NMR Insights into Imbibition Flow During Shut-In Soaking in Fractured Tight Oil Reservoirs

Spontaneous imbibition plays a critical role in enhancing oil recovery. It also helps optimize fracturing fluid design in tight oil reservoirs. Conventional evaluation methods mainly rely on nuclear magnetic resonance (NMR) experiments on intact core samples. These methods can adequately characterize matrix imbibition behavior. However, they generally neglect the presence of artificial fractures. This limits their applicability for guiding hydraulic fracturing optimization in field development. To address this gap, this study introduces artificial fractures into tight sandstone cores from the Sichuan Basin. It integrates scanning electron microscopy (SEM) with online NMR technology. The imbibition performance of an oil-displacement slickwater system was systematically evaluated. The evaluation considered varying agent concentrations, injection rates, and initial oil saturation conditions. The results demonstrate that using a high-concentration oil-displacement fracturing fluid significantly enhances imbibition efficiency. For example, slickwater containing 0.3% oil-displacement agent increased oil recovery by 12.9%. This was compared with slickwater containing 0.1% agent. The high-concentration fluid also substantially improved oil mobilization from small pores. Furthermore, increasing the injection rate proved particularly beneficial for oil recovery across the micropore to mesopore range. The most pronounced improvement was observed in the micropore range. Initial oil saturation serves as another primary controlling factor for imbibition efficiency. As oil saturation increases, imbibition efficiency improves markedly. The optimized slickwater system developed in this study has been successfully applied in Well NC002. It achieved a tested oil production rate of 32 tonnes per day. This field application validates the effectiveness of the proposed approach. It also provides practical technical support for the efficient development of tight oil resources in the Sichuan Basin.