Energy Efficiency Evaluation of EOR in Ultrahigh Water-Cut Reservoirs Based on Oil–Power Ratio and Pore-Scale Analysis

As global energy systems face mounting pressure to enhance oil recovery while minimizing energy input and carbon intensity, optimizing extraction from mature oilfields has become a strategic imperative for both energy security and emissions reduction. In ultrahigh water cut reservoirs (water cut >98%), conventional water injection becomes increasingly inefficient, yet current evaluation methods─focused solely on oil output or water cut changes─fail to reflect the energy performance of recovery strategies. Here, we propose an energy-efficiency-based evaluation framework centered on the oil–power ratio (η), a novel index grounded in mass conservation and energy conversion principles. This metric quantifies the incremental oil output per unit of energy input, offering a physics-informed approach to evaluating six enhancement strategies: water flooding, increased extraction rate, flow line transformation, interfacial tension reduction, displacement fluid viscosity enhancement, and water plugging. Using micro-CT imaging and pore-scale numerical simulations, we reconstruct core-scale pore networks and classify remaining oil into five morphological types─clustered, porous, columnar, film-like, and droplet─for targeted analysis of recovery mechanisms. Our findings show that η not only captures the energy efficiency of different measures but also identifies the optimal timing for their application. Among all tested strategies, water plugging achieves the highest oil–power performance, followed by flow line transformation and tension reduction. Furthermore, strategy-specific mobilization pathways for different residual oil types are revealed, providing mechanistic insights into microscale sweep efficiency. By combining the energy efficiency index with the pore-scale crude oil classification, the oil–power ratio is coupled with the microscopic remaining oil morphology for the first time, and the quantitative evaluation from’ only production’ to’ increasing oil per unit energy consumption’ is realized, which provides a basis for the evaluation of low-carbon oil production in ultrahigh water cut oilfields.