclic “injection–soaking–displacement” process effectively restores formation energy while simultaneously enhancing reservoir permeability and oil recovery. In this study, experiments investigating energy replenishment and permeability enhancement via waterflooding were conducted using a high-temperature, high-pressure online nuclear magnetic resonance (NMR) system. The experiments were designed to evaluate the impact of key operational parameters—including injection pressure, injection volume, soaking time, and the number of “injection–soaking–displacement” cycles—on oil displacement efficiency and to elucidate the underlying displacement mechanisms of this enhanced waterflooding approach. Furthermore, the Sobol index method based on variance decomposition was applied to perform a systematic sensitivity analysis of individual parameters, quantitatively evaluating the influence of each factor on oil recovery. Experimental NMR T2 spectrum data demonstrated that the combined water injection and energy replenishment process increased the contribution of small pores by 7.06% and large pores by 9.68%, confirming significant permeability enhancement. Parametric analysis showed that within an optimal range, the factors influencing oil displacement efficiency ranked by descending effectiveness were the injection volume, number of injection–soaking–displacement cycles, injection pressure, and soaking time. Field implementation of these optimized parameters ultimately achieved a cumulative oil increase of 300 t. The results provide theoretical insights and practical technical guidance for applying waterflooding-based energy replenishment and permeability enhancement in low-permeability and tight reservoirs.