Flow and EOR Behaviors of Shale Oil in the Huff-n-Puff Process Using Water and CO2 Systems: Considering the Elastic Depletion Effect

How to enhance shale oil recovery after the horizontal well and hydrofracturing production becomes increasingly important. Here, the flow and enhanced oil recovery (EOR) behaviors of shale oil during the huff-n-puff process using water and CO₂ systems considering the elastic depletion effect were investigated by the precisely designed physics simulation experiments. During the elastic depletion process, distinct flow behaviors of oil and water were observed for the cores with varying permeability. Notably, cores with low permeability exhibited higher oil recovery than those with high permeability due to the strong forced imbibition effect and extended production time, which is advantageous for spontaneous imbibition to enhance the oil recovery. In the water huff-n-puff process, considering the elastic energy of the formation fluid can significantly enhance oil recovery values, reaching up to 50%, compared to approximately 10% when this energy is not accounted for. The early stage of water huff-n-puff primarily relies on the process itself, while the later stage is dominated by the elastic energy of the formation fluid. The effect of forced imbibition in the initial stage is significantly greater than that of spontaneous imbibition. Moreover, the addition of CO₂ to the water huff-n-puff process markedly increased the recovery rate. When CO₂ and formation water were injected at the same pressure difference, the efficiency of injecting CO₂ first followed by water was higher than that of the reverse order. Furthermore, the injection of carbonated water further enhances the imbibition efficiency, with better huff-n-puff effects observed in the low-permeability cores than in the high-permeability ones.